1
5.1.4 Diseases and Pests of Seedling Roc

Systems 34

5.1.4.1 Insect pests 34

Root weevils 34

Webworms (including cranberry

girdler) 38

European crane fly 40

Dark-winged fungus gnats 42

5.1.4.2 Fungal diseases of roots 44

Fusarium root rot 44

Water molds 46

Cylindrocarpon root rot 48

5.1.4.3 Nondamaging growths on root

systems 48

Root aphids 48

Mycorrhizae 48

5.1.4.4 Abiotic root diseases 49

Temperature stress 49

Waterlogged media 50

5.1.5 Diseases and Pests of Seedling

Shoots and Foliage 51

5.1.5.1 Fungal diseases of shoots 52

Grey mold 52

Fusarium root rot 54

Water molds 54

Other fungal diseases of shoots and 

foliage 54

5.1.5.2 Insects affecting seedling shoots and 

foliage 56

Aphids 56

Spider mites 57

Whiteflies 58

Lygus bugs 58

Thrips 60

5.1.5.3 Abiotic problems affecting seedling 

shoots 61

Fertilizer injury 61

Heat or drought injury 62

Chemical injury 63

Air pollution 63

5.1.5.4 Problems caused by other plants 64

Weeds 64

Cryptogams (moss, algae, and

liverworts) 66

Contents

5.1.1 Introduction 4

5.1.1.1 Definitions 4

Stress, injury, disease, and damage 4

Abiotic versus biotic stresses (pests) 4

Symptoms and signs 5

Disease and pest problems 5

5.1.1.2 Diseases and pests in container tree 

nurseries 6

Pests 6

Abiotic disease 7

5.1.1.3 Disease development in a container 

nursery 8

Factors involved in disease 

development 8

Development of biotic diseases 8

The importance of environment in container nursery 

diseases 10

5.1.1.4 Discussion of pesticides and chemical control 

techniques 12

5.1.2 Diagnosis of Diseases and Pest

Problems 13

5.1.2.1 Identifying diseases and pests in container

nurseries 13

5.1.2.2 Steps in disease and pest diagnosis 13

Assistance with disease and pest 

problems 19

Collection, storage, and shipping of diseased 

samples 19

5.1.2.3 Assessing disease and pest impact 19

5.1.3. Diseases and Pests of Seeds and

Germinants 20

5.1.3.1 Seed disease and pest problems 20

Sowing problems 21

Bird or rodent predation 21

Seed dormancy problems 22

Seed fungus 22

Damping-off 24

5.1.3.2 Diseases and pests of germinants and young

seedlings 29

Postemergence damping-off 29

Heat injury 29

Chemical injury 29

Cutworms 31

Bird damage 32

Sirococcus blight 32

2
3

5.1.8 Pesticide Use 90

5.1.8.1 Legal implications 90

5.1.8.2 Application techniques 90

Seed treatments 90

Growing medium drenches 90

Liquid sprays 90

Baits and aerosols 91

5.1.8.3 Safe use of pesticides 91

5.1.8.4 Pesticide storage 92

5.1.8.5 Disposal of hazardous chemicals 92

5.1.9 Conclusions and

Recommendations 93

5.1.10 References 94

5.1.6 Diseases and Pest Problems During

Seedling Storage 69

5.1.6.1 Problems during sheltered storage 69

Cold injury 69

Winter drying 70

Animal damage 72

5.1.6.2 Problems during refrigerated storage 74

Botrytis storage mold 74

5.1.7 Disease and Pest Management

Strategies 76

5.1.7.1 Philosophy of management instead of

control--integrated pest

management 76

5.1.7.2 Exclusion: preventing pest entry 77

Selecting the site 77

Using certified seeds, transplants, and

cuttings 78

Treating seed 78

Treating growing media 81

Sterilizing containers and growing area 

surfaces 82

Treating irrigation water 84

Maintaining physical barriers 84

Following hygienic practices 84

5.1.7.3 Protection: protecting seedlings from existing; 

pests and abiotic stress factors 84

Modifying the environment 85

Cultural practices 85

Managing microorganisms by using 

suppressive growing media 85

Chemical protection 86

5.1.7.4 Eradication: eliminating pests after disease 

establishment 87

Roguing diseased individuals 87

Removing weeds 88

Chemotherapy: curing diseased seedlings with 

chemicals 88

Disinfecting the growing area 88

5.1.7.5 Resistance: managing pests through

genetics 89
The definition of a pest is subjective, however, because a 

biological stress becomes a pest when it has a negative effect on 

humans or their material goods. Pest-caused diseases interfere 

with the objectives in agricultural systems; in natural ecosystems 

there are no such things as pests (Bohmont 1983). In container 

nurseries, a pest can be defined as any biotic stress factor that can 

cause disease. Many nursery pests do not cause problems in 

natural forests, but in the controlled environment of a container 

seedling nursery, these normally innocuous organisms can cause 

disease (see section 5.1.1.2). The terms pest and pathogen are 

sometimes used synonymously, but the term pathogen is generally 

restricted to microorganisms that cause disease, such as fungi, 

bacteria, and viruses.

4

Abiotic versus biotic stresses (pests). Stresses that can 

produce disease can be either biological (living) or environmental 

(nonliving); environmental stresses include mineral deficiencies, 

damaging weather events such as frosts, mechanical injury, and 

chemical damage (table 5.1.1). Many different biological stress 

agents can cause disease, including fungi, bacteria, viruses, 

animals, and even weeds, which can cause growth loss through 

competition for water, light, or mineral nutrients.

5.1.1 Introduction

Diseases and pests are a continuing source of concern to 

managers of container tree seedling nurseries, and successful 

managers are aware of the various factors that can harm their 

crops. Before we discuss these damaging agents and how to 

identify and manage them, it is necessary to discuss some of the 

common terms used to describe disease.

5.1.1.1 Definitions

Stress, injury, disease, and damage. In container nursery 

management, we are concerned with growing healthy seedlings 

that will survive and grow after outplanting. The concept of "health" 

infers an optimum condition of seedling physiology and 

morphology, and a stress is any factor detracting from that optimum 

state. Plant pathologists make a distinction between injury and 

disease based on the duration of the stress. An injury is the result 

of a single damaging event, such as a severe frost, that causes a 

temporary departure from the normal seedling condition. According 

to the classic definition of disease, a stress factor must cause a 

sustained negative effect on seedling growth before it can cause a 

disease. The distinction between injury and disease, therefore, is a 

matter of duration. The situ ation is confusing, however, because 

some stresses, such as weather or animal damage, can cause 

either injury or disease (fig. 5.1.1). Insect feeding on seedling 

foliage constitutes injury (a single "acute" event) whereas other 

insects, such as aphids, can cause disease because they feed 

slowly over an extended period (a "chronic" problem).

In the broadest definition of disease, any harmful stress, whether 

by living or nonliving agents, that interferes with efforts to produce 

healthy, vigorous seedlings within the standard nursery rotation 

can be considered a disease. A working definition of disease, 

therefore, is any sustained departure from the normal physiological 

or morphological condition that characterizes a healthy seedling. 

Damage differs from injury in that damage infers economic loss to 

the seedling resulting from either abiotic or biotic diseases.
Symptoms and signs. Although the term symptom is commonly 

used to describe both symptoms and signs, there is a particular, 

significant difference between the two terms in plant pathology. A 

symptom is a general term used to describe the physiological or 

morphological response of the host seedling to a stress factor. Disease symptoms can be obvious, such as foliar chlorosis, or more 

subtle, such as reduced growth. Symptoms are present in both 

biotic and abiotic disease. Signs are actual evidence of the causal 

organism and are therefore only found in biotic diseases. For 

example, the fungal mycelia that are sometimes evident on the diseased part of the seedling are signs of disease. Although both 

symptoms and signs are used in disease diagnosis, signs are more 

helpful because they implicate a specific organism or group of 

organisms, whereas symptoms may be caused by a variety of 

biotic or abiotic factors. (See section 5.1.1.3 for examples of 

symptoms and signs.)

Disease and pest problems. A disease or pest becomes a 

problem when it causes extensive economic loss. A pest that is 

causing insignificant losses is not generally considered to be a 

problem because the cost of treatment would exceed the economic 

benefit of control. The definition of a disease or pest problem,

therefore, is subjective in that it involves an assessment of actual 

economic impact, or the potential for economic impact, by the 

nursery manager. The prudent manager will attempt to promptly 

identify all diseases and pests in the seedling crop before they 

reach damaging proportions.

5
There were 19 different types of insects and related pests (such as 

mites) reported in the Container Nursery Survey (table 5.1.3). 

Aphids and cutworms were the most common insects pests, with 

16% and 15% of th e reported occurrences. Root weevils, 

European crane fly, and fungus gnats formed a second group with 

ratings of 8 to 10%. Ten minor insect pests with less than 1 % 

occurrence ratings were also reported, indicating that many 

incidental insects can cause disease in container nurseries. Again, 

a significant number of respondents (15%) reported no major 

insect problems.

5.1.1.2 Diseases and pests in container tree

nurseries

As you can see from the previous discussion, the classic 

definitions of the various causes of seedling disease and the 

distinctions between them are rather vague and subjective. 

Operationally, most container nursery managers distinguish 

between diseases, which are caused by microscopic pathogens 

such as fungi and bacteria, and pests, which are macroscopic 

organisms such as insects or rodents. Following the classic definition of disease, we will consider the injurious effects of abiotic 

stress factors as diseases.

Pests. Many pests are responsible for causing diseases in 

container tree seedling nurseries (table 5.1.1): the standard 

disease agents include fungi, insects, and nematodes as-well as 

less well-known pests such as snails, algae, and weeds. Other 

plant species, such as moss or weeds, are usually not considered 

pests in the traditional sense but, when they compete with the crop 

seedlings for growing space in the container, they cause growth 

reduction or even mortality and therefore constitute a disease.

The Container Nursery Survey asked nursery managers in the 

United States and Canada to report their major disease and pest 

problems and rank them in order of importance. Interestingly, 14% 

of the respondents reported that they had experienced no major 

disease or pest problems; the remaining nurseries, however, listed 

515 different problems. Fungal diseases ranked first, with 38% of 

the responses and, surprisingly, insect pests ranked a close 

second with 36%. Animal damage constituted the third major pest 

category, with 16%, and plant pests (including weeds, algae, 

mosses, and liverworts) the fourth, with 10% of the responses.

A breakdown of the fungal disease category from the Container 

Nursery Survey revealed four principal fungal diseases (table 

5.1.2). This category was dominated by two diseases: grey mold 

(39%) and damping-off (25%), which together accounted for almost 

two-thirds of the disease problems. That relatively few fungal 

diseases were reported in the Survey probably reflects the sanitary 

environment maintained in most container nurseries. In fact, 7% of 

the nurseries reported that they had no major fungal diseases.

6
Biotic pests often cause infectious diseases that can readily 

spread from seedling to seedling; this multiplication effect can 

occur rapidly in the favorable container nursery environment and 

often becomes serious. Many fungal pathogens begin as 

saprophytes, which feed on dead plant tissue, but then spread to 

living tissue and cause disease. Insect pests may build up populations on weed species and subsequently spread to crop 

seedlings.

Abiotic disease. Any environmental stress factor can cause 

disease when it adversely affects seedling growth, and table 5.1.1 

lists some of the more common abiotic stress factors. Several of 

these factors--heat, water, and mineral nutrients--are required for 

normal seedling growth but can induce physiological stress when 

they reach extreme levels. Other abiotic factors-air pollution, for 

example-often originate outside the normal container nursery 

environment. Mechanical injury can occur whenever seedlings are 

handled; it is important to realize that seemingly minor incidents 

can cause damage to succulent container seedlings.

7

Fifteen abiotic diseases were reported in the Container Nursery 

Survey (table 5.1.4). Temperature-related problems accounted for 

60% of the stress factors and nutrient problems, including pH and 

salt injury, were also commonly reported. Low sunlight intensity 

during the winter crop, primarily in northern latitude nurseries, was 

the major light-related problem.

Abiotic diseases are noninfectious because they do not spread 

from one seedling to another. Perhaps the major influence of 

abiotic stresses is that they can weaken the host seedling and 

predispose it to attack by biotic pathogens. Improperly applied 

cultural practices such as irrigation or fertilization can lead to 

abiotic disease.
8

5.1.1.3 Disease development in a container nursery

Container nurseries offer a unique opportunity to study the 

development of disease. Greenhouses are artificial environments 

that are initially pest-free and do not have the "background" pest 

levels that typify traditional bareroot nurseries; therefore, all 

biological pests are introduced. The biotic diseases caused by 

these pests develop in an environment that has none of the 

normal population controls found in the natural environment. 

Abiotic diseases also seem to develop more rapidly in container 

nurseries, probably because of the succulent nature of container 

seedlings.

Factors involved in disease development. Biotic disease has 

traditionally been described using the "disease triangle," which 

illustrates the interrelationships between the pest, the host, and the 

environment (fig. 5.1.3). Although many diseases may appear to 

involve only the host seedling and the biological pest, environmental factors are always involved to some degree. Environmental 

stress may weaken the seedling and predispose it to attack by the 

pest, or a particular environment may favor pest populations, 

enabling them to build-up to harmful levels.

With the elimination of the biological pest, abiotic disease can be 

visualized as a two-way relationship between the host seedling 

and adverse environmental stress (fig. 5.1.3). Abiotic disease may 

develop suddenly as the result of a single injurious climatic incident, such as a freeze, or more gradually as a difficult-to-detect 

growth loss resulting from below-optimum environmental factors, 

such as a mineral nutrient deficiency.

Development of biotic diseases. Because container nurseries 

are artificial systems that do not initially contain any endemic 

diseases, potential disease agents must be introduced into the 

nursery. Pests can enter in many ways: growing media, irrigation 

water, reusable containers, propagation materials (seeds, 

transplants, or cuttings), airborne inoculum, contaminated soil or 

growing medium on tools, etc., and mobile pests that enter the 

growing area directly (see section 5.1.7.2 for more information).

Many people tend to discount abiotic disease and focus instead on 

the more traditional biological disease agents such as fungi or 

insects. Sutherland and others (1982), however, reported that, for 

three of four conifer seedling species, abiotic diseases were more 

common than biotic diseases in disease samples submitted to a 

pathology laboratory (fig. 5.1.2). Consulting pest specialists usually 

find that abiotic diseases are very important in nursery pathology 

because a majority of seedling diseases are either directly caused 

by abiotic factors or are indirectly related to predisposing environmental factors.
The development of a biotic disease in a container nursery can 

best be illustrated with an example such as grey mold disease, 

which is caused by the fungus Botrytis cinerea (fig. 5.1.4). 

Good discussions of this disease are founding Coley-Smith and 

others (1980), Sutherland and Ban Eerden (1980), and James 

(1984).

9
B. cinerea is an aggressive saprophyte that colonizes dead or 

senescent plant material and thus can infect weeds or other plants 

adjacent to the growing area (fig. 5.1.4A). B. cinerea produces 

abundant spores, which can be aerially transported into the 

growing structure through the ventilation system (fig. 5.1.413). This 

airborne inoculum lands on seedling foliage, and these spores can 

germinate and infect senescent or injured foliage in the presence 

of surface moisture (fig. 5.1.4C). The fungus can build up rapidly 

on damaged tissue or in the lower shaded foliage that typically 

becomes senescent after seedling canopy closure. The major 

impact of B. cinerea occurs when the fungus invades the main 

stem of the seedling and causes a canker (fig. 5.1.4D). This canker 

will eventually girdle the stem, causing top dieback or completely 

killing the seedling.

The symptoms of grey mold are foliage necrosis and stem cankers, 

and the signs are grey mycelia and conidiophores on the surface of 

the affected foliage (fig. 5.1.4D). Spores from the infected 

seedlings can become a source of airborne inoculum, resulting in 

secondary spread of the disease later in the growing season (fig. 

5.1.4). The fungus is able to survive periods of adverse conditions 

over winter or between crops by forming special resting structures, 

called sclerotia, on the infected seedling or weed tissue. Sclerotia 

can persist in growing media or plant debris in the growing area

and eventually are able to produce spores when environmental 

conditions become favorable again (fig. 5.1.4E). The production of 

these airborne spores completes the disease cycle (fig. 5.1.4).

The development of other biotic diseases in container nurseries 

is conceptually similar to grey mold in that the pest is introduced 

into an essentially pest-free environment conducive to the build 

up of most biological pests.

The importance of environment in container nursery diseases. 

A container nursery is designed to optimize all the various 

environmental factors that affect seedling growth. Seeds are sown 

in an artificial growing medium that is usually free from pests and 

specifically formulated to produce an ideal chemical and physical 

environment. The growing area is designed to regulate sunlight, 

temperature, and wind, and fully controlled greenhouses have 

automatic environmental controls that maintain ideal levels of 

humidity, temperature, carbon dioxide, and day-length. Moisture 

and nutrient

10
11
The container nursery manager should be aware of this high 

disease potential, be especially vigilant for the appearance of new 

diseases or pests, and design growing schedules that promote 

vigorous, well-balanced growth. Environmental conditions should 

be closely monitored and alarm systems used to guard against 

potentially damaging weather.

5.1.1.4 Discussion of pesticides and chemical control

techniques

A variety of different pesticides are mentioned in this publication, 

but the reader should understand that their mention cannot be 

interpreted as an endorsement or recommendation by the Forest 

Service or the U.S. Department of Agriculture. The information on 

chemi cal control is presented as a review of published literature: 

no new research findings are included, and no specific pesticide 

recommendations can be made. Trade names and active 

ingredient (or common) names of specific chemicals are 

mentioned only to illustrate which pesticides have been used, 

either successfully or unsuccessfully, in the past. We hope that 

this type of information will help nursery managers make informed 

decisions about potential pesticide use.

Because this manual will be used in many different states, 

provinces, and countries, no attempt is made to list which 

chemicals are legally registered for use. Pesti cide registration is 

dynamic, and it would be impossible to keep registration 

information up to date. The most recent complete list of pesticides 

used on seeds and seedlings in tree nurseries can be found in 

Hamel (1983). Before using any pesticide, nursery managers 

should contact their local pesticide specialist for information on 

local registration and use. Modern nursery management requires 

the judicious use of pesticides, and so growers should learn all that 

they can about the action and impact of each specific pesticide 

chemical before implementing its use. Aspects of pesticide use are 

discussed in detail in section 5.1.8.

levels are carefully monitored and maintained at optimal levels. 

Seedling growing density is controlled by the size and spatial 

arrangement of the growth containers. The accelerated seedling 

growth that can be achieved in container nurseries yields crop 

rotations of less than 1 year compared to the 1 to 3 years in the 

typical bareroot operation.

Unfortunately, an environment designed to produce such rapid 

seedling growth also favors the development of many biotic and 

abiotic diseases. Landis (1984) discusses many of the factors 

responsible for the high disease potential in container nurseries:

Favorable climate. The moderate temperatures, high humidity, 

and low wind velocities in a container nursery are ideal for many 

pests.

Very dense stocking levels. Compared to the ideal stocking 

levels for bareroot seedlings of 161 to 269 seedlings/m2 (15 to 

25/ft2), the growing density of container seedlings can be 

1,076/m2 (100/ft2) or higher.

Monoculture. In most container nurseries, the crop consists of 

one species or several closely related seedling species, and 

individual seed lots are composed of seedlings from the same 

genetic base. To aggravate this condition, all the seeds are sown 

at the same time and therefore the seedlings are always at the 

same stage of growth.

Accelerated growth promotes succulence. The rapid seedling 

growth that is achieved in container nurseries often consists of 

relatively large, thin-walled cells that are very succulent; container 

seedlings are therefore quite susceptible to abiotic stresses and 

attack by pests.

Sterile environment. The controlled environment in container 

nurseries is designed for excluding pests: the growing medium is 

considered essentially sterile and the growing surfaces and 

containers are usually sterilized between crops. These conditions 

can lead to rapid disease development and spread because there 

is no complement of natural predators and competitors or 

environmental controls on pest populations-the "biological vacuum" 

discussed by Baker (1957). Insects such as whiteflies that have a 

rapid reproduction potential can build up to damaging levels in a 

short time in container nurseries.

12
13

Most diseases and pests affecting container seedlings cause 

readily identifiable symptoms such as discolored foliage, but many 

problems do not become evident until irreversible injury has 

already occurred. This is particularly true for diseases or insect 

injury of roots because foliar symptoms only develop after the 

roots are severely damaged. Minor stunting or undetectable 

growth loss is especially difficult to diagnose unless the grower 

has some type of growth standards for comparison. Seedling 

measurements, such as shoot height, stem caliper, and total dry 

weight, should be taken regularly and growth curves constructed 

for each seedling species so that "normal" growth patterns can be 

established. Growth of subsequent crops can then be compared to 

these growth standards and potential problems identified.

5.1.2.2 Steps in disease and pest diagnosis

A systematic approach to diagnosis of disease or pest problems is 

most effective, and Bohmont (1983) and Peterson and Smith 

(1975) provide step-by-step procedures. Blanchard and Tattar 

(1981) discuss the technical aspects of disease diagnosis, 

including handling disease specimens, and Streets (1972) presents 

an entire manual on the diagnosis of plant diseases. The following 

procedure is recommended by the author and requires only a 5 or 

10 power hand lens, a sharp knife, and an inquiring, open attitude. 

Streets (1972) states that the most important attribute in disease 

diagnosis is "the ability to observe accurately." If possible, make 

the diagnosis with other members of the nursery staff, especially 

those in charge of day-to-day cultural operations, because they 

may be able to relate the disease symptoms to some recent 

cultural or climatic incident.

5.1.2 Diagnosis of Diseases and Pest

Problems

Diagnosis consists of a systematic search for the causes of a 

disease or pest problem using symptoms, signs, and pattern of 

occurrence (Peterson and Smith 1975). Disease diagnosis in a 

container nursery consists of three phases: 1) identifying a 

disease, 2) diagnosing its true cause, and 3) determining the 

impact of the disease on nursery production.

5.1.:!.1 Identifying diseases and pests in container

nurseries

Identification of diseases and pests requires a certain degree of 

experience and training. Nursery workers need a rudimentary 

knowledge of seedling physiology and morphology before they can 

detect those minor deviations from the normal seedling condition 

that constitute a disease. This knowledge can come from either 

direct experience or formal training but ideally a grower will have a 

combination of both. A formal education in horticulture and 

seedling physiology provides a sound conceptual background, but 

there is no substitute for actual nursery experience. Direct 

"hands-on" experience with container seedlings at all stages of 

nursery culture is necessary to quickly recognize when seedlings 

appear abnormal.

Early detection is extremely important for controlling nursery 

diseases and pests, especially for integrated pest management 

(IPM) programs. Container seedling growers must adopt an 

attitude of vigilance and make periodic inspections of the crop so 

that problems can be identified early. Many pests are difficult to 

eradicate once they become established. Davidson and others 

(1988) recommend three different techniques for monitoring insect 

pests: visual inspection, beating, and trapping devices. Parrella 

(1987) recommends monitoring populations of nursery insect pests 

with yellow sticky cards placed throughout the greenhouse to 

determine if insect pests are present and if possibly harmful 

populations are building up. Baker (1986) presents a useful 

identification guide to greenhouse insects found on yellow sticky 

cards.
1. Check all parts of the seedling for symptoms and 

determine what parts are actually affected. Frequently, foliar 

symptoms are an indication of root disease (fig. 5.1.5), so remove 

the seedling from the container and carefully check the root 

system. Note the symptom pattern on the seedling itself-is one 

part of the shoot or root system affected more than another (fig. 

5.1.6)?

2. Determine whether all species or seedlots within a species 

are equally affected. Abiotic diseases usually affect several 

different species of seedlings, whereas biotic diseases are often 

restricted to one species (table 5.1.5). Environmental stresses are 

non-discriminatory, but biotic pests are often host-specific. 

Exceptions to this general rule do exist, however. Frosts can injure 

one species or ecotype, and there apparently is a genetic 

predisposition to other types of abiotic injury, such as pesticide 

phytotoxicity (fig. 5.1.7). When only a single, occasional seedling is 

affected, the problem is usually genetic (fig. 5.1.8).

14
15
16

3. Note the pattern of the disease symptoms within the 

growing area. Determine if the disease pattern is random or 

regular (table 5.1.5). What sections of the growing area are 

affected and are these areas related to any cultural operation such 

as the irrigation application pattern or to structural features in the 

growing area? Abiotic problems are usually expressed in a regular 

pattern that can be correlated to some cultural factor such as the 

growing tray (fig. 5.1.9) or position on the bench (fig. 5.1.10). Biotic 

diseases often initially show a random distribution because they 

develop from inocula randomly introduced from the air or on seeds 

(fig. 5.1.1 1). Disease "pockets" (fig. 5.1.12) are diagnostic because 

they typically result from the secondary spread of a biotic pest.

4. Check several symptomatic seedlings thoroughly with a 

hand lens for signs of a biotic pest. Fungal mycelia or fruiting 

bodies are sometimes visible on the affected tissue (fig. 5.1.13). 

However, insects in the growing area are often concealed, 

particularly in the growing medium. Collect specimens of any 

potential pests for subsequent identification.
5. Always consider the possibility of abiotic disease. Check 

cultural and weather records and ask nursery workers about any 

unusual incidents. Examine the growing medium for evidence of 

adverse conditions such as waterlogging (sour smell or excessive 

growth of algae) or salinity build-up (salt crust on the container, 

especially around the bottom drainage hole).

17
18

Assistance with disease and pest problems. Experienced 

nursery managers can diagnose common disease and pest 

problems in their nurseries and initiate proven treatments. It is a 

good idea, however, to have this tentative diagnosis confirmed by a 

pest specialist because many nursery diseases are relatively 

complicated and may involve more than one pest or a predisposing 

environmental stress factor. Accurate disease diagnosis is 

essential before proper control measures can be designed and an 

improper diagnosis could lead to additional seedling losses if the 

wrong control treatment was applied.

Many fungal diseases cannot be accurately diagnosed until the 

causal organism is isolated from the diseased tissue and grown 

on an artificial medium because the symptoms of many diseases 

are similar. For example, a particular shoot blight of conifer 

seedlings can be caused by either of two genera of fungi--

Sirococcus spp. or Sphaeropsis spp. These fungi can only be 

differentiated microscopically by examination of the fruiting bodies 

in slides of fresh diseased tissue or of cultures: the spores of 

Sphaeropsis are larger and darker than the spores of Sirococcus 

(fig. 5.1.13). This isolation procedure and taxonomic identification 

require the special laboratory equipment and techniques of a 

trained pest specialist.

Assistance with disease and pest problems is available from a 

variety of sources, including private pest consultants, State and 

Provincial forestry organizations, the extension service of State 

agricultural universities, and Federal forestry organizations. Pest 

specialists in the United States Government are located in the 

regional offices of the State and Private Forestry Division of the 

USDA Forest Service. Canadian nurseries should contact the pest 

specialists at their local office of the Canadian Forestry Service or 

Provincial extension personnel.

6. Establish the disease history. When did the symp toms first 

appear? Is this problem new or has it been observed before? Try 

to correlate these facts with cultural or weather records. Abiotic 

diseases are usually related to a particular damaging incident and 

their symptoms usually develop rapidly, whereas biotic diseases 

develop more slowly and may spread over time if environmental 

conditions are favorable (table 5.1.5).

7. Document your analysis of the disease problem with 

written notes and color photographs if possible. Many times 

disease symptoms or signs will change over time, or other 

saprophytic organisms may colonize the affected tissue and 

obscure the true cause of the problem. Collect diseased seedlings 

to send to a pest specialist for confirmation.

Insect pest problems are particularly difficult to diagnose because 

the insect is often gone by the time the symptoms become 

obvious. Diagnosis of insect-caused root injury is especially 

difficult because many root-feeding insects, such as root weevils, 

have larvae that live in the growing medium and adults that are 

active only at night. Insect problems can often be diagnosed by 

monitoring their populations within the growing area. Because 

they are so mobile and often nocturnal, the presence and 

abundance of many insect pests can be monitored with 

pheromone traps, which attract the adults.

After completing the disease investigation, it is a good idea to 

contact other local container nurseries to see if they have had 

similar problems. It may be that the disease or pest problem has 

already been identified, and effective control measures already 

tested and established
Collection, storage, and shipping of diseased samples.

Most disease and pest diagnoses require careful examination of 

the affected seedlings by a trained pest specialist, and so disease 

samples must be collected and shipped to a diagnostic laboratory. 

Sample collection and handling are discussed by Streets (1972) 

and usually consist of the following steps:

1. Collect samples as soon as the symptoms become evident 

and especially when signs develop. If sample collection is 

delayed, secondary organisms may become established and 

mask the symptoms of the original disease or insect problem. 

Sample the entire seedling so that the pest specialist can 

examine all facets of the disease. Collect a series of seedlings 

showing a gradation of disease, if possible, from healthy to 

severely damaged seedlings; this allows the pest specialist to 

make relative comparisons and estimate the disease impact. 

Leave the seedlings in the containers if possible so that pests 

in the medium or evidence on the containers can be 

examined.

2. Protect samples from deterioration due to heat or drying by 

placing the seedlings in plastic bags and storing them under 

refrigeration. Insects should be placed in a bottle with air holes 

and some plant material for food in the bottom. Make sure that 

all samples are properly identified with species, seedlot, age, 

date, description of the symptoms and signs, and any other 

useful information such as location in the growing area and 

previous cultural practices and weather conditions. Use pencil 

on all labels because ink often smears in humid sample bags.

3. Submit a written description of the disease problem with your 

tentative diagnosis and color photographs of the symptoms if 

possible.

4. Ship samples to the diagnostic laboratory as quickly as 

possible. The best procedure is to contact the pest specialist 

for specific handling and shipping recommendations.

5.1.2.3 Assessing disease and pest impact

The fact that a disease or pest exists in a container nursery does 

not necessarily mean it will affect nursery production, so an 

assessment of its impact is required. A disease or pest problem 

may not be economically serious if it remains at an endemic level 

or can be controlled early enough that the number of seedlings lost 

remains within the normal oversow factors.

The impact of a disease or pest reflects economic loss and can be 

measured in terms of expected growth loss or direct seedling 

mortality. It is simple enough to inventory dead seedlings, but 

growth losses are more difficult to quantify. If the growth loss is 

severe enough, the seedlings will not reach the desired size within 

the normal crop rotation and may have to be culled. If some of the 

diseased seedlings are merely stunted but have the potential for 

recovery, they can be held-over for additional growth and the major 

impact of the disease will be the cost of the additional growing 

time. When the disease is infectious, the seedlings will often have 

to be culled even if the actual infection is relatively minor. Some 

fungal pests such as 8. cinerea can develop into aggressive 

storage molds.

The normal procedure for determining the impact of a disease or 

pest is to conduct an inventory of the affected seedlots and directly 

count or statistically estimate the percent seedling loss. A pest 

specialist should be consulted to train the grading crew to 

recognize diseased seedlings and establish grading standards for 

categories such as "cull," "hold-over," and "shippable." A 

complicating factor with many fungal diseases is that seedlings 

may be infected but do not yet exhibit symp toms; these latent 

infections are extremely difficult to diagnose, even by a trained 

pathologist. Sometimes a second disease survey must be run later 

to identify these latent infections.

Disease and pest impact information can be used to make 

management decisions concerning therapeutic control measures 

for the current crop as well as help to plan preventative control 

measures for future crops.

19
20

Because diseases and pest problems can occur so quickly during 

the germination period, the grower needs a rapid method for 

identification. A damage key of the disease and pest problems of 

seeds prior to emergence is presented in figure 5.1.14. Growers 

can use this key and the pictures and descriptions that follow for 

tentative identification of pre-emergence problems. (Letters in the 

key refer to subheadings in the appropriate section of the text.)

5.1.3.1 Seed disease and pest problems

The germination period is an anxious time for container nursery 

managers while they are waiting for the germinants to appear. 

Conifer seed germination usually takes from 2 to 4 weeks and the 

grower should frequently excavate seed to check for germination 

and disease or pest problems.

If the seed cannot be located under the seed covering, then there 

are two possible explanations (fig. 5.1.14).

5.1.3 Diseases and Pests of Seeds and

Germinants

Seeds and seedlings are most susceptible to diseases and pests 

during the time interval between seed preparation (for example, 

stratification) and when the newly emerged seedling forms woody 

stem tissues. As is the case for many organisms, seedlings are 

least resistant to all forms of stress during their earliest stages of 

development.

Disease and pest identification and diagnosis can be difficult during 

the germination and emergence stage because the germinating 

seed is not visible. Diseases and pest problems can develop and 

spread quickly during this period, and succulent germinants can be 

killed within a few days. During this period, growers must be 

especially alert for problems in order to prevent severe seedling 

losses. Germinating seeds should be checked daily for biotic pests, 

and environmental conditions should be carefully monitored to 

prevent unnecessary abiotic stresses.
A. Sowing problems. Complete absence of the seed in the 

container indicates that there must have been a problem with the 

seed sowing operation. The calibration of the seed sowing 

equipment should be checked for accuracy and precision. If the 

problem is widespread, then resowing may be justified.

B. Bird or rodent predation. It is difficult to quantify the overall 

impact of animal damage because the incidents are generally 

episodic. Bird predation is often due to migrating flocks, which 

can do severe damage in a short time. One nursery that 

participated in the Container Nursery Survey reported that from 

25 to 50% of their sown seed was eaten by goldfinches.

Hosts. Birds and rodents will eat all species of conifer seed but 

prefer the seeds of the large-seeded pines such as white pine, 

sugar pine, and pinyon.

Symptoms/Damage. If the seed cannot be located in the 

container, but the seed covering has been scattered around, then 

bird or rodent predation is a possibility. Rodents may either eat the 

seed and leave the discarded seed coats on top of the container 

(fig. 5.1.15) or remove the seeds and hide them in a cache. Birds 

generally eat the seed immediately, leaving spent seedcoats. 

Rodents feed mostly at night, whereas birds usually feed during the 

day. Rodent predation can occur in open and closed growing 

areas, but bird predation is more common in open compounds. 

Birds also cause clipping injury to emerging seedlings by feeding 

on the seed coat that clings to the cotyledons.

Disease development. Seed predation is usually terminal. 

Seedlings can sometimes recover from clipping injury, although 

severely damaged germinants are weakened and prone to other 

pests such as damping-off fungi.

21
seedlots in British Columbia. The reason for the greater loss in 

bareroot nurseries is probably postsowing spread of the fungus, 

which should be less in container nurseries because intercavity 

spread is difficult (Sutherland and Van Eerden 1980). The true 

extent and impact of this disease is extremely difficult to quantify 

because the infected seed is not decayed and could easily be 

diagnosed as poor quality seed. Sutherland and others (1987) 

present an excellent discussion of the seed fungus as well as color 

photographs.

Disease management. Bird and rodent predation is best 

controlled through prevention: exclusion through proper growing 

area design and use of screens, and elimination of suitable habitat 

around the growing area. If pest exclusion is impossible, trapping 

and baiting will reduce populations. Carlson (1983) recommends 

snap or live traps, or poisoned baits appropriate to the situation. 

Excess seed can be scattered around to attract rodents and keep 

them from reaching the crop seed; finding cracked seeds is a good 

indication that there is a rodent predation problem. Traps or bait 

should be placed on the floor, near potential entry holes. Chemi cal 

seed treatments for repelling birds and rodents have been used, 

but many of these chemicals are phytotoxic and may reduce 

germination.

C. Seed dormancy problems. If the sown seed can be located in 

the container but still has not germinated (fig. 5.1.14), then a 

sample of seeds should be excavated and each seed cut 

longitudinally to examine the seed contents.

A longitudinal section of a typical conifer seed is shown in figure 

5.1.16; the embryo should be clearly visible and the endosperm 

should be firm and completely fill the seedcoat. If the seed does 

not appear normal, then the problem could be poor seed quality, 

and the remainder of the seed lot should be sampled and tested 

for germinability.

If the seed contents appear normal, then the problem may be seed 

dormancy. Many species of seed require cold-moist or other 

stratification treatments to break dormancy; otherwise the seed will 

not germinate. Seed dormancy and treatments to break dormancy, 

including stratification, are provided in volume six in this series, 

and in Seeds of Woody Plants of the United States (Krugman and 

others 1974).

D. Seed fungus. If the bisected seed is not decayed but the 

seeds are covered with a fine fungal mycelium (fig. 5.1.17), the 

problem may be a disease caused by the seed fungus 

Caloscypha fulgens. This pathogen has only been reported in 

container seedling nurseries in British Columbia (Sutherland and 

Van Eerden 1980). It has caused serious disease in bareroot 

nurseries in Ontario, where losses have ranged as high as 95% of 

the viable seed sown (Epners 1964). Sutherland (1984) reported a 

disease incidence of 1 to 5% in infected

22
23

Disease management. Because this disease is usually the result 

of poor cone collection or storage practices, several practices can 

reduce its incidence: 1) collect only cones that have not been on 

the ground for a long time, 2) do not collect cones from squirrel 

caches, and 3) insure proper air drying of cones through proper 

handling and storage practices. Hand picking cones from standing 

trees followed by proper handling should reduce seed fungus 

incidence, and seed orchard collections should be disease-free 

(Sutherland 1986).

Identification of infected seedlots should help prevent the disease, 

and seed-screening techniques are being developed (Sutherland 

and others 1981). For infested seedlots, the following cultural 

practices should reduce losses in the nursery: 1) do not stratify the 

seeds or keep stratification periods as short as possible, 2) sow 

seeds promptly after stratification to reduce the presowing storage 

period, 3) keep greenhouse temperatures warm to promote rapid 

germination, and 4) sow as few seeds per cavity as possible 

(Sutherland 1986). Epners (1964) suggested pelleting seed with 

the fungicide captan, and Salt (1974) found that treating spruce 

seed with thiram effectively reduced seed losses. Because the 

fungus does not affect germinated seed, temperatures and other 

environmental conditions should be maintained at optimum levels 

during the germination period to increase speed of germination 

(Sutherland and Van Eerden 1980).

Hosts. Apparently, most species of conifer seed are susceptible. 

The seed fungus has been isolated from seeds of eastern white 

pine, scotch pine, red pine, and white spruce and has been shown 

to infect jack pine, black spruce, and Norway spruce (Epners 

1964). Sutherland (1979) recovered the pathogen from Douglas-fir, 

Engelmann spruce, white spruce, and grand fir and reported that 

seeds of other conifers, including lodgepole pine and western 

hemlock, are susceptible. Sutherland (1979) stated that one-third 

of all spruce seedlots in British Columbia are infested, but seed 

from species with serotinous cones, such as some ecotypes of 

lodgepole pine, are disease-free (Sutherland 1986).

Symptoms/Damage. This disease is difficult to identify because 

the seed does not germinate and the problem may be confused 

with seed dormancy or pre-emergence damping-off. Unlike 

damping-off fungi, however, the seed fungus does not decay the 

seed; instead, their contents are firm and mummified and the seed 

may be covered with a fungal mycelium (fig. 5.1.17) (Sutherland 

and Van Eerden 1980). Seedlots exhibiting poor germination after 

stratification should be suspect (Sutherland 1986), but positive 

identification of this disease is best made by a pathologist.

Disease development. Caloscypha fulgens is seedborne and 

usually invades cones while they are in contact with forest duff. 

The fungus is especially prevalent in seed lots collected from 

squirrel caches (Sutherland 1979). It can survive for years on dry, 

stored seed (Epners 1964) and can spread among seeds during 

stratification, in cold storage prior to sowing, or after sowing 

(Sutherland and Van Eerden 1980). This fungal pest kills the seeds 

before they start to germinate but does not affect germinated 

seeds or seedlings (Epners 1964). Although the fungus apparently 

spreads in the seedbed in bareroot nurseries, intercavity spread 

should be difficult in container nurseries (Sutherland and Van 

Eerden 1980). A good illustration of the life cycle of Caloscypha 

fulgens is provided in Sutherland and others (1987).
E. Damping-off. Damping-off is a common disease that affects 

seeds, germinants, and young seedlings of many plant species, 

and woody plants are no exception. Traditionally, two different 

types of damping-off are recognized: pre-emergence damping-off, 

which affects seeds and germinants before they emerge, and 

postemergence damping-off, which affects young seedlings until 

their stems become woody. Both forms of the disease occur in 

container nurseries and are caused by the same group of fungi (fig. 

5.1.18). Damping-off was ranked second among the diseases 

affecting container seedlings, with a relative occurrence of 25% 

based on responses during the Container Nursery Survey (table 

5.1.2).

Hosts. All species of seed and seedlings are affected.

Symptoms/Damage. Pre-emergence damping-off is a difficult 

disease to diagnose because the affected seeds are not visible; 

consequently, the losses are often attributed to "poor seed" (Baker 

1957). If the germinants have not emerged after a reasonable 

period, the seed should be excavated and examined; if the seed 

contents are decayed, then damping-off fungi may be involved (fig. 

5.1.14). Sometimes, the germinated seed is killed after the radicle 

of the seed has emerged (fig. 5.1.19).

The classic symptoms of postemergence damping-off (fig. 5.1.20) 

include decay of the seedling hypocotyl at the ground line, causing 

the seedling to topple over. Other stresses can produce symptoms 

similar to fungal damping-off (see section 5.1.5.3), but the 

distinguishing characteristic is the presence of decayed root 

tissue.

Another germinant disease that is usually classed with 

postemergence damping-off is cotyledon blight. This decay of 

the cotyledons develops when seedborne fungi spread from the 

seedcoat during the "birdcage" stage of germination (fig. 5.1.21).

24
25
26

The reason that the more traditional damping-off fungi are not 

being reported from container tree nurseries is probably due to the 

use of soilless growing media, which are generally considered to 

be pathogen-free. Rhizoctonia, Pythium, and Phytophthora spp. 

are primarily spread through contaminated irrigation water or 

growing media, especially from mixes containing soil (Baker 1957). 

McCain (1978) reports that peat is often infested with Pythium and 

Rhizoctonia spp., but Stephens and others (1983) could recover 

neither fungus from non-soil components of artificial growing media 

including peat, perlite, and vermiculite. Coyier (1978) assayed 

perlite and peat moss samples for bacte ria and fungi and found 

that, while perlite was sterile, the peat moss samples contained 

many species of pathogenic and nonpathogenic fungi and bacteria. 

James (1987) has regularly isolated F. oxysporum from the roots of 

weeds growing in greenhouses.

McElroy (1985) has isolated both Pythium and Fusarium spp. from 

unopened bags of container medium; James and Gilligan (1984) 

reported very high populations of both pathogens from one brand 

of growing medium and demonstrated that the recovered 

Fusarium isolates could cause damping-off. Kim and others (1975) 

made fungal isolations from 52 randomly selected samples of peat 

and found that all samples contained Fusarium spp., 29% 

contained Pythium spp., but no Rhizoctonia spp. were isolated 

from any of the peat-based media.

Apparently, seedborne fungi are a primary source of inoculum 

for damping-off fungi in container nurseries. Pawuk (1981) found 

that seedlots of four species of southern pines were infected with 

Fusarium spp. at intensities of 54 to 90% and the same author 

(Pawuk 1978) found that some Fusarium spp. isolates from 

longleaf pine seed caused both pre-emergence and

Disease development. Pre-emergence damping-off is a fungal 

decay of seeds and young germinants and several different fungi 

can be involved. Traditionally, Rhizoctonia has been considered to 

be the major cause of damping-off in ornamental container 

nurseries (Baker 1957); Peterson (1974) states that four fungal 

genera (Pythium, Fusarium, Phytophthora, and Rhizoctonia) "are 

being encountered" in container tree nurseries. A search of the 

literature, however, reveals that only Fusarium spp. have actually 

been implicated as a cause of damping-off of conifer seedlings in 

containers: Fusarium oxysporum (Schlect.) for Douglas-fir (Graham 

and Linderman 1983); Douglas-fir, western larch, grand fir, 

subalpine fir, and ponderosa pine (James 1985a); and Douglas-fir 

and pinyon container seedlings (Landis 1976). Although F. 

oxysporum is most commonly mentioned, other members of the 

genus may also cause damping-off disease of container seedlings. 

James (1985a) recovered F. avenaceum from diseased conifer 

seedlings and Pawuk (1978) demonstrated that four seedborne 

species of Fusarium could produce damping-off of longleaf pine, 

that is, F. moniliforme, F. solani, F. roseum, and F. tricinctum. 

Pythium spp. have also been isolated from diseased container 

seedlings and these fungal pests may be more common than 

generally realized; Peterson (1974) concluded that 

Pythium-infected tissues are often overgrown by other fungi such 

as Fusarium spp.
postemergence damping-off. Graham and Linderman (1983) 

reported that Fusarium oxysporum isolated from Douglas-fir seed 

caused significant pre-emergence damping-off losses. Three 

species of pathogenic Fusarium were isolated from spruce seed 

and seed debris games 1985b). In an earlier study, James (1983) 

recovered Pythium and two Fusarium species from Douglasfir

seed; the Fusarium spp. were isolated from within the seedcoat. 

Fusarium spp. have also been recovered from several species of 

pine seed in container nurseries in the western United States, 

including pinyon and ponderosa pine.

Because container seedlings are spatially isolated in individual 

cells, secondary spread of damping-off is not as severe a problem 

as it is in bareroot nurseries. When more than one seed is sown 

per cell, fungal inoculum introduced on seedcoats can spread to 

adjacent seedlings and cause damping-off or other diseases, such 

as root rot, later in the growing season. Graham and Linderman 

(1983) report that Fusarium spp. grew and sporulated on 

Douglas-fir seedcoats and considered this to be a potential source 

of secondary disease spread. Fungal spores from infected seeds 

could be splashed from container to container during irrigation.

Other damping-off fungi have been isolated from growing medium 

on used containers (Baker 1957) and from dust and soil particles 

from the floors of container nurseries (Stephens and others 1983). 

James (1987) has isolated Fusarium spp. from several different 

types of reusable containers, which had already been "sterilized" 

by conventional techniques (fig. 5.1.22).

27
28

environment of high humidity, low light, and extremely high or low 

temperatures.

Chemical. Because many of the fungi responsible for damping-off 

are seedborne, seeds can be sanitized prior to sowing. Seed 

treatments include standing water soaks, running water rinses, and 

chemical treatments with bleach, hydrogen peroxide, or fungicides. 

If a contaminated growing medium is suspected, chemical 

fumigation or pasteurization can be attempted (see section 5.1.7.2 

for more discussion on seed and growing media treatments). 

Fungicidal drenches can be applied after the disease symptoms 

become evident. This practice is rarely curative: most of the 

damage has already been done by the time the chemical is 

applied. However, drenches can stop secondary spread of the disease. Sutherland and Van Eerden (1980) conclude that drenches 

are seldom effective against damping-off, besides being expensive 

and potentially hazardous to the environment. (See section 5.1.7.4 

for more informa tion on fungicides used to control damping-off.)

Disease management. Cultural. Damping-off is actu ally not as 

common in container nurseries as it is in bareroot nurseries, and 

when it does occur, some environmental or cultural factor is usually 

involved (table 5.1.6). The most important of these factors is seed 

quality: fungal contamination is more common in dirty seed lots, 

and poor quality seed produce weak germinants that are 

particularly susceptible to dampingoff. Reusable containers should 

be carefully cleaned to prevent fungal inoculum from carrying over 

to the next crop. Contaminated growing media are a source of fungal inoculum, and fine-textured mixes often compact and provide 

an ideal environment for damping-off fungi. High pH, either in the 

growing medium or irrigation water, can favor damping-off, but the 

low pH of most sphagnum peat should inhibit this fungal disease 

(Carlson 1983). Oversowing leads to weak germinants that are 

more susceptible to disease. Fertilization with high nitrogen levels 

and over-irrigation can also predispose seedlings to damping-off as 

can a growing
5.1.3.2 Diseases and pests of germinants and young

seedlings.

Once the germinants have emerged, the symptoms and signs of 

disease or pest damage become more apparent. Figure 5.1.23 

provides a damage key to the common disease and pest problems 

of germinants and young seedlings. (Letters in the key refer to 

subheadings in the appropriate section of the text.)

A. Postemergence damping-off. See section 5.1.3.1 for 

discussion of damping-off.

B. Heat injury. Symptoms very similar to postemergence 

damping-off can be caused by high temperatures at the growing 

medium surface. Levitt (1980) reviewed the literature on seedling 

diseases caused by heat stress and reported that stem injury of 

conifer seedlings can result from an exposure to growing media 

temperatures as low as 45 °C (113 °F). Seidel (1986) exposed 2-

to 4-week-old seedlings of ponderosa pine, Douglas-fir, grand fir, 

and Engelmann spruce to a series of time--temperature treatments 

and found that seedlings from all species died from temperatures in 

the range of 48 to 68 °C (118 to 154 °F), depending on exposure 

time.

Heat injury, which sometimes only occurs on the sunny side of the 

seedling, is promoted by dark-colored seed coverings that absorb 

more heat than lighter colors. Boyce (1961) described a disease, 

called "white spot," characterized by white, shrunken, wateryappearing lesions on the south side of the seedling stem just 

above the soil surface. James (1987) states that western larch 

germinants are especially susceptible to heat injury, which can be 

identified by stem swellings at the root collar. Apparently, this 

symptom develops when the lateral meristem is killed and 

basipetally translocated photosynthate causes increased stem 

growth above the damaged area. The distinguishing difference 

between fungal damping-off and heat injury is that the seedling 

radicle shows no sign of decay after heat injury (fig. 5.1.24).

C. Chemical injury. Pesticides can also cause symp toms similar 

to damping-off. Several chemicals, including surfactants, can 

cause these symptoms and it is therefore impossible to identify 

which chemical is responsible for a specific injury. As with heat 

injury, chemical injury can be distinguished from fungal 

damping-off by the lack of decay of the seedling radicle.

29
Certain pesticides can also cause chlorosis, tip-burn, distorted 

cotyledons (fig. 5.1.25), or growth reductions in young, succulent 

seedlings. Captan phytotoxicity is most frequently mentioned in 

the literature. Kozlowski (1986) applied captan at concentrations 

of 250 to 2500 ppm to red pine seed and found that, although 

seed germination was not affected, captan concentrations as low 

as 500 ppm retarded elongation of both shoots and roots, with the 

latter being more severely affected. A captan seed treatment 

caused curled hypocotyls, twisted and chlorotic cotyledons and 

growth reductions in red and jack pine (Carlson and Nairn 1975). 

Denne and Atkinson (1973) tested the effect of captan fungicide 

on several conifer seedlings and found that even a single 

application of this pesticide had a measurable stunting effect on 

shoot and root growth. In a Colorado container nursery, however, 

captan did not injure blue spruce germinants, whereas dicloran 

caused a stem swelling that mechanically weakened the seedling 

stem (fig. 5.1.26).

30
Even chemicals that are not directly applied to seedlings can 

cause injury. Fumes from preservatives used to protect wood from 

decay, such as pentachlorophenol, caused twisting and chlorosis 

of cotyledons and eventual death of red and jack pine seedlings 

(Carlson and Nairn 1975). Arsenic toxicity was diagnosed as the 

cause of needle burn, stunting, and occasional mortality of red 

pine and other conifer seedlings in a Minnesota container nursery. 

The source of the arsenic was cromated copper arsenate used as 

a wood preservative in the bench frames and apparently the 

chemical leached into the paperpot containers around the 

perimeter (Croghan 1984). Herbicide vapors in the air or 

pesticide-contaminated irrigation water can also cause leaf burn or 

distortions. Hanan and others (1978) list specific symptoms for 

several types of herbicide injury to horticultural crops.

31

D. Cutworms. Cutworms are moth larvae that feed on the 

succulent shoots of young seedlings. Sutherland and Van Eerden 

(1980) list three species of cutworms that can cause damage to 

container seedlings, and Matthews (1983) considers the 

variegated cutworm, Peridroma saucia, to be the most common 

species in container nurseries in British Columbia. There are no 

published accounts of the amount of actual damage, but cutworms 

were ranked as the second most important insect pest reported in 

the Container Nursery Survey (table 5.1.3).

Hosts. Cutworms are not host- specific and will feed on most 

species of forest tree seedlings (Sutherland and /an Eerden 1980).

Symptoms/Damage. Cutworms injure young seedlings by feeding 

on succulent foliage and stems of young seedlings ("clipping") (fig. 

5.1.27), often consuming the entire shoot, leaving just a "stump." 

Cutworms can cause chewing injury on the stem, resulting in 

sunken or depressed areas that can resemble fungal lesions. 

Seedlings cut off at the ground line might be mistaken for 

seedlings suffering from damping-off (Palmer and Nicholls 1981).

Most cutworms are large, variably colored, thick-bodied 

caterpillars, up to about 4 cm (1.6 inches) long (fig. 5.1.27). They 

feed mostly at night and hide during the day; the caterpillars can 

sometimes be found under the surface of the growing medium or 

under containers. Shrimpton (1987) states that containers with 

cutworms can be easily spotted: the cutworms in the soil cause the 

container plug to be elevated above the plugs of surrounding 

seedlings. The adult moths, often called "millers," are woolly, 

mottled grey to brown, and about 2 cm (0.8 inch) long. They can 

sometimes be seen flying around the growing area but do no 

damage themselves (Sutherland and Van Eerden 1980).
Pest management. Growing areas should be screened to exclude 

the adult moths. Larvae can be hand-picked out of the containers 

and destroyed, but this is impractical in large operations. Because 

the adult moths are attracted to certain weed species for egg 

laying, keeping the growing area weed-free should help reduce the 

incidence of cutworms (Shrimpton 1987). Chemical controls, such 

as poison baits containing attractants and insectide sprays or 

drenches (see section 5.1.7.4), are usually directed at the larvae. 

Contact insecticides and baits are more effective than protective 

insecticides because the larvae consume very little of the shoot, 

and it is difficult to apply protective insectides to the lower stem 

region (Shrimpton 1987). Sprays to control adult moths were most 

effective when applied late in the day (Sutherland and Van Eerden 

1980).

E. Bird damage. See section 5.1.3.1.

F. Sirococcus blight. A blight is a descriptive term for a disease 

that causes rapid dieback or death of plant tissue (Peterson and 

Smith 1975). Sirococcus blight, caused by the fungus Sirococcus

strobilinus primarily affects spruce seedlings in the northwestern 

United States and coastal British Columbia. This seedborne pest 

is spread through poor cone collection practices, and older cones 

are most likely to be diseased (Sutherland 1986). Within infested 

seedlots, 1 to 3% of the seed may be infested, and secondary 

spread between emerged seedlings in the nursery has resulted in 

losses as high as 40% (Sutherland and Van Eerden 1980). James 

and Gilligan (1985) reported that this fungus caused disease in 18 

of 19 seedlots and was the most damaging disease of spruce 

seedlings in a northern Idaho container nursery. Sirococcus blight 

was the fourth most important disease reported by nursery 

managers during the Container Nursery Survey (table 5.1.2). 

Sutherland and others (1987) present an excellent review of 

sirococcus blight.

Hosts. Although sirococcus blight can affect both spruce and pine 

in container nurseries, it has been most damaging on Sitka and 

white spruce in the north western United States and coastal British 

Columbia (Sutherland and Van Eerden 1980). The pest was shown 

to be seedborne on Sitka, white, and Engelmann spruce in British 

Columbia (Sutherland and others 1981) and is suspected to be 

seedborne on Engelmann spruce in Idaho (James and Gilligan 

1985).

Life cycle. The life cycle of cutworms consists of egg, larval, pupal, 

and adult stages and ranges from one to several generations per 

year, varying with the climate. The adult moths lay eggs in growing 

medium or on plant debris; soon after the eggs hatch, larvae begin 

feeding on plant tissue and continue to grow until pupation. A few 

cutworms can cause significant losses because they can move 

from seedling to seedling and the large larvae consume 

considerable amounts of plant material. Cutworms probably 

overwinter in the pupal stage (Sutherland and Van Eerden 1980).

32
Cultural. Seeds originating from seed orchards should be selected 

to be free from Sirococcus (Sutherland and others 1982). In the 

nursery, diseased seedlings should be rogued as soon as they 

become evident to prevent secondary spread. Manipulating 

environmental conditions such as reducing relative humidity and 

increasing temperature and light may be helpful in minimizing 

disease intensity (Sutherland and others 1980).

33

Symptoms/Damage. The fungus attacks germinants

and very young seedlings and kills primary needles from the base 

upwards; the affected needles become desiccated and turn light 

brown to reddish brown (fig. 5.1.28). The seedlings usually die 

and remain upright (Sutherland and Van Eerden 1980). This 

disease could be mistaken for fusarium root rot (see section 

5.1.4.2) but can be differentiated by looking for the characteristic 

butterscotch- or dark-colored pycnidia on the hypocotyl or the 

base of the cotyledons. On older (4 months) seedlings, S. 

strobilinus can cause a shoot blight or tip dieback that sometimes 

leads to seedling mortality (James and Gilligan 1985).

Disease development. The fungus is mainly introduced into the 

nursery on seed, although spores may be blown in from outside 

the growing area. Once introduced into a container nursery, the 

fungus can apparently spread to adjacent seedlings through 

spores disseminated by irrigation or rain splash. The disease 

pockets that develop often mask the random distribution that is 

characteristic of seedborne pathogens. The disease is favored by 

the cool, moist conditions and low light intensities that often occur 

in Northwest container nurseries (Sutherland and Van Eerden 

1980). A good illustration of the disease cycle of sirococcus blight 

is presented in Sutherland and others (1987).

Disease management. Chemical. Seed assays can be performed 

to detect Sirococcus-infested seedlots (Sutherland and others 

1981), and a recently developed monoclonal antibody assay may 

prove to be a simple, rapid, and accurate diagnostic procedure 

(Mitchell 1986). If infested seedlots can be identified, they can be 

chemically treated prior to sowing, or germinants of infested 

seedlots can be closely monitored for the appearance of the 

disease. Prompt identification will allow fungicides to be applied 

(see section 5.1.7.4) as soon as the disease becomes evident to 

prevent secondary spread of the disease (Sutherland and Van 

Eerden 1980). Benomyl or daconil fungicide drenches applied to 

Sirococcus-infested seedlots have produced conflicting results 

(Matthews 1987). Fungicides should be applied regularly to protect 

new foliage because irrigation or rain may wash the chemicals off 

older seedling foliage.
34

5.1.4.1 Insect pests

Although some insects cause incidental injury when they are 

accidentally introduced into container tree nurseries, some 

species are specific pests of tree seedlings.

A. Root weevils. Although several weevils damage container 

nursery stock, the black vine weevil (Otiorhynchus sulcatus) is one 

of the most damaging (Sutherland and Van Eerden 1980). Both 

the black vine weevil and the strawberry root weevil (O. ovatus) 

were reported from container nurseries in the United States and 

Canada in the Container Nursery Survey. Sutherland and others 

(1982) consider weevils to be the most prevalent, destructive, and 

difficult-to-control insects in British Columbia container nurseries. 

Root weevils

5.1.4 Diseases and Pests of Seedling Root

Systems

Root diseases and pests are at least as common as those of 

seedling shoots and are frequently more damaging. The relative 

importance of root diseases and pests may be underestimated 

because roots are hidden from view, and root problems are often 

expressed through foliar symptoms. Experienced pest specialists 

realize that many foliar symptoms are often indicators of advanced 

root injury.

As in previous sections, a damage key is provided for the 

commonly reported root diseases of established container 

seedlings (fig. 5.1.29). The descriptions in this key should help the 

grower tentatively identify the disease; more information on the 

disease is provided in the sections referred to by the key. (Letters 

in the key refer to subheadings in the appropriate section of the 

text.)
were the third most important insect pest based on responses to 

the Container Nursery Survey, with a relative occurrence of 10% 

(table 5.1.3).

Hosts. Weevils can attack all species of seedlings, but western 

hemlock (Matthews 1983) and spruce seedlings (Shrimpton 1986) 

are parti cularly susceptible.

Symptoms/Damage. Identification of adult weevils can be 

confusing, and Carmean (1986) provides a key to the common 

species. Adults of both the black vine and strawberry root weevils 

are small-from 0.3 to 1.2 cm long (0.1 to 0.5 inch)-but the black 

vine weevil is larger. The color of the adults ranges from tan to dark 

and they have the typical weevil shape (fig. 5.1.30A).

Adult weevils girdle seedling stems, resulting in a neat ring about 6 

mm (0.25 inch) wide just below the point at which the foliage 

begins. Usually only one seedling is attacked at a time. This 

damage usually occurs in early summer, because the adults feed 

before the stem tissues become too woody. Seedlings at the 

outside of a block of containers or at the perimeter of the growing 

area are attacked most frequently (Shrimpton 1986). Adult 

strawberry root weevils can cause more significant damage than 

adult black vine weevils, which cause only minor injury. Adults may 

go unnoticed because they feed at night and hide during the day, 

but they can sometimes be found under containers or other 

material in the growing area.

35
36
37

Pest Management. Chemical. Insecticides can be applied to 

control either the grubs or the adults, although foliar pesticides 

that kill the adults are most effective. Because the timing of these 

applications is critical, monitoring to determine periods of adult 

weevil activity is essential. Foliar pesticides (table 5.1.7) should 

be applied to the seedling foliage soon after finding adults so that 

the adults are killed before females begin laying eggs 

(Sutherland and Van Eerden 1980). Under conditions of 

overlapping generations, such as may occur in greenhouses, 

foliar sprays may need to be applied at 3- to 4-week intervals 

because the toxicity of the insecticides lasts only a few days. The 

spray should be carefully applied to completely cover the 

seedling foliage because weevils often feed out of direct sunlight 

(Capizzi and Green 1984).

Because not all adult weevils are killed by protective foliar 

treatments, a chemical drench (table 5.1.7) may be required to 

control larvae in the growing medium. Capizzi and Green (1984) 

recommend that several drenches can be applied in either the 

spring or fall. Drenches are operationally hard to administer 

because the chemical must uniformly penetrate the growing 

medium for complete control.

Cultural. There are no published cultural controls for this insect.

Weevil larvae are more destructive than the adults. Larvae of both 

species are cream-colored, with brown heads and no legs; larvae 

of the strawberry root weevil are smaller than larvae of the black 

vine weevil (fig. 5.1.30B) (Sutherland and Van Eerden 1980). 

Weevil larvae can sometimes be found within the growing medium, 

where they chew the bark from larger roots and completely 

consume smaller roots (fig. 5.1.31A). Because larvae consume 

increasingly more root tissue as they grow, the supply of water and 

mineral nutrients to the seedling shoot is gradually reduced. Foliar 

symp toms are not usually visible until later in the growing season, 

when the entire seedling can be easily lifted out of the container; 

inspection of these seedlings reveals that the entire lower section 

of the root system has been consumed (fig. 5.1.31 B) (Sutherland 

and Van Eerden 1980). Because the container plug must be dissected to find the larvae, their populations are not routinely 

monitored.

Life history. The life cycles of all weevils affecting container 

seedlings are similar, but the speed of development and number 

of generations per year may be increased by the favorable 

conditions in heated greenhouses. Adults lay their eggs within or 

on the growing medium during the summer. When the larvae 

hatch, they begin feeding on the root system; they may overwinter 

as larvae or pupate into adults, which also can overwinter. 

Interestingly enough, all adult weevils in this genus are females 

(Shrimpton 1987).
B. Webworms (including cranberry girdler). Webworms, the 

larvae of a group of moth-like insects that includes the cranberry 

girdler (Chrysoteuchia topiaria), have been a problem in bareroot 

forest nurseries for several years but have only recently been 

implicated as the cause of container seedling injury. Webworm 

adults (fig 5.1.32A) do no damage, but the larvae (fig. 5.1.32B) feed 

on the stem and upper root system of container tree seedlings. The 

amount of damage caused by webworms is not well known, 

although one severe infestation by a related webworm resulted in 

the loss of several thousand 2 + 0 container spruce in an interior 

British Columbia nursery (Shrimpton 1986). At present, only 

nurseries in Idaho, Montana, and British Columbia have been 

affected by this insect pest.

Hosts. Spruces and Douglas-fir are the only reported hosts, 

although the cranberry girdler attacks other conifers in bareroot 

nurseries.

Symptoms/Damage. The chewing injury of the webworm larvae 

is similar to that caused by other girdlers such as root weevils. 

However, webworms strip the outer bark from the root collar and 

upper roots while weevils consume the lower part of root system 

(fig. 5.1.33A). Damage occurs most often during the month of 

August in British Columbia (Shrimpton 1986). The most 

distinguishing characteristic of webworm damage is the presence 

of white, silky webbing containing bits of growing medium at the 

root collar (fig. 5.1.33B).

Life cycle. Webworm moths become active in late spring and lay 

their eggs in dense stands of plants, so container seedlings 

provide an ideal habitat. The moths, which are pale brown with a 

distinctive snout-like projection from the head, measure from 1.3 to 

2.5 cm (0.5 to 1.0 inch) in length (fig. 5.1.32A). They have a 

characteristic jerky zigzag flight pattern. The larvae hatch in a few 

days and feed until pupation in the fall; the webworm larvae are 

short [0.6 to 1.9 cm (0.25 to 0.75 inch)], thick-bodied, and coarsely 

haired. The larvae overwinter in silken cocoons just under the 

surface of the growing medium and pupate the following spring 

(Metcalf and Flint 1962). Some webworms have more than one 

generation per year, which could be a significant factor in the 

favorable greenhouse environment.

38
39

Pest management. Specific control measures for these insect 

pests have not been established for container nurseries. However 

the following controls, which have been used for cranberry girdler 

in bareroot nurseries, have some applicability. Grass acts as a 

reservoir for webworms, and so populations can be reduced by 

eliminating the amount of grass around nurseries by mowing or 

with herbicides. Susceptible seedlings should be grown as far 

away from open fields as possible (Shrimpton 1983). Screening all 

greenhouse openings would exclude the adult moths from the 

growing area. Insecticides can be used for either the adult or the 

larvae. Diazinon has been applied within the seedling crown to 

control the moths and insecticidal drenches, such as chlorpyrifos, 

have been used against the larvae. Pheromone traps can be used 

to determine webworm occurrence and establish moth activity 

periods (Tunnock 1985).
The larvae have tough, leather-like skin and no distinctive head, 

which helps distinguish them from cutworms. Although the larvae 

are initially small [3 mm (0.1 inch)], they grow rapidly, eventually 

reaching 4 cm (1.6 inches) in length (fig. 5.1.34A). They can

sometimes be found in the upper layers of the growing medium. 

Pupae are brown and spiny and about 3.3 cm (1.3 inches) long, 

and the empty pupal cases are left protruding from the medium by 

emerging adults. Adult crane flies (fig. 5.1.34B) resemble large 

greyish brown mosquitoes with bodies about 2.5 cm (1.0 inch) in 

length, two wings and very long legs (Sutherland and Van Eerden 

1980). Adults are most abundant in late summer (Shrimpton 

1985).

Life history. Under normal conditions in the Pacific Northwest, the 

European crane fly passes through all four life stages (egg, larva, 

pupa, and adult) in 1 year. Adults do not damage seedlings but lay 

eggs on the surface of the growing medium in the late summer or 

early fall. The larvae feed through the fall and winter and pupate 

the following summer. Adults emerge, mate immediately, and lay 

eggs, completing the life cycle (Sutherland and Van Eerden 1980).

Pest management. The decision on whether control is warranted 

should be based on the amount of injury sustained and the 

presence of the adults in the nursery. Adult crane flies can be 

excluded from greenhouses with screening. Although biological 

controls have proven effective in laboratory trials, none have been 

implemented in operational situations (Anonymous 1983). 

Protectant sprays against adult crane flies are ineffective because 

they do not feed on seedlings. Insecticide drenches and baits for 

larvae control may be worthwhile (Sutherland and Van Eerden 

1980) (see section 5.1.7.4). In British Columbia container nurseries, diazinon is applied in the evening during the fall when the 

leatherjackets feed near the surface of the growing medium. An 

estimate of control can be made following these applications 

because the larvae often crawl to the surface before they die. This 

insect is primarily a pest of grasses, and lawns surrounding the 

nursery should also be treated appropriately to elimi nate any 

reservoir populations (Shrimpton 1985).

40

C. European crane fly. The European crane fly, or European 

marsh crane fly (Tipula paludosa) is an introduced pest that has 

only been reported in the Pacific Northwest. The range of this 

insect pest is apparently limited by climate; the southern range of 

the European crane fly is currently around 45° latitude (Oregon to 

Nova Scotia) (Anonymous 1983). Larvae of the European crane fly, 

called "leatherjackets" (fig. 5.1.34), feed on the seedling stem and 

upper part of the root system. This feeding eventually girdles the 

stem and cuts off the water and mineral nutrients from the root 

system, causing foliar wilt and stunting. This insect was ranked 

fourth in the Container Nursery Survey, with a relative occurrence 

of 9.8% based on responses from container nursery managers 

(table 5.1.3). Because of the crane fly's life cycle, the damage 

should be restricted to older container seedlings, especially 2 + 0 

or holdover stock (Sutherland and Van Eerden 1980). Nurseries in 

wet climates are more likely to be affected because the eggs and 

young larvae are susceptible to desiccation (Anonymous 1983). 

This pest could be introduced to new areas because the larvae 

often remain with the seedling plug after it leaves the nursery 

(Shrimpton 1985).

Hosts. There are no published reports of host specificity, but the 

only serious outbreak of damage was on bareroot Sitka spruce in 

British Columbia (Sutherland and Van Eerden 1980).

Symptoms/Damage. This insect is difficult to detect during the 

growing season because the larvae feed under the surface of the 

growing medium, and the damage is usually not noticeable until 

seedlings are lifted for shipping. Leatherjackets create a 

well-defined feeding ring about 2.5 cm (1 inch) wide at the root collar, only consuming the bark. They usually completely girdle the 

seedling stem and may eventually consume some of the upper root 

system. The damage is usually spotty in distribution, because the 

larvae usually attack small groups of 1 to 7 seedlings in a given 

area (Shrimpton 1986).
41
42

D. Dark-winged fungus gnats. These small flies (Bradysia spp.) 

are common nuisances around greenhouses but their importance 

as damaging pests of container tree seedlings has not been 

realized until recently. Actually, adult fungus gnats are harmless; 

the larvae, however, can feed on the roots of young succulent tree 

seedlings, cuttings, or fleshy seeds when conditions are favorable. 

Fungus gnats placed fifth in the ranking of insect pests from the 

Container Nursery Survey (table 5.1.3). Shrimpton (1986) 

considers fungus gnats to be secondary pests, usually only 

attacking seedlings weakened by other pathogens, such as fusarium root rot. The role of these insects in disease transmission has 

always been suspected and now has recently been confirmed for 

verticillium wilt in greenhouses (Kalb and Millar 1986).

Hosts. The larvae normally feed on soil fungi and organic matter, 

but larger larvae can attack healthy root tissue of many plants 

including pine seedlings (Mead 1978). Seeds and cuttings of 

hardwood species and seedlings of many woody species have 

been damaged in western container nurseries.

Symptoms/Damage. The first evidence of infestation by fungus 

gnats is the presence of the adults, which hover around the host 

plants and fly when disturbed. Fungus gnat adults are small, dark, 

mosquito-like flies (fig. 5.1.35A) similar to many other small flies 

common in greenhouses. Shrimpton (1986) trapped small flies in 

British Columbia container nurseries and identified 11 different 

species. It can be difficult for the amateur to differentiate between 

dark-winged fungus gnats and shore flies, which are also common, 

but harmless (table 5.1.8).
Symptoms of injured seedlings include wilting and sudden loss of 

vigor. Examination of affected plants with a hand lens may reveal 

the presence of larvae in the upper layer of the growing media 

(Peck 1982). The larvae are legless, semitransparent worms with 

black heads and range up to 0.5 cm (0.2 inch) in length (Nelson 

1978) (fig. 5.1.35B). The larvae may consume small roots 

completely or just the exterior of the larger roots, leaving just the 

stripped vascular tissue. By the time symptoms become evident, 

damage is usually so severe that control of the larvae is not 

practical. Instead, the adults should be controlled as soon as they 

are noticed.

Life history. Female gnats lay eggs on moist surfaces, preferring 

growing media that are rich in organic matter. Infestations appear to 

be most severe in containers that contain algae or moss, which 

develop in response to overwatering. Eggs hatch in about 6 days, 

and the larvae feed for a couple of weeks and pupate in the 

growing medium. After 5 to 6 days, the adult flies emerge, completing the life cycle (Nelson 1978). Because of their short life cycle, 

populations of dark-winged fungus gnats can build-up rapidly in 

greenhouse environments where organic matter food sources are 

present and warm, moist conditions exist (Peck 1982).

Pest management. Chemical. Insecticides can be used to control 

either larvae or adults. Many common insecticides are effective 

against the adults (see section 5.1.7.4), but it is sometimes 

difficult to reach areas where the adults may be hiding. 

Insecticides can be applied as drenches to control the larvae, but 

all surfaces where the gnats are breeding must be treated (Peck 

1982). Hamlen and Mead (1979) tested 12 common insecticides 

on fungus gnats and found that all were effective; insecticides 

were more effective than growth regulators and surface-applied 

pesticides were as effective as drenches.

Cultural. Cultural control methods involve general greenhouse 

sanitation: removing infested containers, avoiding overwatering, 

controlling algae and mosses, and sterilizing containers and 

surfaces in the growing area (Peck 1982). Lindquist and others 

(1985) report that the type of growing medium affects fungus gnat 

populations and also the efficacy of insecticides: more adult fungus 

gnats emerged and insecticides were less effective in a medium 

containing composted bark. Shrimpton (1986) describes a method 

of monitoring fungus gnats, and potentially controlling them, that 

involves yellow sticky ribbons hung in the greenhouse. The adult 

fungus gnats are attracted to the ribbons and become stuck. 

Parrella (1987) discusses the use of yellow sticky cards in 

ornamental greenhouses, and recommends hanging one card per 

929 m2 (10,000 ft2), for monitoring insect pest populations. 

Shrimpton (1986) states that horticultural greenhouses have successfully reduced greenhouse fly populations by hanging these 

yellow sticky ribbons at a density of one per 0.93 m2 (10 square 

feet). Baker (1986) presents an excellent guide for identifying the 

greenhouse insects trapped on yellow sticky cards.

43
44

Hosts. Most conifer seedlings, including spruces, true firs, pines 

and larch are susceptible to fusarium root rot, but the disease is 

apparently most serious on Douglas-fir.

Symptoms/Damage. The foliar symptoms of fusarium root rot are 

variable: newly infected seedlings typically have scattered chlorotic 

or curled needles (fig. 5.1.36) followed by tip dieback, wilt 

symptoms, and stunting as the disease progresses. The seedling 

foliage often turns a reddish brown just before the seedling dies. 

Diseased root systems show lack of fine root development and 

extensive cortical decay so that the epidermis is easily stripped 

away from the core tissues (fig. 5.1.37). One of the most 

diagnostic signs of this disease is the production of fruiting 

structures (sporodochia) on the seedling stem (fig. 5.1.38A), where 

yellow-orange spore masses are exuded games 1985a, Landis 

1976). These spores are typically multicellular and sickle-shaped 

and can be used to positively identify the fungus (fig. 5.1.38B).

Disease development. Apparently, several different species of 

Fusarium can cause root rot of container tree seedlings, including 

F. oxysporum (Graham and Linderman 1983), F. solani games 

1983, Landis 1976), and F. avenaceum games 1985a). One of the 

main sources of Fusarium inocula in container nurseries is the 

seed, although the fungus has also been isolated from growing 

media, used containers, weeds, and irrigation water. Pawuk (1981) 

isolated only low levels of Fusarium spp. from air and water 

samples around greenhouses but recovered the fungus from four 

species of southern pine seed with infection intensities of 54 to 91 

%. Graham and Linderman (1983) isolated F. oxysporum from 

Douglas-fir seed and James (1983) recov ered F. oxysporum and 

F. solani from the exterior and interior of conifer seed. Seed from 

squirrel-cache-collected cones often are infested with Fusarium 

spp. and other potentially pathogenic fungi games 1986). This 

widespread fungal pathogen has also recently been isolated from 

reusable containers that had already been sterilized (fig. 5.1.22).

5.1.4.2 Fungal diseases of roots

Fungi cause many root diseases of container nursery seedlings. 

Although the advent of soilless growing media has reduced the 

amount of damage, fungal root rots still cause problems for 

container nursery managers. Several species of fungi can cause 

root rot and, although rhizoctonia root rot is one of the main root 

diseases of ornamental plants (Baker 1957), Rhizoctonia spp. 

have not been widely reported in container tree nurseries. 

Instead, Fusarium spp. are the most commonly reported root 

pathogens (Pawuk 1981, James 1985a). Phytophthora root rot, 

caused by several species of Phytophthora, has been identified in 

container tree nurseries only rarely. Its occurrence can usually be 

attributed to poor aeration of the growing medium.

Seedlings affected by root rots may show a variety of aboveground 

symptoms. One of the first indications of root disease is a general 

loss of vigor, followed by needle-tip dieback, needle curling 

("clutching"), chlorosis, and wilting. Even relatively low root 

pathogen levels can severely reduce seedling growth by injuring 

fine root tips and therefore interfering with water and mineral 

nutrient uptake. If root rots are suspected, growers should remove 

several seedlings from the containers and examine the roots. 

Healthy plants should have numerous white root tips, whereas 

diseased roots show various degrees of water-soaking and appear 

brown or black (Cline 1985).

E. Fusarium root rot. Fusarium root rot is one of the most 

common diseases of conifer seedlings in the world and is 

widespread in North American nurseries (Bloomberg 1981, Smith 

1975). Fusarium wilt affects many different horticultural plants 

and is the most important pathological problem of plants grown in 

artificial growing media (Couteaudier and Alabouvette 1981). 

Because this fungus prefers warmer temperatures, heated 

container nurseries are ideal for build-up of this disease.
45
Symptoms/Signs. Water mold fungi cause wilt symp toms, 

followed by chlorosis and stunting. Pythium-infected roots are 

black and water-soaked and are often hollow and collapsed 

(Nelson 1978). Because symptoms develop from the root tips, 

container seedlings affected with pythium root rot often have a root 

system with few lateral roots (fig. 5.1.39). Phytophthora root rot is 

characterized by a distinctive reddish brown discoloration of the 

cambial region of the infected root; in some hardwood species, the 

stain is blue-black or inky colored (Kuhlman and Smith 1975).

Disease development. Water molds are so named because they 

have motile spores that swim in water and therefore thrive in damp 

soils. Unlike many other fungi, they have no airborne spore stage. 

Both Pythium and Phytophthora are favored by wet, poorly 

drained media and cool temperatures. They are able to with stand 

periods of drying by forming thick-walled resting spores (Baker 

1957).

Disease Management. These root rots are more easily prevented 

than controlled. Although water molds can be seedborne, they are 

most often introduced in contaminated irrigation water or growing 

media. Growers, therefore, should check their water sources and 

media. Irrigation water can be tested for Pythium and 

Phytophthora by a "baiting" procedure in which baits of unripe fruit 

(apples or pears) are suspended near the water surface (fig. 

5.1.40). These baits attract motile zoospores, which penetrate the 

fruit and can be subsequently isolated and identified on a selective 

medium (McIntosh 1966). Water molds thrive in wet conditions, 

and so growing media should be formulated to provide good 

aeration and drainage. Fungicidal drenches (see section 5.1.7.4) 

can be used to control water molds, but many of these chemicals 

(e.g., metalaxyl) are only fungistatic, merely stopping the spread of 

the disease, not eradicating it.

Although initial infections are usually random, secondary spread is 

probably due to spores splashed from diseased seed or seedlings 

during irrigation (Graham and Linderman 1983), and disease 

pockets apparently develop as a result of this secondary spread. 

Bloomberg (1981) discusses fusarium diseases in detail and 

reports that high temperatures [25 to 35 °C (77 to 93 °F)] 

stimulate fungal growth and that high nitrogen fertilization appears 

to increase disease losses. Normal-appearing seedlings are often 

infected with the fungus but do not develop foliar symptoms. 

Fusarium is a common rhizosphere inhabitant, and the disease 

only develops when the seedling becomes stressed, by drought or 

heat for example. The cultural practice of moisture-stressing 

seedlings to harden them may thus actually enhance disease 

development (James 1986).

Disease management. Chemical. Chemical control methods can 

be divided into sanitation of containers and surfaces in the growing 

area, seed treatments, growing medium treatments, and fungicidal 

drenches. Because Fusarium spp. can be introduced into container 

nurseries on seed, growers might consider seed treatments prior to 

sowing. Growing media should be assayed to make sure that they 

are pathogen-free, and containers should be carefully cleaned and 

treated with heat or chemical sterilants. Fungicidal drenches (see 

section 5.1.7.4) are commonly used to control fusarium root rot, but 

these treatments function primarily to limit the spread, rather than 

cure the disease.

Cultural. Growers can reduce the impact of fusarium root rot by 

using a growing medium that stimulates healthy root growth and 

discourages pathogens (Couteaudier and Alabouvette 1981) and 

by promptly roguing diseased seedings to prevent secondary 

spread.

F. Water molds. The water mold fungi, species of Pythium and 

Phytophthora, are commonly occurring pathogens that cause 

serious root diseases in many plants. Although both of these 

fungal genera have been consistently linked to root rot of 

ornamental container stock, only Pythium is a serious pest of 

forest tree container seedlings. Peterson (1974) predicted that 

phytophthora root rot will not be a serious pest of container tree 

seedlings because most nurseries use well-drained growing 

media.

Hosts. All seedlings are susceptible to root rots caused by water 

molds.

46
47
48

H. Mycorrhizae. Mycorrhizae--the word literally means 

"fungus-roots"--are complex structures that develop as a symbiotic 

relationship of a fungus and plant roots. These structures can be 

seen macroscopically on the exterior of the container plug and 

have the appearance of a swollen root with fuzzy, variously colored 

external mycelia extending into the surrounding medium (fig. 

5.1.42B). It is easy to distinguish between damaging root-rotting 

fungi and beneficial mycorrhizae because the latter are never 

associated with decay. For a detailed discussion of mycorrhizae 

and their management in container nurseries, refer to chapter 2 of 

this volume.

Cylindrocarpon root rot. Several species of Cylindrocarpon have 

been isolated from tree seedling roots, but only recently have these 

fungi been associated with a serious root disease of container tree 

seedlings. Cylindrocarpon spp. have been isolated from conifer 

seed and roots of both diseased and apparently healthy sc lings 

games 1988); this fungus has also been found peat-based growing 

media (Carter 1988). Cylindrocapon root rot can affect many 

different Northwestern conifer seedlings: disease symptoms 

include root browning, loss of fine root tips, and decay of the root 

exterior (figure 5.1.41). Shoot symptoms are slow to develop, 

however, and therefore seedlings in the early stages of the disease 

may not be culled during seedling grading. Dennis (1988) 

consistently isolated a Cylindrocapon species from the roots of 

diseased Douglas-fir seedlings late in the growing season, during 

seedling storage, and even after outplanting. Apparently, this 

opportunistic fungus functions like Fusarium spp. in that it is a 

normal rhizosphere inhabitant and becomes pathogenic when the 

host seedling is stressed game 1988). Because so little is known 

about this disease control strategies have not been developed at 

the present time, although several operational trials are underway 

in British Columbia (Dennis 1988). Cultural controls effective for 

other root rot fungi, including using a well-drained growing medium 

and minimizing unnecessary stresses, should also be effective 

against Cylindrocarpon.

5.1.4.3 Nondamaging growths on root systems

There are several growths that are often visible on the exterior of 

container plugs that do not indicate disease.

G. Root aphids. One species of aphid--Rhizomaria piceae--feeds 

on the mycorrhizae of spruce container seedlings. The small 

colonies of these insects are covered with a white cottony 

substance and can be seen on the exterior of seedling plugs (fig. 

5.1.42A). They apparently do not damage the seedling (Sutherland 

and Van Eerden 1980) and therefore require no treatment.
Cold injury is a serious problem in container nurseries because 

substantial damage can occur during overwinter storage (see 

section 5.1.6 for more discussion).

49

5.1.4.4 Abiotic root diseases

Roots of container tree seedlings are subject to several 

environmental stress factors that can cause disease.

Temperature stress. Container seedling root systems are more 

exposed to extreme temperatures than bareroot seedlings. 

Direct sunlight can raise root temperatures to damaging levels in 

exposed containers. Heat injury is difficult to diagnose. Using 

white containers to reflect sunlight and cooling with irrigation will 

reduce this hazard.

49
Waterlogged media. One of the drawbacks of using containers is 

that the natural drainage patterns found in field soils are not 

present. Containers develop a perched water table that creates a 

layer of saturated medium at the bottom of the container (see 

volume four of this series). The peat-vermiculite media used in 

most container tree nurseries are particularly liable to compaction, 

which can further aggravate the problem. Waterlogged media 

reduce the necessary gas exchange between the roots and the

atmosphere and can lead to oxygen deficiency.

50

A growing medium that has been overwatered will often develop 

excessive growth of mosses and algae and may smell sour when 

removed from the container. Cauliflower-shaped growths may be 

present on the roots (fig. 5.1.43A); these hypertrophic structures 

are swollen lenticels that develop in response to low levels of soil 

oxygen (Boyce 1961). Lieffers and Rothwell (1986) report that 

black spruce seedlings grown in waterlogged media produced a 

large number of swollen lenticels, which they consider an adaptive 

response to saturated conditions. Tamarack seedlings grown under 

the same conditions did not develop the swollen lenticels, which 

may indicate that some species have a higher tolerance to 

waterlogged conditions than others (Tripepi and Mitchell 1984). 

Another common symptom of waterlogging injury is dark, swollen 

roots that feel soft and spongy (fig. 5.1.43B); these roots are often 

infected with pathogenic fungi.
51

Shoot diseases and pest problems can be caused by fungi, 

insects, or abiotic stresses. Symptoms of many of these diseases 

are similar and so a damage key can be helpful (fig. 5.1.44). Often 

shoot symptoms are caused by root problems and so the root 

system of symptomatic seedlings should also be examined. 

(Letters in the key refer to subheadings in the appropriate section 

of the text.)

5.1.5 Diseases and Pests of Seedling Shoots

and Foliage

Shoot diseases and pests are relatively more important in container 

nurseries than in bareroot nurseries, primarily because of the 

warm, moist environment that is conducive to disease 

development. As an example, grey mold, which requires free 

moisture for spore germination and infection, is much more 

devastating to container seedlings than to bareroot stock. 

Sutherland and others (1982) attribute the importance of shoot diseases in container nurseries to changes in cultural practices, 

especially the use of artificial growing media, which reduces the 

incidence of the soil-borne root diseases so damaging in bareroot 

nurseries. Responses from growers in the Container Nursery 

Survey show that grey mold and other shoot blights make up 49% 

of the reported diseases for container nurseries in the United 

States and Canada (table 5.1.2).
5.1.5.1 Fungal diseases of shoots 

A. Grey mold. The fungus that causes grey mold--Botrytis 

cinerea--is the most damaging pest of ornamental container plants 

(Nelson 1978) as well as the most damaging pest of conifer 

seedlings in container tree nurseries (James 1984). The 

importance B. cinerea was confirmed by the Container Nursery 

Survey, in which growers from the United States and Canada 

ranked it as the most serious pest, with 39% of the total responses 

(table 5.1.2). Mittal and others (1987) have reviewed grey mold 

disease.

Hosts. Grey mold attacks most species of container seedlings but 

certain species are particularly susceptible: redwood and giant 

sequoia (Peterson 1974); western larch, lodgepole pine, 

Engelmann spruce, and ponderosa pine (James and Woo 1984); 

western hemlock and Douglas-fir (Sutherland and Van Eerden 

1980); Scotch pine, and blue spruce (Gilman and James 1980); 

mountain hemlock, noble fir, and Alaska- cedar (Matthews 1983). 

Grey mold is also a serious problem in eastern container nurseries

where most species are affected.

Symptoms/Damage. As the name implies, this disease can be 

identified by the grey, cottony mycelia and spore masses on the 

surface of affected shoot tissue, especially on senescent needles 

of the lower shoot (fig. 5.1.45). Examination of the fungus with a 

hand lens will reveal stalked, spore-producing structures that produce a fuzzy appearance (fig. 5.1.46). As the disease progresses, 

infected shoot tissue becomes watersoaked and brown lesions 

often develop. The fungus may spread to the main stem, where 

cankers eventually gir dle and kill the shoot (fig. 5.1.47) 

(Sutherland and Van Eerden 1980). Because the fungus is an 

aggressive saprophyte, symptoms usually appear first on the 

shaded, senescent foliage at the base of the shoot. The disease is 

most common in the fall when the seedling canopy becomes 

closed, natural light levels are lower, and moisture often 

condenses on seedling foliage (Sutherland and others 1982). 

Under favorable conditions, 8. cinerea can spread rapidly from one 

seedling to another and disease "pockets" can develop within the 

seedling crowns. Peterson (1974) estimates that losses to this 

disease have exceeded 20% in some greenhouses. Mittal and 

others (1978) reported that 40% of a container pine crop (52,000 

seedlings) was killed by grey mold.

52
53

Disease development. The disease cycle for grey mold is 

presented in the introductory section 5.1.1.3 (fig. 5.1.4A-F). B. 

cinerea spores can be introduced into a container nursery in the 

air, on seeds, or in irrigation water (Sutherland and Van Eerden 

1980). Peterson and others (1988) state that the likelihood of 

spores carrying over between crops is low; instead, Botrytis spores 

originate outside the greenhouse and are drawn in by cooling fans. 

They also speculate that Botrytis infections may develop in July or 

August, several months earlier than widely believed. The fungus 

usually invades weak or damaged foliage first and then spreads to 

adjacent healthy tissue. James (1984) reviewed the literature on B. 

cinerea and lists free surface moisture, high humidity, and cool 

temperatures as conducive to infection. When environmental 

conditions become unfavorable, resting structures called sclerotia 

form (fig. 5.1.48) and can persist in soil or plant debris; these 

sclerotia produce spores when favorable conditions return.

Disease management. Reducing grey mold damage requires a 

combination of both cultural and chemical control methods. 

Container nursery managers should strive to make the environment 

less favorable for Botrytis growth and apply protective fungicides to 

limit initial infections (Sutherland and others 1983). The type of 

greenhouse may actually influence development of grey mold: 

fiberglass houses produced an environment that was over 14 times

more favorable for grey mold than the environment in 

plastic-covered greenhouses (Peterson and others 1988). 

Apparently, the fiberglass houses produce taller, more succulent 

seedlings that are prime hosts for Botrytis infection.
The timing of protective fungicide applications is important. These 

chemicals must cover susceptible plant tissue before Botrytis

spores germinate and penetrate the foliage. Because Botrytis

infections are most common in the fall, fungicide applications 

should begin in late summer. Peterson and others (1988) found 

that environmental conditions favoring spore germination were 

most prevalent in July and August in British Columbia tree 

nurseries, earlier than previously thought. Because of the ingrowth 

of new foliage and the rinsing effect of irrigation, protectant 

fungicides should be applied at regular intervals (1 to 2 weeks) 

during the susceptible period.

Botrytis spp. may develop tolerance to fungicides that are used 

repeatedly (Cooley 1981; James and Woo 1984; Gillman and 

James 1980), and so fungicides should be used in rotation during 

the growing season. Tolerance of B. cinerea to the fungicide 

benomyl has been demonstrated in vitro by culturing different isolates of the fungus on agar plates containing varying 

concentrations of the fungicide. Figure 5.1.49 provides a good 

illustration of fungicide tolerance: mycelial growth of B. cinerea

isolate 78--38 shows tolerance to several benomyl concentrations, 

whereas fungal isolate 78--18 is well controlled by the fungicide.

Regardless of the effectiveness of pesticides, chemical control 

of grey mold is virtually impossible without a corresponding and 

coordinated program of cultural control practices.

B. Fusarium root rot. See section 5.1.4.2 for discussion of this 

disease.

C. Water mold. See section 5.1.4.2 for discussion of this 

disease.

D. Other fungal diseases of shoots and foliage.

Although grey mold is by far the major foliar disease of 

container seedlings, there are other foliar diseases caused by 

fungi.

Cultural. Several cultural practices can be used to reduce the 

incidence and extent of grey mold infections (Sutherland and 

others 1983, Cooley 1981):

1. Keep seedlings healthy and vigorous and avoid injuring 

the foliage. Fertilizer-burned or frost-damaged foliage is 

particularly susceptible to Botrytis infection.

2. Avoid overly dense seedling growing levels by selecting a 

container that allows adequate spacing for seedling 

development. Containers can also be placed at a wider 

spacing to allow better air circulation during periods when 

seedlings are especially vulnerable.

3. Reduce the time that seedling foliage is wet by encouraging 

air circulation, irrigating early in the day, using surfactants in 

the irrigation water, providing underbench heating, or 

force-drying foliage with fans.

4. Follow a strict sanitation policy that includes removal and 

destruction of all plant debris, prompt roguing of infected 

seedlings, and sterilization of containers and growing area 

surfaces between crops.

Chemical. All fungicides registered for the control of grey mold are 

protectants that must be applied before infection takes place. 

There are a number of chemicals registered for controlling B. 

cinerea on ornamental plants, but not all of these are registered for 

tree seedlings. New chemicals are also continually being developed, so growers should monitor trade publications and check with 

a certified pesticide specialist for up-to-date information. McCain 

(1987) lists the latest chemi cals used for controlling Botrytis

cinerea. A partial list of fungicides used for B. cinerea control in 

container tree nurseries (table 5.1.9) reveals three things: first, 

there is a considerable difference in effectiveness among the six 

chemicals; second, some seedling species are much more 

susceptible to B. cinerea infection than others; and third, certain 

fungicides (e.g., dicloran, in this table) provide better protection on 

some species than others.

54
55
Fusiform rust. Fusiform rust (caused by Cronartium fusiforme) is 

the most severe disease problem in bareroot nurseries in the 

southern United States and poses a serious threat to southern 

pines grown in containers in open compounds. This disease can 

be controlled with carefully scheduled applications of the systemic 

fungicide triadimefon, which can be applied as a seed treatment, 

soil drench, or foliar spray (Rowan 1983). It also can be controlled 

culturally: delaying sowing until after mid-June avoids the spore 

dispersal period (Barnett 1987).

Sirococcus shoot blight. Sirococcus shoot blight is primarily a 

disease of germinants (see section 5.1.3.2) but can cause a stem 

canker and tip dieback of older seedlings later in the growing 

season. Sirococcus spp. spores are apparently seedborne but can 

also originate from trees adjacent to the nursery or in nearby 

shelterbelts (Sutherland and Van Eerden 1980).

Rhizoctonia. Rhizoctonia is a common pathogen of ornamentals 

but is apparently rare in forest nurseries. It has been observed on 

longleaf pine, where it causes a foliage blight; the mycelium is 

plainly visible and spreads from seedling to seedling when the 

foliage remains wet for extended periods (Pawuk 1981).

Melampsora rust. Melampsora spp., which require both a 

conifer and a poplar host to complete their life cycles, have

caused minor damage to Douglas-fir and lodgepole pine 

seedlings in British Columbia nurseries (Sutherland and Van 

Eerden 1980).

Colletotrichum acutatum. Colletotrichum acutatum is a newly 

reported pathogen that causes dieback of terminal and lateral 

shoots of western hemlock container seedlings. This disease was 

previously unknown in North America and its damage potential 

has not been established (Hopkins and others 1985).

5.1.5.2 Insects affecting seedling shoots and foliage

Many insects feed on the foliage and shoots of container tree 

seedlings, causing different types of injury (fig. 5.1.44). 

Diagnosis of insect injury can be difficult because these pests 

are very mobile and therefore not readily associated with the 

damage.

E. Aphids. Several aphid species have been identified in forest 

nurseries, incuding giant conifer aphids (Cinara spp.). Aphids 

were listed as the most common insect pest of container nurseries 

in the United States and Canada, based on the Container Nursery 

Survey (table 5.1.3). Although Sutherland and Van Eerden (1980) 

consider aphid damage to be minimal in British Columbia 

nurseries, Johnson (1965) reported growth losses as high as 70% 

in bareroot Douglas-fir seedlings infested by giant conifer aphids. 

These aphids also caused significant distortion of the terminal 

shoots of outplanted Scotch pine seedlings (Holopainen and 

Soikkeli 1984). Giant conifer aphids are often attracted to seedling 

grafts on tree improvement stock (Shrimpton 1987). Besides 

these growth effects, the mere presence of aphids on container 

seedlings implies poor nursery management practices to the 

seedling user.

Hosts. Many seedling species can be affected.

Symptoms/Damage. Giant conifer aphids (fig. 5.1.50) are 

relatively large, with long legs and dark bodies, and are usually 

visible on seedling twigs (Sutherland and Van Eerden 1980). Eggs 

are black, approximately 1 mm (0.04 inch) long, and there is 

usually only one per needle (Shrimpton 1987). Foliar chlorosis is 

the most obvious symptom of infestation; another symptom is the 

honeydew that aphids produce, which attracts other insects, such 

as ants and wasps.

Life cycle. The life cycle of most aphids is complex, and 

reproduction can be either sexual and asexual. Because aphids 

produce many generations per year, their populations can build up 

rapidly. When conditions become unfavorable, aphids become 

winged or lay eggs that survive overwinter (Nelson 1978).

56
Pest management. It is impossible to exclude aphids from 

container nurseries, but they can be managed by prompt 

identification and treatment with insecticides (see section 5.1.7.4). 

If aphids are not controlled at the nursery, they can lay eggs that 

overwinter on the foliage. Aphids are very difficult to control in the 

egg stage, and so infested seedlings could be sent to the planting 

site (Shrimpton 1987).

F. Spider mites. Mites are arthropods, but not insects, because 

they have 8 legs in their adult forms. There are many species of 

mites that can damage container crops, but the two-spotted or red 

spider mite is one of the most damaging of all greenhouse pests 

(Nelson 1978). Spider mites are not common on conifers but are a 

serious pest of many hardwood species, especially under warm, 

dry conditions (Matthews 1983, Sutherland and Van Eerden 

1980).

Hosts. Mites are not host specific and have been recorded 

on both hardwood and conifer seedlings.

Symptoms/Damage. Red spider mites are difficult to detect 

because they are so small. The first evidence of an infestation is 

chlorotic stippling of seedling foliage, but the best sign of their 

presence is the fine webbing that they produce. This webbing may 

eventually cover the foliage (fig. 5.1.51). When mite infestations 

become heavy, leaves appear scorched. The small red mites are 

often visible, under a hand lens, on the underside of the leaves.

Life cycle. The complete life cycle of spider mites can take only 

10 to 20 days and is accelerated by low humidity and warm 

temperatures. Mites form several resting stages that, together with 

the egg stage, are resistant to environmental stresses (Nelson 

1978).

Pest management. Mites are too small to be excluded from 

growing areas and are usually noticed first near vents, where they 

have been blown in. Miticides, such as those containing dicofol, 

can be applied as aerosols or sprays, but these are only partially 

effective because the egg and resting stages are resistant to 

pesticides. Miticide applications must be made as often as 2 days 

apart because many different stages of mites may be present at 

any one time. A good discussion of miticides is provided by Nelson 

(1978).

57
Hosts. Many conifer species are potential hosts for this insect 

pest and Shrimpton (1985) notes a preference for pine seedlings. 

The most serious damage in the United States has been to 

bareroot Douglas-fir in Oregon (Overhulser and others 1986), and 

the recent incidence of "bushy-topped" loblolly pine seedlings in 

bareroot nurseries in the southern United States has also been 

attributed to lygus bugs (South 1986).

Symptoms/Damage. Adult lygus bugs are true bugs, mottled 

yellow-green to reddish brown, 7 cm (0.25 inch) long, with flat oval 

bodies (fig. 5.1.53A); the wingless nymphs (fig. 5.1.53B) are 

smaller, varying from 1 to 6 mm (0.04 to 0.24 inch), and resemble 

aphids (Shrimpton 1985). Adults and nymphs feed by sucking plant 

juices and introduce a toxic saliva into the plants, which causes 

distorted needles, stem lesions, and deformed tops (fig. 5.1.53C). 

This feeding injury often results in bud abortion, multiple tops, and 

bushy-topped seedlings (fig. 5.1.53D) (Overhulser and others 

1986). Up to 20% of conifer and bareroot seedlings have been 

attacked in British Columbia nurseries (Shrimpton 1985).

G. Whiteflies. The greenhouse whitefly--Trialeurodes

vaporariorum--is a small [2 mm (0.08 inch)] insect with four wings 

that resembles a miniature moth (fig. 5.1.52). Although whiteflies 

are a major pest of ornamental greenhouse crops, they ranked 

eighth compared to other insect pests reported in the Container 

Nursery Survey (table 5.1.3). Parella (1988) presents a detailed 

discussion of the biology and taxonomy of the greenhouse 

whitefly.

Hosts. Many seedlings are affected but infestations of 

broad-leaved species are most common.

Symptoms/Damage. The first sign of a whitefly problem is the 

adults, which fly up when the host plant is disturbed. Immature 

whiteflies resemble scale insects that are oval-shaped and 

flattened against the underside of the leaf. Both adults and 

immature white flies suck plant juices, causing a yellow stippling of 

leaves; they also excrete honeydew, which often promotes growth 

of black sooty molds (Nelson 1978).

Life cycle. Depending on temperature, the whitefly life cycle takes 

from 4 to 5 weeks. The eggs hatch into flat scale-like crawlers that 

are transparent to yellow-green. After several growth stages, the 

nymphs pupate into winged adults (Nelson 1978).

Pest management. Because whiteflies thrive in the warm, humid 

environment of greenhouses, there are no effective cultural 

controls except to remove infested plants. Chemical sprays, 

particularly resmethrin and kinoprene, were effective against 

whiteflies but had to be applied two or three times in sequence to 

control overlapping generations (Nelson 1978).

H. Lygus bugs. Lygus bugs have only recently been identified as 

serious pests of conifer seedlings (Overhulser and others 1986). 

Although several different lygus bugs are pests of agricultural 

crops, only Lygus hespurus and the tarnished plant bug, L. 

lineolaris, have been identified in forest nurseries. Lygus bugs are 

sucking insects that feed on young, succulent plant tissue, such 

as terminal shoots and buds, and cause stunting and multiple 

leaders.

58
59
60

Life History. Lygus bugs overwinter as adults and feed and lay 

eggs on plants as soon as the weather warms. Eggs hatch after 10 

to 14 days, and flightless nymphs begin to feed on plant tissue. The 

adult insects are active fliers and readily move from one crop to 

another. Three to four generations are possible per year in warm 

climates (Overhulser and others 1986).

Pest Management. The presence of lygus bugs is often difficult to 

confirm; trapping has been used in agriculture but a test of several 

traps in British Columbia netted few insects (Shrimpton 1985). 

Chemical control is more effective on the flightless nymphs, 

because adult lygus bugs are highly mobile and therefore difficult to 

control with insecticides. In British Columbia, foliar applications of 

diazinon and cypermethrin during the summer months were 

effective (Shrimpton 1985). Overhulser and others (1986) found 

that multiple applications of fenvalerate, acephate, and endosulfan 

between July and September reduced damage by 80 to 90% in 

bareroot nurseries.

Thrips. These very small insects [1 mm (0.04 inch)] feed on a 

variety of greenhouse crops including tree seedlings. Thrips can 

go through the various stages of their life cycle (figure 5.1.54A) in 

as little as 2 weeks, and their populations can thus build-up very 

rapidly in a greenhouse environment. Adults are passively carried 

into the growing area through the ventilation system and lay eggs 

on the seedling foliage (Nelson 1978). Nymphs have 

piercing-sucking mouthparts, and their feeding causes small 

necrotic spots on the foliage, followed by twisting and curling of 

the shoot tips (figure 5.1.54B). It is difficult to see these insect 

pests, and symptomatic foliage should be carefully examined with 

a hand lens, or they can also be detected by tapping the damaged 

foliage over white paper (Nelson 1978). The extent of thrips 

damage in container tree nurseries is unknown but was severe 

enough on Douglas-fir seedlings in one Idaho nursery to warrant 

control measures. Common insecticides are effective in controlling 

this pest (see section 5.1.7.4).
Hosts. All species of seedlings are susceptible, and young, 

succulent germinants are particularly prone to injury.

Symptoms/Damage. Hallett (1982) describes the following 

symptoms of fertilizer injury:

1. Needle mortality in conifers, either needle-tips scattered 

randomly throughout the crown or entire needles in the 

mid-shoot position (fig. 5.1.55A). Hardwood seedlings 

exhibit a marginal leaf scorch (fig. 5.1.55B).

2. Chlorosis, needle-tip burn, or seedling mortality that 

corresponds to irrigation patterns.

3. Blisters on succulent shoot tissues.

61

5.1.5.3 Abiotic problems affecting seedling shoots

As discussed in section 5.1.1.2, abiotic diseases are often more 

common than those caused by biological pests. Because 

seedling shoots are directly exposed to cultural practices and the 

ambient environment, they are often injured by abiotic stress 

factors.

I. Fertilizer injury. Chemically, fertilizers are salts and can 

therefore cause salt injury to seedling foliage or roots. Although 

fertilizer burn of roots is common, the symptoms are usually 

expressed through foliar symptoms. Direct injury to seedling 

foliage can sometimes occur, especially when granular fertilizer is 

applied to containers as a top dressing (Sutherland and Van 

Eerden 1980), or when concentrated liquid fertilizers are applied 

without a clear water rinse. Sutherland and others (1982) consider 

fertilizer burn to be the most common abiotic disease of container 

seedlings in British Columbian container nurseries, primarily 

because it often disposes seedlings to attack by Botrytis spp.
62

Symptoms/Damage. Common symptoms of drought injury include 

withered foliage and a browning or drying of the needle tips or leaf 

margins (fig. 5.1.56). Drought injury is often difficult to distinguish 

from fertilizer burn, although the latter usually affects individual 

needles whereas drought stress can affect the entire shoot. 

Hardwood foliage senesces more rapidly when drought stressed, 

and individual leaves abscise from the base toward the tip of the 

shoot (Tinus 1987). Direct heat can also cause injury to seedling 

tissue. High heat levels can develop at the surface of the growing 

medium from direct and reflected sunlight. This intense heat often 

causes a girdling injury to sucEnvironmental conditions are very important in the development of 

fertilizer burn: damage is most likely to occur under hot, dry 

conditions, which accelerate evapotranspiration and concentrate 

salts in the growing medium or on the foliage.

Disease management. Fertilizer burn can be reduced by the 

following cultural practices:

1. Use proper fertilization techniques:

Apply fertilizers as liquids rather than as dry fertilizer 

incorporations or top-dressings.

Use frequent applications of diluted liquid fertilizer rather than a 

few concentrated fertilizer applications.

Follow liquid fertilizer applications with a clear water 

rinse.

2. Avoid unnecessary drought stress, particularly after 

fertilization.

3. Use a growing medium that is buffered by a high cation 

exchange capacity, and has good porosity to promote 

leaching.

J. Heat or drought injury. These two stresses are considered 

together because they often occur under similar environmental 

conditions, that is, high temperatures that cause excessive 

transpiration (heat injury to germinants is discussed in section 

5.1.3.2).

Hosts. All container tree seedlings are affected, although 

shade-tolerant species are most susceptible.
culent stem tissue. Although this injury normally occurs in younger 

seedlings before stem lignification, the constriction symptoms may 

not be manifested until later in the growing season, when normal 

cambial growth does not occur. The abrupt change between 

healthy and injured stem tissue produces a constriction just above 

the surface of the growing medium (fig. 5.1.57).

Disease management. Heat or drought injury can be minimized 

by proper nursery design and cultural practices:

1. Avoid excessively high temperatures or high sunlight levels, 

especially with succulent plants. Shade-tolerant species, such 

as western hemlock, can suffer foliage damage if moved from 

shade to direct sunlight without an acclimatization period 

(Matthews 1987).

2. Use a growing medium that contains a mixture of particle 

sizes to supply adequate moisture, yet has good aeration 

and leaching properties.

K. Chemical injury. See section 5.1.3.2.

L. Air pollution. This damage category includes all types of 

aerially induced chemical injury. Forest container nurseries are 

not normally established in areas that are subject to traditional 

sources of industrial air pollution, although more and more urban 

areas are becoming seriously polluted. Pollution damage often 

occurs in valleys where pollutants are trapped by air inversions 

and accumulate to damaging levels. Hanan and others (1978) 

discuss sources and types of air pollution damage to horticultural 

plants and also list indicator species that can be used to 

document exposure to major pollutants.

The most recent and comprehensive treatment of air pollution 

injury in container tree nurseries is provided by Scarratt (1985), 

who states that the most common pollution sources originate 

within the nursery itself. Heating furnaces, gasoline and diesel 

generators, and nursery vehicles produce several gaseous 

pollutants including carbon monoxide, carbon dioxide, sulfur 

dioxide, nitrous oxides, and ethylene. Sulfur dioxide and ethylene 

are considered to be the most serious threats to container 

seedlings and can be damaging in concentrations of 1 ppm or 

less (Mastalerz 1977).

63
3. Make periodic inspections of heating systems for leaking 

gases and adjust burners for peak combustion efficiency.

4. Never operate gasoline or diesel-powered vehicles in growing 

areas or where their exhaust gases could be sucked into 

building vents.

5.1.5.4 Problems caused by other plants

Plants are not generally considered to be disease-causing agents 

but, in keeping with the strict definition of a disease, plants such 

as weeds or liverworts compete with the crop seedlings for light, 

water, and mineral nutrients and can cause reduced growth or 

even death.

Weeds. Biologically, there is no such thing as a weed On a 

management basis, however, any plant that grows where it is not 

wanted is a weed (Bohmont 1983). In a container tree nursery, any 

noncrop plant that grows in the containers or even on the floor of 

the growing area is considered a weed.

Weeds are considered pests because

1. They reduce seedling growth or even kill seedlings by 

competing for light, water, and mineral nutrients. Because 

weeds germinate and grow much more rapidly than most 

woody plant species, they can easily outcompete the crop 

seedlings for the limited growing space in the container.

2. They serve as reservoirs for insect and disease pests. 

Whitcomb (1988) reports that aphids often build up 

populations on weeds such as bittercress (Cardamine 

pennsylvanica) and spider mites can be found on sorrel 

(Oxalis spp.).

3. Their presence in forest nurseries gives the impression of overall poor management.

Weed control is more critical in container fa cilities than in bareroot 

nurseries because weeds can easily dominate the volume of 

growing medium in small containers. Weeds are a serious problem 

in container nurseries in which seedlings are germinated in 

enclosed areas and then taken to open compounds for the 

remainder of the growing season.

Hosts. Susceptibility varies between seedling species. In Ontario 

greenhouses, jack pine was most severely damaged; tamarack, 

European larch, eastern white pine, and red pine suffered 

intermediate damage; and northern white- cedar, Japanese larch, 

and black spruce were least affected (Scarratt 1985).

Symptoms/Damage. Air pollution injury always occurs on the 

foliage because gases enter plants through the stomates. 

Symptoms vary with the type of pollutant and the seedling species. 

In Ontario nurseries, conifer seedlings exhibited varying degrees of 

cotyledon and needle tip necrosis and curling, whereas birch seedlings showed marginal and interveinal chlorosis and necrosis. The 

most insidious symptom, however, is growth loss. Scarratt (1985) 

reported growth loss exceeding 40% in height and 85% in dry 

matter production in jack pine seedlings.

Disease development. Air pollution damage can be either chronic 

or acute. Individual incidents of exposure to high levels of 

pollutants can cause acute injury in which symptoms develop 

immediately after exposure to the pollutant. Chronic injury in which 

undetected growth losses are caused by continued exposure to 

sublethal levels of air pollution, is probably more common. Scarratt 

(1985) demonstrated that seedlings exposed to chronic pollution 

did not exhibit visible symptoms but suffered growth losses.

Disease management. There are no treatments that can 

alleviate the effects of air pollution injury once it has occurred, 

but several cultural practices can help prevent pollution losses 

(Scarratt 1985):

1. Do not locate oil furnaces inside, or in structures

immediately adjacent to, growing areas.

2. Install heating units and carbon dioxide generators so that 

they have adequate air supplies for efficient combustion and 

sealed vents for exhaust gases. Heating systems that use 

fossil fuels should be located downwind from the growing 

areas and away from depressions in which exhaust fumes 

could accumulate.

64
Weed management. Cultural. Cultural control of weeds includes 

screening vents and irrigation water intakes to exclude weed 

seeds, cleaning soil or growing medium from shoes and 

equipment before they are brought into the growing area, and 

hand weeding. Hand removal of weeds from the containers is 

most practical and economical when the weeds are still small, 

e.g., during the thinning operation. Once weeds become 

established in the container, their roots dominate the growing 

medium and hand weeding can uproot the crop seedling. The 

weed population produces a reservoir of weed seeds around the 

nursery that can be reduced by burning or mowing the weeds 

before they go to seed. All weeds should be physically removed 

from the growing areas between crops; growing compounds with 

paved floors or fabric ground coverings will have fewer weed 

problems. Whitcomb (1988) concludes that herbicides should only 

be considered as part of an overall weed control program that 

emphasizes sanitation.

Chemical. Chemical control of weeds in container nurseries can 

be divided into two categories: selective herbicides that are 

applied directly to the containers, and nonselective chemicals 

that are used to eliminate weeds around the growing area.

Herbicides are not as commonly used in container nurseries as in 

bareroot nurseries because the high organic content of most 

growing media inactivates many chemicals and the succulent 

seedlings are more susceptible to phytotoxic injury. Heat 

pasteurization or chemical fumigation of the growing medium (see 

section 5.1.7.2) will eliminate weed seed contamination, although 

most commercial media should be weed-free. Pre-emergence 

herbicides are not used in container tree nurseries although some 

postemergence or contact herbicides are being tested on 

seedlings in outside growing areas. Hallett and Burns (1983) 

tested a variety of postemergence chemicals and found that only 

glyphosate, when applied late in the growing season, provided 

acceptable weed control without seedling injury (table 5.1.10). 

Ahrens (1985) reported

65

Hosts. All species of seedlings are susceptible.

Symptoms/Damage. Weeds generally germinate much more 

rapidly than woody plant species and are easily recognized once 

they develop true leaves, although there could be some confusion 

during the cotyledon stage. Growers must become familiar with the 

common weed species in their area so that they can be promptly 

removed.

There are few published reports that quantify the detrimental 

effects of weeds on container tree seedlings, but Whitcomb 

(1988) reviews literature that reports growth losses of 24 to 44% 

due to weed competition in ornamental container stock.

Weed development. Because of the predominant use of artificial 

growing media in container nurseries, weeds are usually not a 

serious problem in enclosed growing areas. Growing media are 

sometimes contami nated, however, when weed seeds are 

wind-blown into opened bags of medium or cached in closed bags 

by mice. Weed seeds can also be introduced into growing areas by 

wind, or in soil on equipment or workers' feet. Because some 

weeds have very small seeds, they can be distributed by the 

irrigation system in container nurseries with surface water sources.

Weeds that are present in the growing medium germinate within a 

few days after the containers are placed in the growing area and 

develop rapidly. Weed seeds that are blown or carried into the 

greenhouse can show up anytime during the growing season but 

are generally not a problem in the containers after seedling crown 

closure. Weeds grow extremely well in the favorable nursery 

environment and can become a serious problem in unpaved 

growing compounds.
glyphosate phytotoxicity in peat-based growing media, so growers 

should be cautious using this chemical. One of the real problems 

with postemergence herbicides is that crop seedlings vary in their 

susceptibility to phytotoxic injury. Gilreath and Gilreath (1986) 

tested four postemergence herbicides on 17 different species of 

woody plants and found that each herbicide was phytotoxic to at 

least one crop species.

Chemical control of weeds around the growing area is usually done 

between seedling crops because it is difficult to apply herbicides 

uniformly under benches and some herbicides can volatilize and 

injure seedlings. Three different types of herbicides can be used on 

noncrop areas: pre-emergence, postemergence (contact), and soil 

sterilants (table 5.1.10). Because preemergence herbicides are 

only effective on germinating weeds and do not kill established 

plants, all existing weeds must be removed before application. 

Contact herbicides are generally nonselective, that is, they kill all 

plants on contact. Soil sterilants are pre-emergence herbicides that 

are effective for extended periods of time, but some weed species 

are tolerant of these chemicals (table 5.1.10).

Using any chemical in the enclosed environment of a container 

nursery is potentially hazardous, and nursery managers are 

cautioned to read the herbicide label carefully and implement 

small-scale tests before attempting a full-scale chemical weed 

control program.

66

Cryptogams (moss, algae, and liverworts). Cryptogams is a 

collective term for plants that reproduce from spores instead of 

seeds. These lower plants thrive in the container nursery 

environment and cause serious problems in containers when they 

completely cover the surface of the growing medium and interfere 

with water and nutrient infiltration. Liverworts (fig. 5.1.58A) and 

larger mosses (fig. 5.1.58B) can also physically overshadow small 

seedlings and compete with them for light. The exact amount of 

damage caused by these plant pests is difficult to determine and 

varies from nursery to nursery, but Ross and Puritch (1981) state 

that damage is increasing, especially in older growing facilities. 

Liverworts have proven to be especially troublesome in older 

growing stock types, such as 2 + 0 seedlings; the liverworts 

become established during the first year and then outcompete the 

seedlings during the second year. The problem is most serious in 

coastal container nurseries where the cool, moist climate favors 

the establishment and growth of liverworts (Shrimpton 1987).

Hosts. All species of seedlings can be affected but 

slower-growing conifers such as spruces and true firs seem 

particularly vulnerable.
Symptoms/Damage. All these plants can be seen with the naked 

eye. Ross and Puritch (1981) identified five bryophytes (true 

mosses and liverworts) and nine algal species from container 

nurseries in British Columbia and list some keys for culturing and 

identifying species. Haglund and others (1981) report that mosses 

can eventually choke out small seedlings and cause stunting and 

chlorosis. Unless these plant pests become established early in 

the season, they are progressively shaded-out as soon as the 

seedling grows large enough to completely shade the surface of 

the growing medium. Mosses and liverworts are more of a problem 

in open growing areas and shadehouses, probably because of the 

difficulty of complete sanitation between crops.

Cryptogam development. Moss and algae develop from residual 

material on reused containers and from water or airborne spores. 

Ross and Puritch (1981) found algae, but no spores of mosses or 

liverworts, in irrigation water and did not isolate any cryptogams 

from peat or growing media samples. They report that the major 

sources of contamination were used containers and airborne 

inocula, which are easily spread by the greenhouse ventilation 

system; some species are able to spread vegetatively as well as 

from spores. Cryptogams can develop rapidly on the surface of the 

growing medium under the moist growing environment of a 

container nursery. Algae and moss develop rapidly whereas 

liverworts and lichens are much slower growing.

67
Chemical. Several chemicals have been used to control 

cryptogams, including fungicides and surfactants, but few 

materials are registered specifically for that purpose. There are 

several chemicals that have controlled algae on greenhouse 

surfaces including copper sulfate and calcium hydroxide; a 

bromine compound (Agribrom®) was found to be effective in 

controlling algae in greenhouses and is apparently not phytotoxic 

to ornamental crops (Anonymous 1987). Ross and Puritch (1981) 

recommend filtering the air used for cooling and eliminating 

cryptogams from greenhouse surfaces and containers. The 

benches and floors of the growing area should be thoroughly 

cleaned between rotations, and reusable containers should be 

disinfected between rotations (see section 5.1.7.2).

Most herbicides that would effectively control crypto gams are also 

phytotoxic to tree seedlings. Pawuk (1983) tested 11 different 

chemicals for control of algae and concluded that the fungicides 

maneb and dichlone were the most effective and yet were not 

phytotoxic to shortleaf pine seedlings. Haglund and others (1981) 

initiated a moss control test with chemical treatments involving 

fungicides and surfactants, alone or in combination. X77® was the 

least phytotoxic of the eight surfactants tested, and a tank mix of 

X77 surfactant and captan fungicide gave "virtually complete" 

moss control. This solution was applied at a rate that thoroughly 

wetted the moss, and the authors suggested that applications be 

made in the late afternoon on a cloudy day because phytotoxic 

injury is more severe in bright sunlight. Dodine (Cyprex®) is a 

fungicide used for control of liverworts in British Columbia 

nurseries; it is applied to cover the liverworts and penetrate the 

surface layer of the growing medium (Shrimpton 1987). Obviously, 

any potential chemical control method should be carefully reviewed 

and tested before being attempted operationally.

Many of the common disinfectants such as chlorine bleach can be 

used to control cryptogams in noncrop areas (see section 5.1.7.2). 

Matthews (1983) reports that container nurseries in British 

Columbia use specially formulated soaps to reduce moss and 

algae growth on floors, pallets, and used containers. Peck (1984) 

reports that a cryptocidal soap (Safer's De Moss®) controlled 

mosses on greenhouse surfaces and retarded subsequent 

infestations.

68

Cryptogam management. Cultural. Cryptogams can be controlled 

by encouraging vigorous early seedling growth and limiting the 

source of the inocula with proper sanitation practices. Several 

cultural practices inhibit development of cryptogamic plants. Using 

a light-colored seed covering that completely covers the surface of 

the growing medium inhibits the growth of these plant pests. Many 

container nurseries in British Columbia use a special "forestry 

sand" for a seed covering which has drastically reduced the 

liverwort and moss problem (Sutherland 1987). Hallett and Burns 

(1983) specifically mention a lime-based grit covering, and perlite 

has also been effective. Promoting rapid seed germination and 

early-season seedling growth will ensure that cryptograms will not 

become established in containers. Reducing irrigation frequency 

will allow the seed covering to dry out and lower the humidity at the 

growing medium surface, which should discourage growth of 

cryptogams (Hallett 1982). Gravel floors of growing areas are 

especially subject to the development of cryptogams, which should 

be physically removed or killed with chemicals before the crop is 

sown.
5.1.6 Diseases and Pest Problems During

Seedling Storage

The type of storage conditions will determine the types of disease 

problems that will be encountered. Originally, all container 

seedlings were either shipped directly from the growing area or 

were held in shadehouses or other sheltered storage structures 

until shipment. Many of the same diseases that occurred in the 

growing area could carry over into the shadehouse. With the 

advent of refrigerated storage, however, fungal diseases such as 

storage molds became a more serious problem.

5.1.6.1 Problems during sheltered storage

Many of the diseases discussed in the previous sections can also 

be encountered in shadehouses, although disease severity is 

usually diminished because of the less-favorable environment. 

Container tree seedlings must be hardy and dormant if they are to 

be stored over winter, thus making cultural practices to induce 

hardiness and dormancy crucial (see chapter on hardiness in 

volume six of this series). Seedlings in sheltered storage are 

susceptible to two major types of overwinter damage: cold injury 

and winter drying.

Cold injury. Whenever tree seedlings are held overwinter, they 

are susceptible to cold injury, and container seedlings are 

particularly vulnerable because of their extended period of 

succulence. One of the major problems with the rapid growth rates 

achieved in container nurseries is the difficulty in slowing growth 

and inducing dormancy and cold hardiness. Cold injury is directly 

related to seedling dormancy and cold hardiness. The root systems 

of container seedlings are particularly vulnerable to cold injury 

because the roots are less hardy than other seedling tissues and 

are exposed in containers.

Hosts. All species of container seedlings are suscepti ble, but 

species or ecotypes from low elevations, coastal areas, and 

southern latitudes are most likely to be injured.

Symptoms/Damage. Foliar cold injury is initially expressed as 

pale, water-soaked tissue that eventually turns from 

straw-colored to brown or bright red (fig. 5.1.59A), depending on 

the seedling species and degree of injury. Symptoms develop 

relatively soon

69
Disease management. Losses to cold injury can be significantly 

reduced by carefully planned and executed cultural practices:

1. Develop growing schedules that include an adequate 

hardening period.

2. Avoid moving seedlings out of enclosed growing structures 

until they are adequately cold hardy or all danger of frost has 

passed.

3. Protect nonhardy seedlings with irrigation, supplemental 

heat or protective coverings.

4. Try to grow seedlings that are adapted to the local 

environment; exotic species or seed sources will require 

special handling and protection.

5. Monitor seedling hardiness with cold hardiness tests to 

determine when seedlings are ready for storage. Glerum 

(1985) summarizes the latest technology in cold hardiness 

testing.

Winter drying. Winter drying is actually desiccation injury and 

occurs whenever seedlings are exposed to drying conditions, 

generally wind or direct sunlight; damage is most severe when the 

growing medium and roots remain frozen for extended periods. 

Seedlings can even become desiccated when they are stored 

under frost-free refrigeration without proper packaging. Winter 

drying is not directly related to seedling dormancy or cold 

hardiness.

Hosts. Conifer seedlings are most susceptible, although 

hardwoods could be damaged in extreme circumstances (Boyce 

1961).

Symptoms/Signs. Winter drying is often referred to as winter burn 

because the affected foliage often turns bright red and appears 

scorched (fig. 5.1.59B). All exposed foliage is susceptible, but 

generally the needle tips are most commonly injured; buds within 

the symp tomatic foliage may not be damaged and often produce 

normal shoots after outplanting. Tip dieback or even seedling 

mortality can occur in extreme cases. Winter drying can be 

distinguished from cold injury by foliage color and timing of 

symptom expression.

70

after exposure to freezing temperatures, usually within a couple of 

weeks, and meristematic tissues such as shoot tips show damage 

first. This characteristic helps distinguish cold injury from winter 

desiccation, which usually affects all exposed foliage and develops 

over a longer time period. Buds can sometimes be killed by frosts 

that do not injure hardier foliage (fig. 5.1.60A). The most insidious 

form of cold injury is cambial damage, which can easily be 

overlooked because the bark must be removed to expose the 

symptoms. Cold-damaged cambial tissue turns varying shades of 

brown, and this damage may occur in intermittent patches along 

the stem (fig. 5.1.60B). Roots are particularly susceptible to cold 

injury because they do not harden as much as the shoots, and cold 

damage to container seedling root systems can be common during 

overwinter storage. Overwintering losses can be severe. Hallett 

(1984) reports that 11.8 million container seedlings (38% of the 

total crop) were lost in the Maritime Provinces in the winter of 

1982-83.

Disease development. Cold injury can develop from a single frost 

or during an extended period of cold weather. Damage is most 

common in the late fall or early spring, when seedlings are entering 

or coming out of dormancy. Young Scotch pine seedlings showed 

wide individual variation in cold tolerance when exposed to a 

simulated frost. Some showed injury after 2 hours at-4.5 °C (24 

°F), whereas others remained tolerant at -7.5 °C (20 °F) 

(Holopainen 1988). Symp tom expression may be delayed during an 

extended period of cold weather but can be accelerated by bringing 

seedlings into a warm environment. Cambial or root injury may 

cause delayed bud break or foliar wilting after the seedlings are 

returned to a growth promoting environment.
Disease development. Compared to bareroot stock, container 

seedlings are more susceptible to winter drying because of the 

limited amount of moisture reserves in small containers. 

Desiccation can become severe when seedlings in frozen growing 

media are exposed to sunlight or drying winds. Winter drying is 

slower to develop than cold injury, usually requiring weeks rather 

than days. Seedlings that are exposed around the perimeter of the 

storage area are most severely damaged, especially when the root 

systems are not properly insulated. In areas with snow cover, 

shoots that protrude above the snow often show winter burn.

Disease management. Proper storage structure design and 

some preventative cultural practices can reduce winter drying 

losses:

1. Protect seedlings from direct exposure to sun and

wind with sheltering structures.

2. Group seedlings together on the ground with some

sort of insulating material around the perimeter.

3. Cover seedling foliage with mulches or sheeting material; 

complete snow cover provides an ideal type of protection.

71
72

Deer. Watchdogs may discourage deer in a small storage area, 

but fencing is a more common approach. Although tall exclusion 

fences are sometimes used, deer are able to jump many such 

structures. The best system is a permanent, low-impedance 

electrical fence, but proper design is important. Byford (1987) 

describes a 5-strand deer fence with a low-impedance electrical 

charger that is both effective and cost-efficient. Temporary fences 

are sometimes used; these consist of a single-strand electrical 

fence that repels deer by shocking them after they have been 

attracted to an electrically charged bait.

Chemical repellents are another option for deer control, and there 

are two basic types: contact repellents and area repellents. 

Contact repellents are applied directly to the seedlings, and the 

deer are repelled by the taste of the chemical. Frequent 

reapplications of contact repellents are often necessary because 

the chemicals wash off the foliage and any new shoot growth is 

unprotected. Area repellents are applied on or near the seedlings 

and repel deer by smell alone. In a comprehensive te st in 

Connecticut, a wide variety of deer repellents were tested including 

human hair, hot sauce, and some conventional chemicals (thiram, 

Magic Circle Deer Repellent®, Big Game Repellent®, and 

Hinder®). Results were variable between the study areas, but the 

researchers found that, under heavy browsing pressure, none of 

the repellents were very successful (Byford 1987).

Rabbits. Controls for these common animal pests include 

live-trapping, fencing, or chemical repellents. Three-foot-high 

chicken-wire fences are very effective in excluding rabbits but 

must be secured to the ground to prevent rabbits from going 

under the fence. Most rabbit repellents (e.g., thiram) are taste 

repellents, but they suffer the same limitations as the deer 

repellents (Byford 1987).

Animal damage. Seedlings stored in outdoor holding areas are 

subject to animal predation from rodents and larger animals, such 

as rabbits and deer. Ironically, structures for overwintering 

container seedlings are an ideal environment that protects small

animals from their natural enemies.

Hosts. All species of seedlings are susceptible.

Symptoms/Damage. Rodents leave small teeth marks (fig. 

5.1.61 A) on the lower stem, whereas rabbits usually clip the 

entire stem off at a smooth 45° angle. Deer usually browse 

seedlings, clipping the seedling shoot; because deer have no 

upper incisors, deerbrowsed tissue has ragged edges (Byford 

1987). Tracks and fecal droppings often help in identifying the 

specific pest.

Animal management. Protecting container seedlings in sheltered 

storage is very difficult. Byford (1987) presents a series of 

questions for determining if animal control in nurseries is justified:

1. Is a control program worth the effort? Is the economic loss 

or nuisance greater than the cost of control?

2. Is there some way to keep the animals from getting

to the seedlings, e.g., screens or fencing?

3. Can the offending animals be repelled from the site? Can 

chemicals, visuals, or sound be used to keep animals from 

the growing area?

If none of the above actions are effective, then the final step is to 

remove the problem animals with trapping or chemical poisons. 

There are specific controls for some common animal pests.
Rodents. Cats are often used in greenhouses to control small 

rodents but they are less effective in large outside seedling storage 

areas. Fencing is obviously ineffective for rodents; it is difficult and 

expensive to build a completely rodent-proof structure. Snap 

trapping is some times used to control rodent populations although 

chemical baits are usually the cheapest and most effective means 

of dealing with large numbers of these pests (Carlson 1983). 

Byford (1987) discusses two types of rodent poisons: single-dose 

rodenticides (zinc phosphide), which kill the animals after one 

feeding, and anticoagulants (warfarin, coumafuryl), which are only 

effective after continued ingestion of the poison. Anticoagulants are 

safer around nontarget animals and often more effective because 

"bait shyness" will not develop. Anticoagulant poisons must be 

provided in

plentiful supply for several weeks to be effective. Byers and 

Carbaugh (1987) tested the efficacy of several rodenticides in 

laboratory and field tests and found that zinc phosphide pellets ZP 

Rodent Bait AG® gave better control of pine voles (Microtus 

pinetorum) and meadow voles (M. pennsylvanicus) than other 

similar products. If possible, place baits both under the containers 

and on the container surface, especially around the perimeter of a 

group of seedlings (fig. 5.1.61 B). Any pesticide should be carefully 

placed to minimize the possibility of accidentally poisoning other 

animals, especially pets.

73
74

5.1.6.2 Problems during refrigerated storage

As with bareroot seedlings, storage mold of container seedlings is 

a principal concern in refrigerated storage. Whenever seedlings 

are stored under refrigeration, storage mold is inevitable; the 

question is not whether mold will develop, but when it will occur 

because the likelihood of storage mold increases with storage 

time. Many different fungi can cause molding of bareroot stock, 

most of which are introduced on soil particles; because container 

seedlings are grown in artificial growing media, this major source 

of fungal inoculum has been eliminated. The major storage mold 

fungus of container stock is Botrytis cinerea, the same pathogen 

that is responsible for grey mold in growing stock.

Botrytis storage mold. Botrytis storage mold can be a serious 

problem in stored container stock because the fungus can spread 

unobserved in the cool, moist environment of the storage 

container. Often the problem is not identified until the seedlings 

are shipped to the planting site and by that time the entire storage 

container is usually affected and the seedlings ho,, t be 

destroyed. Storage molds are particularly cost',, because stored 

seedlings are at their maximum ,i.~. and the nursery manager has 

usually committed stock to a customer.

Hosts. The same seedling species that are susceptible to grey 

mold in the nursery are vulnerable to botrytis storage mold: 

redwood, giant sequoia, western larch, lodgepole pine, ponderosa 

pine, Engelmann spruce, western hemlock, Douglas-fir, Scotch 

pine, blue spruce, mountain hemlock, noble fir, and Alaska- cedar.

Because B. cinerea is not very host- specific, probably all conifer 

and hardwood seedlings are susceptible.
Symptoms/Damage. Storage molds usually develop in the 

interior of the storage bundle, so that several storage containers 

need to be opened and checked throughout. The grey, cottony 

mycelium and fruiting bodies (fig. 5.1.62) can be seen on the 

foliage or stem; in advanced cases, the infected foliage becomes 

soft, water-soaked, and spongy.

Disease development. Botrytis storage mold is almost always the 

result of a pre-existing infection that originated during the nursery 

period, especially on species like Douglas-fir and western hemlock. 

Seedlings that are damaged by fertilizer burn, cold, or mechanical 

injury are particularly vulnerable (Sutherland and others 1982). 

Surface moisture accelerates the spread of B. cinerea, and mold 

severity is directly related to the temperature and the length of 

storage. Because B. cinerea thrives under cold temperatures, it 

grows well under refrigerated storage conditions.

Disease management. Cultural. Sutherland and others 

(1982) list the following cultural controls for botrytis storage 

mold:

1. Prevent grey mold infections from developing in

the nursery.

2. Carefully inspect seedlings during prestorage grading and rogue-out diseased individuals.

3. Store seedlings, especially those with any dead or

damaged foliage, for the shortest time possible.

4. Make certain that seedling foliage is dry before

storage.

5. Store seedlings at as cold a temperature as possible. Where 

feasible, frozen storage [-1 to -2 °C (about 30 °F)] is 

recommended because all free moisture on the seedling 

foliage is turned into ice crystals, which apparently suppresses 

the spread of B. cinerea.

6. Inspect stored stock frequently, especially seedlots with dead 

or damaged foliage, so that diseased seedlings can be rogued 

before the disease spreads through the entire storage 

container.

7. Insure that seedlings are properly hardened-off prior to storage 

(see the chapter on hardening in volume six).

75

Chemical. Protective fungicides that are applied to seedlings prior 

to storage may be of some value, although Sutherland and Van 

Eerden (1980) reported that results have been erratic. Fungicides 

that are registered for grey mold (table 5.1.9) can either be applied 

as a spray late in the growing season or as a spray or dip 

immediately before storage. Because many fungicides persist on 

the seedling foliage during handling, shipping, and outplanting, 

nursery and field personnel may be exposed to prolonged contact 

with storage fungicides. Therefore, these chemicals should be 

used cautiously and only as a last resort.
5.1.7 Disease and Pest Management

Strategies

The management of nursery diseases and pests is covered in two 

places in this publication: what follows here is a general discussion 

of nursery disease and pest management, whereas specific control 

measures are included in the discussion of the individual pest 

problems.

5.1.7.1 Philosophy of management instead of

control--integrated pest management

Nursery managers have traditionally talked about "controlling" a 

disease. This usually involved waiting for the disease to occur and 

then spraying some toxic chemical on the already dead or dying 

seedling. Today, we take a more scientific approach to nursery 

disease control that involves a variety of cultural and chemical 

measures that are designed to manage the disease (fig. 5.1.63). 

Nursery disease management should be a series of interrelating 

processes that are incorporated into the entire spectrum of 

container nursery culture, rather than as a knee-jerk reaction to a 

specific pest. Often, a single control measure will not be effective

because most nursery diseases are very complex and require an 

integrated management approach using several different control 

measures. Chemical pesticides (fig. 5.1.64) should be just one part 

of a comprehensive pest management program that integrates 

both chemical and cultural control measures.

Integrated pest management (IPM) is a concept that has been 

proposed for some time but has only recently been implemented in 

some ornamental container nurseries (Davidson and others 1988). 

IPM can be defined as the selection, integration, and 

implementation of a pest control program that is based on 

predicted economic, ecological, and sociological consequences 

(Botrell 1979). Nursery managers will recognize that most of the 

individual components of an IPM program are not new: IPM 

includes standard cultural, mechanical, and chemical controls as 

well as biological controls and host genetic resistance. The main 

difference is the organization of the different pest control strategies 

into a single, comprehensive program--IPM is a systems approach 

rather than an incident approach.

76
Selecting the site. Disease prevention begins with nursery site 

selection. Sutherland and others (1982) state that nursery location 

is one of the two most criti cal factors in determining disease 

incidence. McCain (1978) recommends that container nurseries 

not be established in locations where host-specific pathogens such 

as fusiform or gall rusts are a problem. Another solution is to 

remove the primary or alternate rust hosts from a reasonable 

distance around the nursery during nursery development. 

Matthews (1983) reports that, to control the spread of European 

pine shoot moth in British Columbia, regulations require that 

infested pines be eliminated for 100 m (328 feet) around nurseries. 

He also mentions that shrubs and grass surrounding nurseries can 

harbor insect pests including thrips, aphids, weevils, and the 

European crane fly. Any unnecessary structures or vegetation 

around a nursery site should be removed because they provide 

cover for seed-eating rodents and birds.

77

Perhaps the most important selling point of an IPM program is the 

sociological-political aspect. All forms of agriculture, including 

container tree nurseries, are coming under increasing scrutiny 

concerning pest control activities, especially the use of pesticides. 

Progressive nursery managers should begin to integrate their current pest management treatments into an IPM program framework, 

as well as consider the newest pest management options, such as 

the use of suppressive growing media (see section 5.1.7.3).

The concept of disease management has been presented in 

several different ways; the following treatment is based on the four 

phases of disease management presented by Smith (1970) and 

Bohmont (1983): exclusion, protection, eradication, and resistance. 

This approach is preferable to the traditional division of disease 

management into chemical and cultural control, which separates 

these two control options rather than integrating them. The 

four-phase approach allows both chemical and cultural controls to 

be discussed together and is also applicable to both biotic and 

abiotic diseases. Exclusion and eradication can only be applied to 

biotic disease agents whereas protection and resistance can be 

used for both biotic and abiotic problems.

5.1.7.2 Exclusion: preventing pest entry

The container nursery environment is initially pest-free and so the 

most logical approach to disease management is to attempt to 

prevent diseases by excluding disease agents from the growing 

area. Diseases are much easier to prevent than to cure. Moody 

(1984) states the case well when he says that all disease controls 

are based on one criterion-prevention. Excellent discussions on 

disease prevention through sound sanitation practices are provided 

in Nelson (1978) and Hanan and others (1978).
With existing container nurseries, however, we need to identify 

how potential pests generally get into the growing area:

1. Wind--Airborne spores, seeds, or insects can be

introduced through the ventilation system.

2. Water--Fungus and cryptogam spores and weed seeds 

can be introduced through the irrigation water.

3. Growing media--Most commercial mixes are considered 

"essentially sterile," but potentially harmful fungi have been 

isolated from some types of growing media or their 

components.

4. Containers--Reusable containers may contain residual 

growing medium that harbors pest propagules or moss or 

algae from the previous crop.

5. Surfaces in the growing structure--Floors, benches, and other 

surfaces in the growing area may harbor pests from the 

previous seedling crop.

6. Propagation materials--Seed, transplants, or cuttings are 

sometimes infected before they reach the nursery.

7. Transported soil and growing medium--Infested materials 

can be carried into the growing area on tools, equipment, or 

on shoes of workers or visitors.

8. Mobile pests--Insects, birds, and rodents can enter

the growing area directly.

Using certified seed, transplants, and cuttings. One of the most 

effective ways of excluding pests from nurseries is to use only 

seed, transplants, and cutting materials that are certified to be pest 

free. Hanan and others (1978) discuss some of the horticultural 

practices that are used to prevent pest spread through propagation 

materials; these practices are not as applicable for forest tree seed 

and transplants but there are some examples. In British Columbia, 

certain nurseries are in the balsam woolly adelgid ("aphid") 

infestation zone and thus Abies spp. seedlings grown in these 

nurseries must be treated with insecticides and be certified pest 

free before they can be shipped to other nurseries outside this zone 

(Matthews 1983). To overcome this problem, Abies spp. are 

normally grown in nurseries outside the infestation zone.

Although there is not an operational pest-free seed certification 

program for forest tree seed at the present time, certain 

precautions during seed collection and handling can lower the 

incidence of disease. Any cone or seed handling practice that 

keeps seed clean will reduce the possibility of introducing 

seedborne pathogens into the nursery. Cones collected from the 

ground, and from squirrel caches in particular, are considerably 

more dirty than cones collected directly from the tree. Sutherland 

(1979) showed that the incidence of the seed fungus Caloscypha 

fulgens is significantly higher in cones collected from squirrel 

caches. Pathogenic species of Fusarium were observed to be 

more prevalent on seed lots from ground-collected cones in north 

Idaho and western Montana (James 1986).

Treating seed. Many different pathogenic organisms, including 

fungi, bacteria, viruses, and nematodes, can be carried on seed. 

Andersen and Leach (1961) provide a good discussion on testing 

seeds for pathogens. Tree seed has been shown to be infected 

with several fungal pathogens, and even nonpathogenic fungi can 

cause seed handling problems during stratification. Kliejunas 

(1985) reported that, even though none of the fungi were known 

pathogens, stratification bags of Jeffrey pine seed had extensive 

mold development, with groups of seed bound together with 

mycelia. Several different types of seed treatments are available.

78
Chemical sterilization. Chemical sterilization of seeds with 

sodium hypochlorite or hydrogen peroxide has shown variable 

results. The concentration of the chemical solution and the 

treatment time are the most important factors affecting chemical 

seed treatments. The major problem with treating seeds with 

strong disinfectants is that solutions strong enough to kill surface 

contaminants are often phytotoxic to the seeds. The effectiveness 

of chemical seed treatments also varies between different types of 

seeds: species with thin seedcoats may be more easily damaged 

than those with thick seedcoats, and rough- surfaced seeds such as 

those of firs (Abies spp.) are particularly difficult to treat.

Hydrogen peroxide. A 40-minute soak in laboratory grade (30%) 

hydrogen peroxide was effective in surface-sterilizing Douglas-fir 

seeds for research purposes, although Fusarium spp. propagules 

sometimes survive the treatment because they are carried internally in the seed (Graham and Linderman 1983). Barnett (1976) 

recommends using 30% hydrogen peroxide

Cold water soaks. Standing water soaks are common 

presowing or prestratification seed treatments but this practice 

greatly reduced germination and did not remove all seedcoat 

contaminants in a trial with Douglas-fir seed (James 1983).

Water rinses. A 48-hour running water rinse has been advocated 

as a way to remove seedcoat-contaminating fungi. Seeds can be 

loosely enclosed in open-mesh bags and placed in a water tank 

with vigorously moving water, which should wash away most 

surface contaminants. James and Genz (1981) compared this 

seed treatment to the standard standing water soak and reported 

a significant increase in germination and reduced fungal and 

bacterial contamination of ponderosa pine seed (table 5.1.11).

Hot water soaks. Hot water soaks have been recommended as a 

seed treatment for ornamental species. The seeds are placed in 

cotton or plastic-mesh bags and immersed in hot water at 49 to 53 

°C (120 to 127 °F for 30 minutes, and then cooled in running tap 

water (Handreck and Black 1984).

79
Merthiolate. Jobidon and Thibault (1980) reported that bleach and 

hydrogen peroxide did not work well with papery, porous seeds 

such as those of American green alder, and found that a 15-minute 

treatment in 1 % aqueous merthiolate gave very satisfactory 

results.

Obviously, more work is needed on different solution 

concentrations and varying treatment intervals. Growers should 

test any potential seed treatment on a small scale before 

attempting it operationally.

Fungicide seed treatments. The literature is contradictory on the 

benefits of using fungicides to control seedborne fungi. Bloomberg 

(1981) reviewed the literature on seed treatments for controlling 

Fusarium spp. and lists captan, thiram, and benomyl as seed 

fungicides but cautions that phytotoxicity may outweigh disease 

control benefits. Pawuk (1979) tested the effectiveness of 15 

different fungicide seed treatments on three species of southern 

pines, and found that captan and thiram were the least phytotoxic 

of the chemicals and that phytotoxicity varied between species, 

with slash pine being the most sensitive species. Belcher and 

Waldrip (1972) reported that thiram reduced fungal contamination 

without reducing slash pine germination, even with treatments as 

long as 30 minutes in a 6% thiram solution. James (1983) found 

that captan effectively eliminated fungal contaminants on 

Douglas-fir seed but was severely phytotoxic. Kliejunas (1985) 

treated seeds of several western conifers with captan or thiram 

and reported that neither chEmical significantly reduced 

germination, with the exception of thiram on white fir and 

incense-cedar. Carlson (1983) concluded that, even though thiram 

and captan may reduce losses to damping-off, they may inhibit 

germination or cause growth distortions. (See section 5.1.3.2 for 

more information on chemical injury caused by phytotoxicity.)

Fungicide seed treatments may be ineffective because the activity 

spectrum of many fungicides is narrow, resistant strains of the 

pathogen may develop, and the pesticides may be washed off the 

seed by frequent irrigation. The harmful effects of seed treatments 

may be more serious in container nurseries because the biological 

and chemical buffering capacities of arti ficial growing media are 

very low (Sutherland and Van Eerden 1980).

for seeds of southern pines, with soaking times ranging from 15 

minutes for shortleaf pine to 1 hour for longleaf pine. Several 

studies report variable results for other species: longleaf pine 

(Campbell 1982), and lodgepole pine, ponderosa pine, and 

Engelmann spruce (Fuller and Hildebrand 1985). James (1983) 

found that a 3% hydrogen peroxide treatment for 64 hours reduced 

most seedcoat contamination with only a negligible effect on 

germination of Douglas-fir seed. James and Genz (1981) found that 

ponderosa pine seed treated with 3% hydrogen peroxide for 5 

minutes and then rinsed showed significantly increased 

germination and decreased seedcoat contamination (table 5.1.11). 

Carlson (1983) recommends a 15-minute agitated soak in 3% 

hydrogen peroxide, followed by a cool, clear water rinse.

Sodium hypochlorite (household bleach). Saeur and Burroughs 

(1986) provide an excellent discussion of the various aspects of 

treating corn seed with sodium hypochlorite. They report that the 

strength of commercial brands of household bleach can vary from 

5.25 to 6.00%, and that effective surface disinfection depends on 

the pH, formulation, and strength of the solution. They also found 

that the effectiveness of sodium hypochlorite seed treatments is 

affected by pathogen concentration and seedcoat 

properties-spores trapped in seedcoat cracks or air bubbles may 

not be reached by the disinfectant.

For conifer seed, James (1983) reported that a 2-hour soak in a 

5.25% bleach solution effectively removed pathogenic fungi from 

the seedcoat but greatly reduced germination of Douglas-fir. 

James and Genz (1981) found that a 5-minute soak in 5.25% 

bleach followed by a thorough rinse significantly reduced seedcoat 

contamination of ponderosa pine without affecting germi nation 

(table 5.1.11). Wenny and Dumroese (1987) recommend a 

10-minute soak in an agitated bleach solution-2 parts stock laundry 

bleach (5.25% sodium hypochlorite) to 3 parts water-followed by a 

thorough rinse in running tap water. This seed treatment reduced 

losses to damping-off fungi such as Fusarium spp. on many 

thick- coated conifer seeds, such as Douglas-fir and pines, but is 

not used on species with thinner seed coats, such as true firs, 

larch, or spruces. Kliejunas (1985) treated seed of several western 

conifers with a dilute bleach solution (0.525%) and reported 

reduced germination and poor control of seed mold fungi.

80
81

Heat treatments. Several heat sources can be used for 

pasteurizing growing media: moist heat from steam, aerated 

steam, hot water, or microwave ovens; or dry heat from flame or 

electric pasteurizers. Regardless of the heating method, it is 

important to maintain the entire mass of growing medium at a 

uniform temperature that exceeds the thermal death points of the 

various nursery pests. Pests vary in their ability to tolerate high 

temperatures (fig 5.1.65), but most can be elimi nated by being 

subjected to a temperature of 60 to 82 °C for at least 30 minutes 

(table 5.1.12). Excessive temperatures can cause problems by 

eliminating benefi cial soil organisms and producing toxic chemical 

compounds. Heat treatment requires expensive application 

equipment but is cost effective if steam-generating equipment is 

readily available (fig. 5.1.66).

Treating growing media. A very effective way to exclude disease 

organisms from the container nursery is to eliminate them in the 

growing medium. Heat pasteurization or chemical fumigation were 

common greenhouse operations when soil was used as part of the 

growing medium, but these practices are less common now that 

soilless growing media are the standard. According to the 

Container Nursery Survey, only 5% of the container nurseries in 

the Unites States and Canada routinely treat their growing media.

Although most growers assume that artificial growing media such 

as peat-vermiculite mixes are sterile, recent evidence indicates that 

this may not always be true. The inorganic components of most 

mixes such as vermiculite or perlite are initially sterile because of 

their exposure to high temperatures during processing. However, 

the organic components of growing media may sometimes contain 

pathogenic fungi, for example, Fusarium or Pythium spp. in peat 

(see section 5.1.3.1).

Many manufacturers of commercial growing media treat their 

product before packaging, but it would be wise to specify that the 

media be pest-free when purchasing commercial media. Container 

nursery managers should have their growing media assayed for 

pathogenic fungi after mixing or before purchasing commercial 

products. Most nursery pest laboratories or seed processing firms 

can provide pathogen assays at a reasonable cost. Compared to 

the potential losses from infested media, the costs of assays or 

treatment are quite reasonable; Mastalerz (1977) estimates that the 

cost of pasteurization is only 1 to 2% of the total crop production 

costs.

Treating growing media to eliminate pathogenic organisms is 

called sterilization, although pasteurization is a better term 

because no operational media treatment completely eliminates all 

living organisms. Basically, there are two methods for treating 

growing media: heat and chemical fumigants. Excellent 

descriptions of these processes are provided in Baker and 

Roistacher (1957), Bunt (1976), Mastalerz (1977), Ball (1985), and 

Lawson and Horst (1987).
82

Chemical fumigants. A variety of chemical fumigants are available 

for treating growing media, but they vary in their effectiveness 

against different disease organisms (table 5.1.12). All of these 

fumigants are biocides, that is, extremely toxic to all organisms; 

most are also restricted-use pesticides and should be handled with 

extreme caution. McCain (1978) concludes that methyl bromide is 

the most useful fumigant for treating container growing media and 

recommends using a relatively thick (4 mil) plastic tarp to 

completely contain the gas. The main advantage of chemical 

fumigants is their low capital cost and the fact they can be applied 

without expensive application equipment; however, they must be 

applied under a tarp and the treated medium must be ventilated 

for an extended period before planting (table 5.1.12). A good 

discussion of commonly used fumigants in ornamental container 

nurseries is provided in the Ball Redbook (Ball 1985), and McCain 

(1987) presents an excellent discussion on chemical fumigants 

used in ornamental greenhouses in California.

Sterilizing containers and growing area surfaces.

Reusable containers usually contain some residual growing 

medium or pieces of roots that could contain pathogenic fungi. 

Seedling roots grow into the pores of containers with 

rough-textured walls, such as Styroblocks®, and remain after the 

seedling plug has been extracted (fig 5.1.67). Cryptogams 

(liverworts, moss, and algae) also grow on containers and are very 

difficult to remove from reusable containers.

Benches, floors, and other surfaces in the growing area often 

serve as reservoirs for biotic pests, particularly cryptogams and 

weeds. Containers should be treated with steam, washed with 

surface disinfectants, or chemically fumigated between crop 

rotations (fig. 5.1.68). Nelson (1978) lists several disinfectants that 

can be used on containers, benches, and other surfaces in the 

growing area (table 5.1.13). Matthews (1983) mentions use of a 

pestcidal soap for cleaning containers or greenhouse surfaces. 

Northwestern container tree nurseries have recently been testing 

another disinfectant, called sodium metabisulfite, which releases 

sulfur diox ide when mixed with water (Sturrock 1988). McCain 

(1977) also stresses the importance of sanitation in container 

nurseries and lists common disinfectants.

Weeds should be hand-pulled or killed with herbicides between 

crops (see table 5.1.9 for a list of low-risk herbicides).
83
Treating irrigation water. Surface sources of irrigation water can 

contain pathogenic fungi such as Pythium or Phytophthora spp., 

weed seeds, and cryptogam spores. Irrigation water should be 

tested prior to use and treated if a problem exists (see volume four

of this series for more information).

Maintaining physical barriers. Mobile pests such as insects, 

birds, and rodents must be physically excluded from the growing 

area with screening or fencing. Air intake vents can be screened to 

prevent weed seeds and insect pests from entering the growing 

area. Gravel, asphalt, or cement floors prevent weeds from 

becoming established and make cleanup between crops much 

easier.

84

Following hygienic practices. Greenhouse workers should 

clean soil from their shoes before entering the growing area. 

Equipment and tools can be sterilized with steam or surface 

disinfectants between uses, and transfers of plant material 

between different sections of the nursery should be carefully 

monitored.

5.1.7.3 Protection: protecting seedlings from existing

pests and abiotic stress factors

This second phase of a disease management program is needed 

when excluding the biotic pest from the growing area is not 

practical, and for minimizing the adverse effects of abiotic stress 

factors.
85

Managing microorganism populations by using suppressive 

growing media. The concept of managing microorganism 

populations to enhance plant growth is relatively new because it 

has only recently become operationally possible with the advent of 

artificial growing media. In the near future, nursery managers may 

be able to create "suppressive" growing media that are specially 

formulated to resist disease organisms. Suppressive soils can be 

created by introducing beneficial organisms into the growing 

medium or by using growing medium components that suppress 

disease organisms. Linderman (1986) gives an excellent overview 

of the possibilities of this technology.

One of the practical possibilities of microbial management is the 

suppression of pathogenic organisms through the introduction of 

beneficial organisms. Lawson and Horst (1987) state that most 

artificial growing media contain very low populations of beneficial 

microorganisms. Before any organism can be artificially introduced 

into a growing medium, the existing microbial population must be 

reduced by some cultural treatment, such as chemical fumigation 

or heat pasteurization. Heat pasteurization has some advantages 

because some beneficial organisms are not killed, and may 

actually be favored, by heat treating at lower temperatures (Baker 

1970).

Once the growing medium has been fumigated or pasteurized, the 

next step is to introduce the desired microbes into the growing 

medium, by treating either the seed or the medium (Lawson and 

Horst 1987). Among the microorganisms that may have potential 

usefulness are mycorrhizal fungi, fungi that are antagonistic to 

pathogens, rhizobacteria, and free-living microbes such as 

nitrogen-fixing bacteria. The benefits of inoculating seedlings with 

mycorrhizae have been well established, and inoculation 

techniques for container seedlings have been recently developed 

(see chapter 2 in this volume). Some soil-inhabiting fungi (for 

example, Trichoderma spp.) are actively antagonistic to pathogenic 

fungi because they compete with the pathogens for substrate, and 

some are mycoparasites on pathogens, such as Fusarium spp. 

Another possibility is to introduce a nonvirulent isolate of a 

normally pathogenic organism, such as Fusarium oxysporum. 

Growth promoting rhizobacteria, such as Pseudomonas spp.

Modifying the environment. Unfortunately, the environment in a 

container nursery is ideal for many biotic pests, especially fungi, 

which benefit from the warm temperatures and high humidities. 

Container nurseries should be designed to provide the proper 

range of environmental conditions that favor seedling growth, 

without allowing damaging climatic stresses to develop. Alarm 

systems should also be installed in the growing area to alert the 

grower of adverse environmental conditions.

Seedling storage is one of the most crucial periods for 

development of both biotic and abiotic problems. Botrytis storage 

mold thrives in the cold, moist environment of refrigerated storage 

but can be successfully controlled by storing seedlings at slightly 

below freezing temperatures. Container seedlings under sheltered 

storage are subject to winter drying and must be properly protected 

(see section 5.1.6 for more details).

Cultural practices. Many cultural practices can minimize the 

impact of seedling diseases even after they have become 

established in the nursery. The organic components used in 

artificial growing media can affect disease incidence and 

development. Selection of a growing medium that is porous and 

well-drained will reduce root rots, and using a medium that 

contains composted hardwood bark may reduce the impact of 

certain pathogenic fungi (Hoitink 1980). Hoitink and Kuter (1985) 

reported that some sources of "light" peat moss harbor organisms 

that are suppressive to pathogenic fungi, whereas "dark" peat 

sources do not. Lindquist and others (1985) found that populations 

of fungus gnats varied significantly between 20 different types of 

artificial growing media.

Containers that create high seedling growing densities can favor 

diseases such as grey mold because of the high humidites that 

occur in the dense seedling canopies. Cultural practices that create 

one germinant per cavity (use of high germination seed, accurate 

sowing calculations, and precision sowing) should produce more 

vigorous seedlings and may reduce damping-off losses. Most 

foliage diseases, particularly grey mold, can be reduced by any 

cultural practice that minimizes the time that the seedling foliage is 

wet: encouraging air movement, spacing containers, irrigating early 

in the day (McCain 1978); and controlling temperatures and relative 

humidity so that condensation does not occur (Nelson 1978).
Chemical protection. Many of the currently used fungicides are 

protectants that form a chemical barrier between the seedling and 

the pathogen (table 5.1.14). Protectants prevent fungal spores 

from germinating or kill the germinating hyphae before they 

penetrate the host tissue (Smith 1982). Systemic fungicides are 

absorbed into the foliage or roots and are then transported 

throughout the seedling, but only a few protectants fall into this 

category (table 5.1.14). Some fungicides are able to kill pathogenic 

fungi after they have initiated an infection; these chemicals, called 

eradicants, are discussed in section 5.1.7.4. Chemical protection 

against pathogenic fungi must be accomplished by residual 

pesticides that persist on the plant and prevent attack by the 

fungus. An effective protectant fungicide should, according to 

Bohmont (1983):

1. Remain active for a relatively long time.

2. Have good adhesive properties and resist washing-off.

3. Have good spreading properties (usually aided by

addition of wetting agents).

4. Resist breakdown in sunlight.

5. Have a broad-spectrum effectiveness against the target 

fungus to avoid build-up of resistant strains. Smith (1982) 

reports that several pathogenic fungi have developed 

resistance: Botrytis cinerea is now resistant to benomyl (fig. 

5.1.49) and iprodione.

6. Not be overly phytotoxic to the crop seedling.

On an operational basis, protectant pesticides must be applied so 

that they completely cover the seedling foliage, which can be 

difficult in the dense canopies usually found in container tree 

nurseries. This problem can be partially alleviated by mounting a 

bar in front of the spray head to separate the foliage and allow the 

fungicide to penetrate the canopy. Extended protection requires 

that the pesticide be applied at regular intervals to cover new 

foliage and make up for the chemical that has washed off.

and Bacillus spp., enhance plant growth through antagonism of 

pathogenic organisms or through production of growth-regulating 

substances. Free-living microbes can also affect plant growth 

indirectly through effects on other microorganisms in the root zone. 

The most practi cal solution may be a combination of these 

treatments (Linderman 1986).

When the desired organisms have been introduced, they must be 

culturally encouraged by management of the growing medium 

environment. Managing beneficial microorganisms involves an 

understanding of both their ecological requirements and 

interactions with other microbes.

Another way to create a suppressive growing medium is to 

incorporate special components, such as composted tree bark, into 

the medium. Lawson and Horst (1987) state that hardwood bark 

suppressed a wider range of pathogenic fungi than pine bark and 

that beneficial organisms were most abundant in composts older 

than 1 year. Hoitink (1980) reports that species of Phytophthora, 

Pythium, Rhizoctonia, Botrytis, and other pathogenic fungi were 

killed by the heat generated during the composting process. Some 

ornamental growers are now using a 4:1 (v/v) mixture of composted 

bark and peat moss for the organic medium component to create a 

pathogen-suppressive growing medium. Three mechanisms have 

been proposed for this suppressive effect: 1) bark-based media are 

coarser than peat-based media, resulting in improved aeration, 2) 

bark composts support high levels of microorganisms that are 

antagonistic to pathogens, and 3) water extracts from hardwood 

bark seem to have fungicidal properties (Hoitink 1980).

The practical application of microorganism management in the 

growing medium is still some years away, but the concept has 

tremendous potential for container tree nurseries and more 

research in this area is justified.

86
Roguing diseased individuals. Roguing involves prompt removal 

and destruction of the diseased seedling and associated materials, 

such as the growing medium or containers, that may also harbor 

inoculum. This practice will not save the original host seedling but 

will halt secondary spread (fig. 5.1.63). Most fungal diseases, 

including grey mold, sirococcus blight, and fusarium rot, are able to 

spread from seedling to seedling, particularly in the container 

nursery environment. Some insects, such as aphids, can build up 

high population levels on the original host and spread to adjacent 

seedlings. Effective roguing requires prompt disease identification, 

so growers need to make frequent surveys of the growing area and 

train workers to identify and report diseased seedlings.

87

5.1.7.4 Eradication: eliminating pests after disease

development

This phase of disease management is by far the most difficult to 

achieve because once a biotic pest is established in the growing 

area, it can be almost impossible to eradicate. Because most 

pests exist in close association with their hosts, it is almost 

impossible to kill them without damaging the host seedling. In 

addition, once the disease has progressed past a certain point, 

the host seedling has usually suffered enough physical injury or 

growth loss so that it is not saleable.
Commercial fungicides differ in their effectiveness against fungal 

pathogens. Most chemicals (e.g., meta laxyl) are effective against 

one particular group of path ogens such as the water mold fungi 

(Pythium and Phytophthora) but provide no control of other fungal 

pests. Some fungicide formulations contain a mixture of two 

chemicals (e.g., Banrot®) and are used on a wide variety of fungal 

pathogens (table 5.1.15). A good discussion of common 

fungicides, their application rates, target pathogens, cost and other 

useful information can be found in the Ball Red Book (Ball 1985); 

McCain (1987), Smith (1982), and Hallett (1984) also present good 

discussions on fungicide use in container nurseries. Nursery 

managers should carefully check pesticide labels and consult with 

pesticide specialists to determine which pesticides should be 

applied against specific fungi.

Although most fungus control chemicals are advertised as 

fungicides (which kill fungi), many are actually fungistats (which 

inhibit the growth of the fungus). Baker (1985) states that 

fungicidal drenches such as PCNB or fenaminosulf will inhibit but 

not eliminate Rhizoctonia, Pythium, and Phytophthora and that 

frequent applications may be necessary to be effective. He also 

points out that fungistats suppress disease symptoms and merely 

postpone the problem until the seedling is transplanted or sold, at 

which time the fungus may become active again.

Chemotherapy is easier with insecticides and animal poisons that 

can be applied around the crop seedlings and eliminate the pest 

without phytotoxic effects on the host. Commonly used insecticides 

in container tree nurseries are listed in table 5.1.16. Hallett (1984) 

lists pesticides that are in operational use in container tree 

nurseries in the Maritime Provinces.

Disinfecting the growing area. Pest organisms should be 

eradicated from the growing area as soon as possible if the 

control treatments will not harm the crop seedlings. The best time 

to completely disinfect the growing area is between crops. (See 

section 5.1.7.2 for more discussion on this topic.)

Removing weeds. Besides being unsightly and spreading seeds 

within the growing area, weeds should be immediately controlled 

because they can serve as reservoirs for nursery pests such as 

aphids, spider mites, Fusarium spp., and Botrytis cinerea.

Chemotherapy: curing diseased seedlings with chemicals.

Chemotherapy involves applying a pesti cide to a diseased seedling 

to kill the pest without damaging the host. In general, this is much 

more difficult for pests such as fungi, weeds, and cryptogams 

(liverworts, moss, and algae) than for insects because the former 

are plants living in close association with the host plant; it is difficult 

to kill one plant without injuring the other. Also, most fungal 

pathogens exist partially or completely within the host plant tissue 

and are therefore difficult to reach with fungicides (Bohmont 1983).

Eradicants are fungicides that destroy pathogenic fungi after an 

infection has occurred. Protectant fungicides (see previous section) 

are ineffective once the pathogen has penetrated the host seedling 

(Smith 1982). Some eradicant fungicides are systemics because

they are absorbed into the leaf tissue or roots and are 

physiologically transported throughout the plant, where they can 

attack the pest. Few commercial fungicides are true systemics, 

although benomyl and triadimefon are reported to have some 

systemic activity. Fungicides commonly used in container tree 

nurseries are listed in table 5.1.14.

88
5.1.7.5 Resistance: managing pests through genetics

The final phase of the disease management process involves the 

genetic selection of seedlings for immunity, resistance, or 

tolerance to disease. A plant is considered immune when it 

remains free of disease symptoms in the presence of a disease 

agent and predisposing environmental conditions. Resistance 

involves the inherent ability of a plant to restrict disease 

development after a pathogen has initiated an infection. 

Tolerance concerns the ability of a plant to remain relatively 

healthy in spite of an established disease (Smith 1970).

Breeding for resistance to specific pathogens is an established 

practice in horticulture and has been attempted successfully for 

some forest diseases such as white pine blister rust. Tree seedling 

nurseries in northern Idaho are growing western white pine 

seedlings from seed collected from parent trees that have been 

shown to be resistant to blister rust. It is probably not practical to 

genetically select seedlings to resist nursery diseases because 

many of these diseases are unique to the nursery environment. 

Many nursery diseases could be avoided, however, by selecting 

for rapid germination and early seedling growth.

89

This listing does not constitute a recommendation: these specific 

products were reported in the Container Nursery Survey and may not be 

currently registered. Other products may be available as well. Consult with 

a pesticide specialist and check labels for information on registered uses 

and application rates.

Source: Container Nursery Survey.
Seed treatments. Pesticides such as captan and thiram are 

applied to seeds to control fungal diseases such as damping-off. 

The chemicals are usually applied to larger conifer seeds as a 

dust, or are pelleted on smaller seeds using a methyl cellulose or 

latex sticker. The amount of fungicide varies from 113 to 226 g (4 

to 8 ounces) of 50% chemical per 0.45 kg (1 pound) of seed; 

approximately 57 g (2 ounces) of sticker is required to apply 113 g 

(4 ounces) of fungicide to 0.45 kg (1 pound) of conifer seed 

(Hanson and others 1961). The literature contains conflicting 

evidence on the benefits of chemical seed treatments, and 

because many of these pesticides have proven phytotoxic, growers 

should test any potential seed treatment (see section 5.1.5.3).

Growing medium drenches. The objective of a pesticide drench 

is to completely saturate the growing medium with a fungicide or 

insecticide, usually applying the chemical through the irrigation 

system. Drenches applied through the irrigation system require a 

substantial amount of pesticide because the solution is usually 

applied until some degree of leaching occurs. Because of the large 

amount of pesticide used, drenches are expensive and may be 

environmentally hazardous because of the surplus runoff. 

Fungicides such as fenaminosulf, benomyl, or PCNB are often 

applied as drenches for control of root-rotting fungi. Insecticide 

drenches are often used for control of root insects, including 

weevils and the European crane fly. Because of the heavy 

amounts of irrigation that are normally applied in container 

nurseries, drenches must be repeated at 6- to 8-week intervals to 

maintain the proper pesticide concentration in the growing medium 

solution (Nelson 1978).

Liquid sprays. Spraying is the most common pesticide application 

method used in container nurseries and numerous pesticides are 

formulated for mixing with water and spraying onto the plant. 

Emulsifiable concentrates (EC) are oily preparations with an 

emulsifying agent that keeps the liquid chemical suspended in the 

water solution. Wettable powders (WP) contain very small particles 

that must be suspended in the aqueous solution with constant 

agitation. WP formulations generally tend to be less phytotoxic 

than EC formulations of the same pesticide (Nelson 1978).

90

5.1.8 Pesticide Use

Pesticides are an accepted part of nursery culture, but the nursery 

manager should be aware of proper application, storage, and 

disposal of these chemicals, which when used inappropriately can 

be hazardous to workers, the public, and the environment.

5.1.8.1 Legal implications

Pesticides are regulated by several federal and state laws. In the 

United States, the major federal legislation is the Federal 

Insecticide, Fungicide, and Rodenticide Act (FIFRA) as amended, 

which is administered by the United States Environmental 

Protection Agency. Because pesticide laws are constantly 

changing, nursery managers should check the label for current 

application information and consult with pesticide specialists to 

make sure that they are operating within the current laws, policies, 

and regulations.

5.1.8.2 Application techniques

Pesticides can be applied in many different ways: dusts, aerosols 

("smokes"), mists, granules, drenches, dips, baits, or liquid sprays. 

The choice of application method will depend on the type of 

pesticide, the characteristics of the target pest, the susceptibility of 

the crop seedlings to phytotoxicity, the environmental conditions, 

the equipment available, and the legal restrictions (Hanan and 

others 1978, Nelson 1978). In container tree nurseries, pesticides 

are applied as seed treatments before the growing season; as 

growing medium drenches, liquid sprays, or baits during the 

season; and as baits or aerosols during the seedling storage 

period.

New pesticide application technology is being studied in container 

nurseries (Lindquist 1986). Foam applicators add a foaming agent 

to the pesticide-water mixture to provide better coverage and less 

spray drift; they may also use less pesticide. Low-volume 

applications include thermal pulse-jet applicators, rotary atomizers, 

and electrostatic applicators. The latter devices place an electric 

charge on spray droplets as they are emitted, which potentially 

gives better coverage of lower leaf surfaces and less drift. 

Container nursery managers should stay abreast of the latest 

pesticide application technology.
Because some seedling foliage is covered with a waxy cuticle that 

repels water, surfactants (also called wetting agents or spreaders) 

are often added to the pesti cide solution to lower the surface 

tension of the liquid, thus increasing foliar coverage by the 

pesticide. Check the label because some pesticides already 

contain a surfactant; for example, there are EC formulations in 

which the emulsifying agent is itself a surfactant (Nelson 1978). 

Surfactants are commercially available, but because some are 

phytotoxic, they should all be tested for possible phytotoxicity 

before operational use. An excellent discussion of the properties of 

surfactants and a comparison of commercial brands is provided by 

Bohmont (1983).

Liquid sprays can be applied through the irrigation system or with 

hand or mobile tank sprayers. WP formulations should be premixed 

in a bucket to ensure that no solids plug the injector pump or spray 

nozzles (Nelson 1978). Pesticide sprays should be applied at high 

pressure to produce small droplets and force the chemical down 

into the seedling canopy. Some nurseries attach a bar in front of 

the spray boom to separate the seedlings and allow the pesticide to 

reach the lower shoot. Pesticides are usually applied late in the 

day, after a thorough irrigation, to allow the chemical maximum 

contact with the foliage. Many pesticide sprays are applied as 

protectants and so should be reapplied at regular intervals to 

ensure that new growth is covered and to replace chemicals that 

are washed off by irrigation. A good discussion of pesticide 

application equipment and methods is presented by Bohmont 

(1983).

Baits and aerosols. Pesticide baits are sometimes used to control 

rodents during the seed germination period and during sheltered 

storage. All poison baits should be placed in locations where they 

will be encountered by the pests but not by pets and other 

nontarget animals.

Aerosols are sometimes used to control botrytis storage mold in 

sheltered storage. Chlorothalonil is available in a formulation called 

Exotherm Termil® , which is used in enclosed storage buildings 

(Nelson 1978). Insecticidal aerosols can only be used in closed 

greenhouses.

5.1.8.3 Safe use of pesticides

All pesticides are poisons and should therefore be handled with 

caution; nursery managers must realize that any pesticide has both 

a benefit and an inherent danger (Powell 1984). Fungicides and 

herbicides are generally less hazardous to workers than 

insecticides. The relative toxicity rating of a pesticide is called the 

LD50, which is the milligrams of the chemical required per kilogram 

of body weight to kill 50% of a test population. Pesticides are given 

three different ratings to cover the three major routes of accidental 

contact: oral, dermal, and inhalation. Hanan and others (1978) and 

Nelson (1978) list the LD50 ratings for the pesticides commonly 

used in container nurseries and give the following general rules for 

handling pesticides:

1. Purchase and use proper clothing and protective gear. Despite 

the discomfort in hot, humid greenhouses, insist that all 

employees wear protective clothing when handling, mixing, 

and applying pesticides. Employees should never work alone 

when applying pesticides.

2. Read the label and be sure you understand the information 

before using the chemical. The label contains information on 

toxicity, brand name, common and chemical names, 

ingredients, uses, and application instructions and rates. 

Safety precautions and antidotes will also be listed.

3. Choose a pesticide formulation that lowers worker exposure. 

The pesticide formulation affects ease of handling and worker 

exposure. Powell (1984) states that wettable powder 

formulations are particularly hazardous because of the fine dust 

that is produced during handling, and liquid formulations can 

splash. Flowable granules, water-dispersible granules, and dry 

applied granules or pellets present a lower exposure hazard.

4. Be particularly careful when mixing pesticides. Powell (1984) 

reports that over 90% of pesticide exposure cases are 

associated with the preparation and initial mixing operations. 

Protective clothing, including respirators, needs to be used 

during mixing operation as well as during application.

91
5.1.8.4 Pesticide storage

Pesticides should be stored in well-ventilated, locked closets or 

rooms that can be maintained between 4 to 32 °C (39 to 90 °F). 

The door should be clearly marked with a sign indicating that the 

room contains hazardous pesticides and it is a good idea to keep 

an up-to-date list of the contents posted as well (Nelson 1978). A 

fire extinguisher and a telephone with a list of emergency phone 

numbers should be readily available. Many government 

organizations have specific regulations for pesticide storage. Local 

fire departments should be notified of pesticide storage locations 

and potential hazards.

Pesticides should be stored in their original, labeled containers, 

never (even temporarily) in unmarked containers. Pesticide 

containers should be checked periodically for leakage. An 

up-to-date inventory should be kept on all stored chemicals that 

lists date of acquisition, time and amount of use, and current 

balance (Singer 1980).

All chemicals, including pesticides, have a recommended shelf life. 

Information provided by the manufacturers is helpful but is usually 

based on optimal storage conditions. The principal factors that can 

reduce shelf life are temperature, moisture, and light. Some 

pesticides can be ruined by freezing and others degrade when 

temperatures exceed 100 °C (212 °F). All pesticides should be 

kept in waterproof containers and should be stored on pallets to 

keep them from getting wet. Peck (1984) presents an excellent 

discussion on aspects of pesticide shelf life and storage, and lists 

the advertised shelf lives for many common pesticides that are 

used in forest tree nurseries.

5.1.8.5 Disposal of hazardous chemicals

Disposal of empty pesticide containers and surplus pesticides is 

regulated by law, and all waste pesticides should be considered 

hazardous to the public and the environment. Contact local 

authorities including extension agents or regional Environmental 

Protection Agency officials to determine the proper disposal 

method. Detailed instructions are provided by Bohmont (1983) and 

Singer (1980).

92

5. Use the proper equipment when applying pesticides. Spray 

nozzles should be selected for the specific purpose. For 

example, to apply a protective fungicide to seedling foliage, 

select a nozzle that produces a fine spray that covers the 

foliage, not one that mists or produces large droplets that 

splash.

6. Never smoke, drink, or eat in an area where pesticides are 

stored, mixed, or applied. Wash thoroughly after handling any 

chemical.

7. Be aware of the major symptoms of pesticide poisoning and 

the acceptable antidotes. Pesticide applicators should be 

trained in first aid, and emergency numbers including poison 

control centers should be posted close to the telephone. 

Report any incidents and adverse health effects immediately 

to the local safety officer.

The pesticide program should be assigned to one or two specially 

trained and certified applicators who are responsible employees 

(Nelson 1978); this is important, not just for safety, but for accurate 

recordkeeping. Although only specially trained personnel should be 

allowed to handle pesticides, all employees should be trained in 

pesticide safety and what to do in an emergency. Excellent reviews 

of the subject are provided by Singer (1980) and Bohmont (1983).
5.1.9 Conclusions and Recommendations

Diseases and pests are a source of continuing concern in container 

tree nursery management. Nursery managers must be able to 

identify major diseases and pests quickly and accurately, before 

they can inflict significant damage to the seedling crop. Although 

biological pests are always present, abiotic stress agents often 

cause more loss in container tree nurseries. Disease and pest 

problems are often more cata strophic in container nurseries 

because the ideal seedling growing environment also favors many 

pest organisms, and actively growing, succulent seedlings are 

more susceptible to abiotic stresses.

Disease diagnosis requires a certain degree of experience and 

training, and nursery workers should be trained to spot new 

diseases or pests quickly, as well as incidents of abiotic injury. 

Early detection should be encouraged through regular inspections 

of the crop, because most nursery diseases and pests become 

more difficult to eradicate once they have become established. All 

nursery workers should receive annual training in disease and pest 

identification and control; laborers who are in the growing area 

daily have the best chance to spot potential problems before they 

can intensify or spread. The damage keys and color photographs in 

this publication can be used to help identify common nursery 

diseases and pests. Although nursery managers should make 

tentative diagnoses of disease and pest problems, they should 

confirm their conclusions with a trained nursery pest specialist.

Nursery managers should adopt a philosophy of disease and pest 

management instead of control. By correctly identifying the disease 

or pest and understanding their life cycles, growers can use a 

variety of cultural and chemical measures to manage the problem 

and limit their losses. Chemical pesticides should be just one 

option in a comprehensive pest management program that 

integrates both chemical and cultural controls. This program should 

include the four necessary aspects of a complete pest 

management program: exclusion--preventing pest entry; 

protection--protecting seedlings from existing pests or abiotic 

stress factors; eradication-eliminating pests after disease establishment; and resistance-managing diseases and pests through 

genetics. Because container nurseries are initially free from 

disease and pest organisms, managers should put most of their 

efforts into preventing the damaging agents from gaining entry into 

the nursery. Environmental control systems should be regularly 

monitored to insure against malfunction, and cultural practices 

periodically reviewed and adjusted to minimize the development of 

harmful abiotic stresses.

93
Bloomberg, W.J. 1981. Disease caused by Fusariurn in forest 

nurseries. In: Nelson, P.E.; Tousson, T.A.; Cook, R.J. Fusarium: 

diseases, biology and taxonomy. University Park, PA: The 

Pennsylvania State University Press: 178-187.

Bohmont, B.L. 1983. The new pesticide user's guide. Reston, 

VA: Reston Publishing Co. 452 p.

Bottrell, D.R. 1979. Integrated pest management. Washington, 

DC: Council on Environmental Quality. 120 p.

Boyce, J.S. 1961. Forest pathology. New York: 

McGraw-Hill Book Co. 572 p.

Bunt, A.C. 1976. Modern potting composts: a manual on the 

preparation and use of growing media for pot plants. University 

Park, PA: The Pennsylvania State University Press. 277 p.

Byers, R.E.; Carbaugh, D.H. 1987. Efficacy of rodenticides for 

control of orchard voles. Journal of the American Society for 

Horticultural Science 112(2): 267-272.

Byford, J.L. 1987. Combat wildlife damage with commonsense 

control methods. American Nurseryman 165(8): 91-94, 96, 98, 

100.

Campbell, T.E. 1982. The effects of presoaking longleaf pine 

seeds in sterilants on direct seeding. Tree Planters' Notes 

33(1): 8-11.

Capizzi, J.; Green, J. 1984. Root weevil update. 

Ornamentals Northwest Newsletter 8(2): 22-23.

Carlson, L.W. 1983. Guidelines for rearing containerized conifer 

seedlings in the Prairie provinces. Inf. Rep. NOR-X-214E. 

Edmonton, AB: Environment Canada, Canadian Forest 

Service, Northern Forest Research Centre. 64 p.

Carlson, L.W.; Nairn, L.D. 1975. Pentachlorophenol and captan 

effects on containerized red and jack pine seedlings. Tree 

Planters' Notes 26(1): 32-34.

Carmean, D. 1986. Identifying common root weevils. 

Ornamentals Northwest Newsletter 10(1): 8-9.

Carter, A. 1988. Personal communication. Surrey, BC.

Cline, M.N. 1985. Fungal diseases in roots: their symptoms 

and treatments. American Nurseryman 161(11): 41-42, 

44-45.

Coley-Smith, J.R.; Verhoeff, K.; Jarvis, W.R. 1980. The biology of 

Botrytis. London: Academic Press. 318 p.

Cooley, S.J. 1981. Fungicide tolerance of Botrytis cinerea isolates 

from conifer seedlings. Portland, OR: USDA Forest Service, 

Pacific Northwest Region; 13p.

Couteaudier Y.; Alabouvette, C. 1981. Fusarium wilt in soilless 

culture. Acta Horticulturae 126: 153-158.

5.1.10 References

Ahrens, J.F. 1985. Herbicides for forest nurseries. In: Lanteigne, 

L.J.; Hallett, R.D., eds. Canadian Forestry Service-Maritimes 

Workshop Proc. 4. Proceedings, Canadian Forest Tree Nursery 

Weed Control Workshop; 1985 July 15-16; Fredericton, NB. 

Fredericton, NB: Canadian Forestry Service: 3-4.

Andersen, A.M.; Leach, C.M. 1961. Testing seeds for seedborne 

organisms. In: Seeds, the yearbook of agriculture. 

Washington, DC: U.S. Department of Agriculture. 591 p.

Anonymous. 1983. European crane fly. Ornamentals Northwest 

Newsletter 7(1):19-21; 1983.

Anonymous. 1987. Clean greenhouses make more green. 

Greenhouse Manager 5(10): 44; 1987.

Baker, J. 1986. Insects found on yellow sticky cards. North 

Carolina Flower Growers Bulletin 30(1): 10-13.

Baker, K.F. 1985. The obligation of the plant propagator. The 

International Plant Propagators' Society Combined Proceedings 

(1984) 34: 195-203; 1985.

Baker, K.F. 1970. Selective killing of soil microorganisms by 

aerated stem. In: Toussoun, T.A.; Bega, R.V.; Nelson, P.E., 

eds. Root diseases and soil-borne pathogens. Berkeley, CA: 

University of California Press: 234-239.

Baker, K.F. 1957. Damping-off and related diseases. In: Baker, 

K.F. The U.C. system for producing healthy container-grown 

plants. Calif. Agric. Exper. Sta. Ext. Serv. Manual 23. 

Parramatta, Australia: Australian Nurserymen's Association 

Ltd.: 34-51.

Baker, K.F.; Roistacher, C.N. 1957. Heat treatment of soil. In: 

Baker, K.F. The U.C. system for producing healthy 

container-grown plants. Calif. Agric. Exp. Sta. Ext. Serv. 

Manual 23. Parramatta, Australia: Australian Nurserymen's 

Association Ltd.: 123-137.

Ball, V. 1985. Ball red book-greenhouse growing. 14th ed. 

Reston, VA: Reston Publishing Co. 720 p.

Barnett, J.P. 1987. Personal communication. USDA Forest 

Service, Pineville, LA.

Barnett, J.P. 1976. Sterilizing southern pine seeds with hydrogen 

peroxide. Tree Planters' Notes 27(3): 17-19.

Belcher, E.W., Jr.; Waldrip, B.T., Jr. 1972. Effect of thiram on 

seed mold and germination of slash pine seed. Proceedings of

the Association of Official Seed Analysts 62: 91-93.

Blanchard, R.O.; Tattar, T.A. 1981. Field and laboratory guide to 

tree pathology. New York: Academic Press. 285 p.

94
Hallett, R.D. 1984. Forest nursery practice in the Maritimes. In: 

Hallett, R.D.; Cameron, M.D.; Murray, T.S., eds. Proceedings, 

Reforestation in the Maritimes, 1984 symposium; 1984 April 

3-4; Moncton, NB. Fredericton, NB: Canadian Forestry Service, 

Maritimes Forest Research Centre. 188 p.

Hallett, R.D. 1982. Personal communication. Canadian 

Forestry Service, Fredericton, NB.

Hallett, R.D.; Burns, T.W. 1983. Postemergence application of 

herbicides to control weeds in containerized tree seedling 

crops. Tech. Note 77. Fredericton, NB; Environment Canada, 

Canadian Forestry Service, Maritimes Forest Research Centre. 

8 p.

Hamel, D.R. 1983. Forest management chemicals: a guide to 

use when considering pesticides for forest management. 

Agric. Handbk. 585. Washington, DC: U.S. Department of 

Agriculture. 645 p.

Hamlen, R.A.; Mead, F.W. 1979. Fungus gnat larval control in 

greenhouse plant production. Journal of Economic 

Entomology 72: 269-271.

Hanan, J.J.; Holley, W.D.; Goldsberry, K.L. 1978. 

Greenhouse management. New York: SpringerVerlag. 

530 p.

Handreck, K.A.; Black, N.D. 1984. Growing media for ornamental 

plants and turf. Kensington, NSW, Australia: New South Wales 

University Press. 401 p.

Hanson, E.W.; Hansing, E.D.; Schroeder, W.T. 1961. Seed 

treatments for control of disease. In: Seeds: the yearbook of 

agriculture. Washington, DC: U.S. Department of Agriculture: 

272-280.

Hoitink, H.A.J. 1980. Composted bark, a lightweight growth 

medium with fungicidal properties. Plant Disease 64: 142-147.

Hoitink, H.A.J.; Kuter, G.A. 1985. Effect of composts in container 

media on diseases caused by soilborne plant pathogens. Acta 

Horticulturae 172: 191-198.

Holopainen, J.K. 1988. Growth and visible responses of Scots 

pine (Pinus sylvestris L.) seedlings to simu lated summer frost. 

European Journal of Forest Pathology 18(2): 85-92.

Holopainen, J.K.; Soikkeli, S. 1984. Occurrence of Cinara pini 

(Homoptera, Lachnidae) in Scots pine (Pinus sylvestris) 

seedlings with disturbed growth. Annales Entomologici 

Fennici 50: 108-110.

Hopkins, J.C.; Lock, W.; Funk, A. 1985. Colletotrichum acutaturn, a 

new pathogen on western hemlock seedlings in British 

Columbia. Canadian Plant Disease Survey 65(1): 11-13.

95

Coyier, D.L. 1978. Pathogens associated with peat moss used for 

propagation. International Plant Propagators' Society Combined 

Proceedings 28: 70-72.

Croghan, C.F. 1983. Forest pest management nursery report. 

Field Note 84-1. St. Paul MN: USDA Forest Service, 

Northeastern Area, State and Private Forestry. 8 p.

Davidson, J.A.; Cornell, C.F.; Zastrow, M.E.; Alban, D.C. 1988. 

Making the pilot fly-Maryland researchers implement a trial 

IPM program in eight nurseries. American Nurseryman 

167(10): 51-53, 55-58, 60.

Denne, M.P.; Atkinson, L.D. 1973. A phytotoxic effect of captan on 

the growth of conifer seedlings. Forestry 46(1): 49-53.

Dennis, J. 1988. Cylindrocarpon study-1988. Seed and Seedling 

Extension Topics 1(1): 5. [Victoria, BC: British Columbia 

Ministry of Forests]

Epners, Z. 1964. A new psychrophilic fungus causing germination 

failure of conifer seeds. Canadian Journal of Botany 42(12): 

1589-1604.

Fuller, L.R.; Hildebrand, D.M. 1985. Effects of cold stratification 

and hydrogen peroxide treatments on seeds of three Rocky 

Mountain conifer species. Tech. Rep. R2-32. Lakewood, CO: 

USDA Forest Service, Rocky Mountain Region, Timber, Forest 

Pest and Cooperative Forestry Management. 12 p.

Gillman, L.S.; James, R.L. 1980. Fungicidal tolerance of Botrytis 

within Colorado greenhouses. Tree Planters' Notes 31(1): 

25-28.

G i I reath, P. R.; G i I reath, J. P. 1986. Response of 17 species of 

container-grown woody landscape and foliage plants to four 

postemergence herbicides. Journal of Environmental 

Horticulture 4(2): 52-55.

Glerum, C. 1985. Frost hardiness of coniferous seedlings: 

principles and applications. In: Evaluating seedling quality: 

principles, procedures, and predictive abilities of major tests: 

Proceedings of a workshop held at Oregon State University; 

1984 October 16-18; Corvallis, OR: Forest Research 

Laboratory: 107-123.

Graham, J.H.; Linderman, R.G. 1983. Pathogenic seedborne 

Fusarium oxysporum from Douglas-fir. Plant Disease 67: 

323-325.

Haglund, W.A.; Russell, K.W.; Holland, R.C. 1981. Moss 

control in container grown conifer seedlings. Tree Planters' 

Notes 32(3): 27-29.
Jobidon, R.; Thibault, J.R. 1980. Optimal surface sterilization of 

Alnus crispa seeds with merthiolate. Forest Science 26(3): 

483-484.

Johnson, N.E. 1965. Reduced growth associated with infestations 

of Douglas-fir seedlings by Cinara species (Homoptera: 

Aphidae). Canadian Entomologist 97: 113-119.

Kalb, D.W.; Millar, R.I. 1986. Dispersal of Verticillium albo-atrum 

by the fungus gnat (Bradysia impatiens). Plant Disease 70(8): 

752-753.

Kennedy, M.K.; Helgesen, R.G. 1973. Distinguishing 

characteristics of two flies common to greenhouse crop 

production. New York State Flower Industries Bulletin 40: 

4-5.

Kim, S.H.; Forer, L.B.; Longenecker, J.L. 1975. Recovery of plant 

pathogens from commercial peat products. Proceedings of the 

American Phytopathological Society 2: 124.

Kliejunas, J. 1985. Effect of selected chemicals on mold 

development during conifer seed stratification and on 

subsequent germination. For. Pest Mgmt. Rep. 85-35. San 

Francisco, CA: USDA Forest Service, Pacific Southwest 

Region. 7 p.

Kozlowski, T.T. 1986. Effects on seedling development of direct 

contact of Pinus resinosa seeds or young seedlings with 

captan. European Journal Forest Pathology 16(2): 87-90.

Krugman, S.L.; Stein, W.I.; Schmitt, D.M. 1974. Seed biology. In: 

Schopmeyer, C.S., ed. Seeds of woody plants in the United 

States. Agric. Handbk. 450. Washington, DC: U.S. 

Department of Agriculture: 5-40.

Kuhlman, E.G.; Smith, R.S., Jr. 1975. Phytophthora root rot, 

Phytophthora cinnamoni Rands. In: Peterson, G.W.; Smith, 

R.S. Jr., eds. Forest Nursery Diseases in the United States. 

Agric. Handbk. 470. Washington, DC: U.S. Department of 

Agriculture: 17-18.

Lambe, R.C.; Lapp, N.A.; Hadden, C.; Jones, R.K. 1982. 

Fumigants and nematicides. In: Jones, R.K.; Lambe, R.C., 

eds. Diseases of woody ornamental plants and their control 

in nurseries. Publ. 286. Raleigh, NC: North Carolina 

Extension Service: 107-109.

Landis, T.D. 1984. The critical role of environment in nursery 

pathology. Dubreuil, S. H., comp. 31st Western International 

Forest Disease Work Conference Proceedings; 1983 August 

22-26; Coeur d' Alene, ID. Missoula, MT: USDA Forest Service, 

Cooperative Forestry and Pest Management: 27-31.

James, R.L. 1988. Diseases of conifer seedlings associated 

with Cylindrocarpon species: a review. Nursery Dis. Notes 

76. Coeur d' Alene, ID: USDA Forest Service, Forest Pest 

Management. 14 p.

James, R.I. 1987. Personal communication. USDA Forest Service, 

Missoula, MT.

James, R.L. 1986. Diseases of conifer seedlings caused by 

seed-borne Fusarium species. In: Shearer, R.C., comp. 

Proceedings, conifer tree seed in the Inland Mountain West; 

1985 August 5-6; Missoula, MT. Gen. Tech. Rep. INT-203. 

Ogden, UT: USDA Forest Service, Intermountain Research 

Station: 267-271.

James, R.L. 1985a. Studies of Fusarium associated with 

containerized conifer seedling diseases: 2. Diseases of western 

larch, Douglas-fir, grand fir, subalpine fir, and ponderosa pine 

seedlings at the USDA Forest Service Nursery, Coeur d' Alene, 

Idaho. For. Pest Mgmt. Rep. 85-12. Missoula, MT: USDA 

Forest Service, Northern Region; 1985. 7 p.

James, R.L. 1985b. Pathogenic Fusarium on spruce seed from the 

Towner Nursery, North Dakota. For. Pest Mgmt. Rep. 85-23. 

Missoula, MT: USDA Forest Service, Northern Region. 9 p.

James, R.L. 1984. Diseases of containerized conifer seedlings. 

Dubreuil, S.H. comp. 31st Western International Forest Disease 

Work Conference Proceedings; 1983 August 22-26; Coeur d' 

Alene, ID. Missoula, MT: USDA Forest Service, Cooperative 

Forestry and Pest Management: 17-23.

James, R.L. 1983. Fusarium root disease of containerized 

seedlings at the Montana State Nursery, Missoula. Missoula, 

MT: USDA Forest Service, Cooperative Forestry and Pest 

Management. 9 p.

James, R.L.; Genz, D. 1981. Ponderosa pine seed treatments: 

effects on seed germination and disease incidence. For. Pest 

Mgmt. Rep. 81-16. Missoula, MT: USDA Forest Service, 

Northern Region. 13 p.

James, R.L.; Gilligan, C.J. 1985. Containerized Engelmann 

spruce seedling diseases at the USDA Forest Service Nursery, 

Coeur d' Alene, Idaho. For. Pest Mgmt. Rep. 85-17. Missoula, 

MT: USDA Forest Service, Northern Region. 15 p.

James, R.L.; Gilligan, C.J. 1984. Studies of Fusarium associated 

with containerized conifer seedling diseases: pathogenicity 

tests of isolates from the Alpine Nursery, Kalispell, MT. For. 

Pest Mgmt. Rep. 84-14. Missoula, MT: USDA Forest Service, 

Northern Region. 29 p.

James, R.L.; Woo, J.Y. 1984. Fungicide trial to control botrytis 

blight at nurseries in Idaho and Montana. Tree Planters' Notes 

35(4): 16-19.

96
Mead, F.W. 1978. Darkwinged fungus gnats, 8radysia spp., in 

Florida greenhouses (Diptera: Sciaridae). Entomol. Circ. 186. 

Gainesville, FL: Florida Department of Agriculture and 

Consumer Service, Division of Plant Industry. 4 p.

Metcalf, C.L.; Flint, W.P. 1962. Destructive and useful insects, 

their habits and control. New York: McGrawHill Book Co. 1,087 

p.

Mitchell, L.A. 1986. Monoclonal antibodies recognizing the 

seed-borne fungal pathogen Sirococcus strobilinus. In: 

Shearer, R.C., comp. Proceedings, Conifer Tree Seed in the 

Inland Mountain West; 1985 August 5-6; Missoula, MT. Gen. 

Tech. Rep. INT-203. Ogden, UT: USDA Forest Service, Intermountain Research Station: 272-278.

Mittal, R.K.; Singh, P.; Wang, B.S.P. 1978. Botrytis: a hazard to 

reforestation. European Journal of Forest Pathology 17: 

369-384.

Moody, E.H., Sr. 1984. Sanitation: a deliberate, essential exercise 

in plant disease control. Proceedings of the International Plant 

Propagators' Society 33: 608-613.

Moody, E.H., Sr.; Smith, G.E. 1982. Sanitation: plant health from 

start to finish. In: Jones, R.K.; Lambe, R.C. eds. Diseases of 

woody ornamental plants and their control in nurseries. Publ. 

286. Raleigh, NC: North Carolina Extension Service: 84-100.

Nelson, P.V. 1978. Greenhouse operation and management. 

Reston, VA: Reston Publishing Co. 518 p.

Overhulser, D.L.; Morgan, P.D.; Miller, R. 1986. Control and impact 

of Lygus damage on 1-0 Douglas-fir seedlings. In: Landis, T.D. 

Proceedings: Western Forest Nursery Council and 

Intermountain Nursery Association, combined meeting; 1986 

August 12-15; Tumwater, WA. Gen. Tech. Rep. RM-137. Fort 

Collins, CO: USDA Forest Service, Rocky Mountain Forest and 

Range Experiment Station: 153-157.

Palmer, M; Nicholls, T. 1981. How to identify and control cutworm 

damage on conifer seedlings. Leaflet HT-51. St. Paul, MN: 

USDA Forest Service, North Central Forest Experiment Station.

Parrella, M.P. 1988. Whiteflies-there's not just one, but several, 

culprit species attacking world crops. Greenhouse Manager 

6(11): 80-81, 83-88.

Parrella, M.P. 1987. Yellow sticky cards reveal pest problems. 

Greenhouse Manager 5(11): 169-170, 172.

Pawuk, W.H. 1983. Fungicide control of algae in containers. Tree 

Planters' Notes 34(4): 5-7.

97

Landis, T.D. 1976. Fusarium root disease of containerized tree 

seedlings. For. Insect Dis. Mgmt. Bio. Eval. R2-76-16. Lakewood, 

CO: USDA Forest Service, Rocky Mountain Region. 7 p.

Lawson, R.H.; Horst, R.K. 1987. Sterilization controls soil-borne 

diseases. Greenhouse Manager 5(10): 97-100.

Levitt, J. 1980. Responses of plants to environmental stresses. 

I. Chilling, freezing, and high temperature stresses. New 

York: Academic Press. 497 p.

Lieffers, V.J.; Rothwell, R.L. 1986. Effects of depth of water table 

and substrate temperature on root and top growth of Picea 

mariana and Larix laricina seedlings. Canadian journal of 

Forest Research 16(6): 1201-1206.

Linderman, R.G. 1986. Managing rhizosphere microorganisms in 

the production of horticultural crops. HortScience 21(6): 

1299-1302.

Lindquist, R.K. 1986. Efficient application of pesticides studied. 

Greenhouse Manager 5(6): 118, 120.

Lindquist, R.K.; Faber, W.R.; Casey, M.L. 1985. Effect of various 

soilless root media and insecticides on fungus gnats. 

HortScience 20(3): 358-360.

Mastalerz, J.W. 1977. The greenhouse environment. New York: 

John Wiley & Sons. 629 p.

Matthews, R.G. 1987. Personal communication.

Matthews, R.G. [19831. Seedling production for crown lands in 

British Columbia: guidelines for commercial container nurseries. 

Victoria, BC: British Columbia Ministry of Forests, Silviculture 

Branch. 45 p.

McCain, A.H. 1987. California greenhouse disease management. 

Berkeley, CA: University of California, Cooperative Extension. 

64 p.

McCain, A.H. 1978. Nursery disease problems: containerized 

nurseries. In: Gustafson, R.W., ed. Proceedings, Western 

Forest Nursery Council and Intermountain Nurseryman's 

Association, combined meeting; 1978 August 7-11; Eureka, 

CA. San Francisco, CA: USDA Forest Service, State and 

Private Forestry: B-139-B-142.

McCain, A.H. 1977. Sanitation in plant propagation. 

International Plant Propagators' Society Combined 

Proceedings 27: 91-93.

McElroy, F.D. 1985. Personal communication.

McIntosh, D.L. 1966. The occurrence of Phytophthora spp. in 

irrigation systems in British Columbia. Canadian Journal of 

Botany 44: 1591-1596.
Salt, G.A. 1974. Etiology and morphology of Geniculodendron 

pyriforme Gen. Et Sp. Nov., a pathogen of conifer seeds. 

Transactions of the British Mycological Society 63(2): 339-351.

Sauer, D.B.; Burroughs, R. 1986. Disinfection of seed surfaces 

with sodium hypochlorite. Phytopathology 76(7): 745-749.

Scarratt, J.B. 1985. Greenhouse managers: beware combustion 

fumes in container greenhouses. Forestry Chronicle 61(4): 

308-31 1.

Schopmeyer, C.S., tech. coord. 1974. Seeds of woody plants in 

the United States. Agric. Handbk. 450. Washington, DC: U.S. 

Department of Agriculture. 883 p.

Seidel, K.W. 1986. Tolerance of seedlings of ponderosa pine, 

Douglas-fir, grand fir, and Engelmann spruce for high 

temperatures. Northwest Science 60(1): 1-7.

Shrimpton, G.M. 1987. Personal ommunication.

Shrimpton, G.M. 1986. Some insect pests of conifer seedlings in 

British Columbia. In: Landis, T.D., tech. coord. Proceedings: 

Western Forest Nursery Council and Intermountain Nursery 

Association, combined meeting; 1986 August 12-15; 

Tumwater, WA. Gen. Tech. Rep. RM-137. Ft. Collins, CO: 

USDA Forest Service, Rocky Mountain Forest and Range 

Experiment Station. 164 p.

Shrimpton, G.M. 1985. Four insect pests of conifer nurseries in 

British Columbia. In: Landis, T.D., comp. Proceedings: Western 

Forest Nursery Council and Intermountain Nurseryman's 

Association, combined meeting; 1984 August 14-16; Coeur d' 

Alene, ID. Gen. Tech. Rep. INT-185. Ogden, UT: USDA Forest 

Service, Intermountain Research Station: 119-121.

Shrimpton, G.M. 1983. Biology and control of some insect pests 

in British Columbia Forest Service seedling nurseries. Victoria, 

BC: Simon Fraser University. 131 p. M.P.M thesis.

Singer, J. 1980. Pesticide safety: guidelines for personnel 

protection. Davis, CA: USDA Forest Service, Forest Insect and 

Disease Management. 45 p.

Smith, L.D. 1982. Fungicides for nursery ornamentals. In: Jones, 

R.K.; Lambe, R.C. eds. Diseases of woody ornamental plants 

and their control in nurseries. Publ. 286. Raleigh, NC: North 

Carolina Extension Service: 1 O 1-106.

Smith, R.S., Jr. 1975. Fusarium root disease. In: Peterson, G.W.; 

Smith, R.S., Jr., eds. Forest nursery diseases in the United 

States. Agric. Handbk. 470. Washington, DC: U.S. Department 

of Agriculture: 9-10.

Pawuk, W.H. 1982. Diseases of container-grown southern pine 

seedlings and their control. In: Guldin, R.W.; Barnett, J.P., eds. 

Proceedings, Southern Containerized Forest Tree Seedling 

Conference; 1981 August 25-27; Savannah, GA. Gen. Tech. 

Rep. SO-37, New Orleans, LA: USDA Forest Service: 47-50.

Pawuk, W.H. 1981. Potting media affect growth and disease 

development of container-grown southern pine. Res. Note 

SO-268. New Orleans, LA: USDA Forest Service, Southern 

Forest Experiment Station. 4 p.

Pawuk, W.H. 1979. Fungicide coverings affect the germination 

of southern pine seeds. Tree Planters' Notes 30(1): 3-4.

Pawuk, W.H. 1978. Damping-off of container-grown longleaf 

pine seedlings by seedborne fusaria. Plant Disease Reporter 

62: 82-84.

Peck, K. 1984a. Pesticide shelf life and storage. Hummert's 

Quarterly 8(2): 4-5.

Peck, K. 1984b. Greenhouse use of herbicides. Hummert's 

Quarterly 8(1): 3.

Peck, K. 1982. Fungus gnats. Hummert's Quarterly 6(4): 3.

Peterson, G.W. 1974. Disease problems in the production of 

containerized forest tree seedlings in North America. In: Tinus, 

R.W.; Stein, I., Balmer, W.E., eds. Proceedings, North 

American Containerized Forest Tree Seedling Symposium; 

1974 August 26-29; Denver, CO; Great Plains Agricultural 

Council Pub]. 68: 170-172.

Peterson, G.W.; Smith, R.S., Jr., eds. 1975. Forest nursery 

diseases in the United States. Agric. Handbk. 470. Washington, 

DC: U.S. Department of Agriculture. 125 p.

Peterson, M.J.; Sutherland, J.R.; Tuller, S.E. 1988. Greenhouse 

environment and epidemiology of grey mould of 

container-grown Douglas-fir seedlings. Canadian Journal of 

Forestry Research 18: 974-980.

Powell, C.C., Jr. 1984. Safe field use of pesticides requires an 

understanding of common exposure hazards, American 

Nurseryman 159(2): 105-106.

Ross, R.L.M.; Puritch, G.S. 1981. Identification, abundance, and 

origin of moss, liverwort, and algal contaminants in 

greenhouses of containerized forest nurseries. Canadian 

Journal of Forest Research 11: 356-360.

Rowan, S.J. 1983. Time between application of Bayleton and 

irrigation not critical for fusiform rust control. Tree Planters' 

Notes 34(1): 16-17.

98
Smith, W.H. 1970. Tree pathology: a short introduction. New 

York: Academic Press. 309 p.

South, D.B. 1986. The "tarnished plant bug" can cause loblolly 

pine seedlings to be "bushy-topped." Rep. 27. Auburn, AL: 

Auburn University, Southern Forest Nursery Management 

Cooperative. 6 p.

Stephens, C.T.; Herr, L.J.; Schmitthenner, A.F.; Powell, C.C. 

1983. Sources of Rhizoctonia solani and Pythium spp. in a 

bedding plant greenhouse. Plant Disease 67: 272-275.

Streets, R.B. 1972. The diagnosis of plant diseases: a field and 

laboratory manual emphasizing the most practical methods 

for rapid identification. Tucson, AZ: University of Arizona 

Press. 130 p.

Sturrock, R. 1988. Personal communication. Victoria, BC: 

Canadian Forestry Service.

Sutherland, J.R. 1987. Personal communication. Victoria, 

BC: Canadian Forestry Service.

Sutherland, J.R. 1986. Influence of diseases on seed production. 

In: Shearer, R.C., comp. Proceedings, conifer tree seed in the 

Inland Mountain West symposium; 1985 August 5-6; Missoula, 

MT. Gen. Tech. Rep. INT-203. Ogden, UT: USDA Forest 

Service, Intermountain Research Station: 260-266.

Sutherland, J.R. 1984. Two seedborne diseases of conifers: the 

seed or cold fungus and Sirococcus blight. In: Proceedings, 

31st Annual Western International Forest Disease Work 

Conference; Coeur d' Alene, ID; 1983 August 22-26. Missoula, 

MT: USDA Forest Service, Cooperative Forestry and Pest 

Management: 6-8.

Sutherland, J.R. 1979. The pathogenic fungus Caloscypha fulgens 

in stored conifer seeds in British Columbia and relation of its 

incidence to ground and squirrel-cache collected cones. 

Canadian Journal of Forest Research 9: 129-132.

Sutherland, J.R.; Van Eerden, E. 1980. Diseases and insect pests 

in British Columbia forest nurseries. Joint Rep. 12. Victoria, BC: 

British Columbia Ministry of Forests and Canadian Forestry 

Service, Pacific Forest Research Centre. 55 p.

Sutherland, J.R.; Lock, W.; Benson, L.E. 1982. Diseases and 

insects and their management in container nurseries. In: 

Scarratt, J.B.; Glerum, C.; Plexman, C.A. eds. Proceedings of 

the Canadian Containerized Tree Seedling Symposium; 1981 

September 14-16; Toronto. Sault Ste. Marie, ON: Canadian 

Forestry Service, Great Lakes Forest Research Centre: 

215-223.

Sutherland, J.R.; Lock, W.; Farris, S.H. 1981. Sirococcus blight: a 

seed-borne disease of container-grown spruce seedlings in 

coastal British Columbia forest nurseries. Canadian Journal of 

Botany 59: 559-562.

Sutherland, J.R.; Miller, T.; Quinard, R.S. 1987. Cone and seed 

diseases of North American conifers. N. Am. For. Comm. Publ. 

1. Victoria, BC: Ministry of Supplies and Services. 77 p.

Tinus, R.W. 1987. Personal communication. USDA Forest 

Service, Flagstaff, AZ.

Tripepi, R.R.; Mitchell, C.A. 1984. Stem hypoxia and root 

respiration of flooded maple and birch seedlings. Physiologia 

Plantarum 60: 567-571.

Tunnock, S. 1985. Suppression of cranberry girdler damage in 

beds of Douglas-fir seedlings, Coeur d' Alene Nursery, Idaho 

Panhandle National Forest-1984. For. Pest Mgmt. Rep. 85-4. 

Missoula, MT: USDA Forest Service, Northern Region. 7 p.

Wenny, D.L.; Dumroese, R.K. 1987. Germination of seeds 

surface sterilized with bleach. Tree Planters' Notes 38(3): 

18-21.

Whitcomb, C.E. 1988. Plant production in containers, second 

ed. Stillwater, OK: Lacebark Publications, 633 p.

99