Alternatives to Preplant

Soil Fumigation for

Western Forest

Nurseries 

Diane M. Hildebrand, Jeffrey K. Stone, Robert L. James, and Susan J. Frankel

United States

Department of

Agriculture

Forest Service

Pacific Northwest

Research Station

General Technical 

Report

PNW-GTR-608

April 2004
Authors

This publication reports research involving pesticides. It does not contain recommendations for their use, nor does it imply that the uses discussed here have been registered.

Appropriate state or federal agencies, or both, must register all uses of pesticides

before they can be recommended.

CAUTION: Pesticides can be injurious to humans, domestic animals, desirable

plants, and fish or other wildlife—if they are not handled or applied properly. Use all

pesticides selectively and carefully. Follow recommended practices for the disposal of

surplus pesticides and pesticide containers.

Diane M. Hildebrand is a plant pathologist, U.S. Department of Agriculture, Forest

Service, Westside Forest Insect and Disease Service Center, 16400 Champion Way,

Sandy, OR 97055; Jeffrey K. Stone is an associate professor, Oregon State

University, Department of Botany and Plant Pathology, Corvallis, OR 97331-2902;

Robert L. James is a plant pathologist, U.S. Department of Agriculture, Forest

Service, Northern Region, 3815 Schreiber Way, Coeur d’Alene, ID 83814-8363; and

Susan J. Frankel is a plant pathologist, U.S. Department of Agriculture, Forest

Service, Pacific Southwest Region, 1323 Club Drive, Vallejo, CA 94592. The Pacific

Northwest Research Station is publishing this paper as part of a cooperative effort.
Hildebrand, Diane M.; Stone, Jeffrey K.; James, Robert L.; Frankel, Susan J.

2004. Alternatives to preplant soil fumigation for Western forest nurseries. Gen.

Tech. Rep. PNW-GTR-608. Portland, OR: U.S. Department of Agriculture, Forest

Service, Pacific Northwest Research Station. 27 p.

Field trials at six bare-root forest tree nurseries in the Western United States compared

cultural treatments including timing and depth of sowing; bare fallow (with and without

periodic tilling); organic amendments including sawdust, composts, and cover crops;

mulches including pine needles, sawdust, and rice straw; and fumigation with methyl

bromide/chloropicrin or dazomet. Measured effects included population levels of

potential soil-borne pathogens (species of Fusarium and Pythium), disease incidence,

seedbed density, and sizes of conifer seedlings. Several nonfumigation treatments

resulted in production of seedlings with densities and sizes similar to or better than

those produced in beds treated with chemical fumigation. Results varied within the

nurseries depending on conifer species, field history, and disease presence. Beneficial

cultural practices included (1) incorporation of slowly decomposing organic soil

amendments, e.g., aged sawdust with additional nitrogen provided later to seedlings;

(2) bare fallowing with periodic tilling, and bare fallowing without periodic tilling plus

supplemental weed control; and (3) sowing of conifer seed earlier and more shallow

than sown conventionally, and covering seed with a nonsoil mulch such as aged

sawdust or hydromulch.

Keywords: Methyl bromide, chloropicrin, dazomet, conifer seedlings, bare fallowing,

Fusarium, Pythium.

Abstract
Contents 1 Introduction

2 Materials and Methods

3 Bend Pine Nursery

4 J. Herbert Stone Nursery

4 Coeur d’Alene Nursery

5 Lucky Peak Nursery

5 Placerville Nursery

6 Humboldt Nursery

7 Measurement of Seedling Density and Mortality

7 Measurement of Seedling Size

8 Soil Sampling

8 Statistical Analyses

8 Results

8 Bend Pine Nursery

10 J. Herbert Stone Nursery

12 Coeur d’Alene Nursery

14 Lucky Peak Nursery

16 Placerville Nursery

18 Humboldt Nursery

20 Discussion

24 Acknowledgments

24 Metric and English Equivalents

24 Literature Cited
Many forest tree nurseries use soil fumigation prior to sowing to reduce populations of

pathogenic fungi, insects, nematodes, and weeds. Methyl bromide with chloropicrin

formulations are among the most commonly used fumigants (Smith and Fraedrich

1993), and are generally considered the best for achieving a uniform, vigorous crop

of quality seedlings (Cordell 1983). Where soil-borne fungal pathogens are important,

the fumigant formulation with 67 percent methyl bromide and 33 percent chloropicrin

(MBC) is preferred (Cordell et al. 1989). Control of soil-borne diseases and weeds is

the main reason nursery managers use soil fumigation (Boyd 1971). In some cases,

nursery managers use fumigants to protect their crops from catastrophic diseases

that do not occur every year, such as charcoal root rot, caused by Macrophomina

phaseolina (Tassi) Goidanich (Frankel et al. 1999). Growing seedlings without fumigation is considered to increase the risk of crop failure. Dazomet is an alternative

fumigant, and its granular form may be easier and safer to handle than gaseous

formulations containing methyl bromide (James and Beall 1999, Kelpsas and

Campbell 1994).

Chemical fumigation is expensive (costing approximately $1,500 per acre) and may

be hazardous to human and environmental health. Methyl bromide is thought to have

a high potential to deplete stratospheric ozone and is scheduled to be phased out for

soil fumigation in the United States after January 2005 (USDA 1999). The amount of

methyl bromide available for soil fumigation on January 1, 2001, was 50 percent of the

1991 production level (USDA 2000). Reliance on chemical biocides for production of

forest seedlings is inherently risky over the long term because such chemicals may

become restricted owing to environmental concerns (Smith and Fraedrich 1993).

Fumigation disrupts soil microbiota, eliminating both beneficial and detrimental organisms

(Munnecke and Van Gundy 1979). Opportunistic pathogens, including Fusarium spp.,

may be among the first micro-organisms to recolonize fumigated soil, from residual

survivors in roots and debris, or air-borne soil, or soil fragments on equipment

(Vaartaja 1967). Although Trichoderma spp. tend to quickly reinvade (Danielson and

Davey 1969, Gandy and Chanter 1976, Vaartaja 1967), populations of beneficial

micro-organisms, including those antagonistic to pathogens, may develop more slowly.

Ectomycorrhizal fungi can recolonize soil within the first growing season. Tanaka et al.

(1986) found that mycorrhizal development in 1+0 and 2+0 Douglas-fir (Pseudotsuga

menziesii (Mirb.) Franco) was not affected by preplant soil fumigation with MBC when

compared to nonfumigated soil. Fusarium spp. have been detected on conifer seed,

and this source may contribute to reinfestation of fumigated soil (Axelrood et al. 1995).

Populations of many opportunistic pathogenic fungi can increase rapidly under favorable

conditions (Hansen et al. 1990), which may include the absence of natural competitors

or antagonists owing to fumigation. Hence, disruption of microbial communities owing

to fumigation may result in the need for continued fumigation for disease control. On

the other hand, fumigation also can produce microbial and nutrient conditions beneficial

for seedling growth (Danielson and Davey 1969).

Expression of seedling diseases involves interactions among the host, pathogen, and

the environment. Likelihood of infection depends on the susceptibility of the host, the

population level of pathogen (inoculum) present, and the virulence of the pathogen.

Environmental conditions affect susceptibility of the host to infection, disease expression, and vigor of the pathogen. Even after the pathogen infects host tissue, disease

expression may be delayed or reduced if environmental conditions are not favorable

1

Introduction
2

for disease development. A delay between initial infection and the expression of disease symptoms (incubation period) often occurs. For Fusarium root disease, infection

often takes place quite early, soon after seed germination (Hamm 1990, James et al.

1991). For seedlings that do not succumb early to damping-off, the root disease is

expressed later, usually by midsummer, when temperatures and moisture stress begin

to adversely affect the host (Hamm 1990). Disease can be avoided by preventing or

reducing infection, or by preventing or reducing disease expression. The goal of disease

control is to suppress activity by pathogens and favor conditions for optimum host

vitality by manipulating the seedling environment.

The most effective means of disease control is to prevent infection, which can be

done by reducing pathogen populations or by providing physical or temporal barriers

to infection. Preplant chemical fumigation reduces pathogen populations by killing

pathogenic organisms. An alternative approach is to reduce pathogen populations by

removing the available food base and encouraging competitive saprobes. Many

pathogenic fungi, including Fusarium spp., can function as facultative saprobes,

enabling them to persist in soil on organic residues when living hosts are not available.

“Green manures” from cover crops incorporated into soil tend to stimulate increases in

populations of these facultatively saprobic pathogens because they flourish on the

abundant simple organic substrates available in crop residues (Hansen et al. 1990,

James et al. 1996, Stone and Hansen 1994). Weeds and weed residues also can provide nutritional resources for facultative saprobes. Few facultative saprobes, however,

are able to grow on more complex organic polymers, e.g., cellulose and lignin. One

approach to disease control, therefore, is to eliminate cover crops, maintain weed

control, and supplement soils with sawdust or other complex organic substrates to

favor soil saprobes over pathogens and help improve soil tilth.

Differences among nurseries in local climate, topography, soil characteristics, crop

species, disease and other pest problems, and management history can influence

disease management, making it difficult to implement a single disease management

strategy that would be effective for several nurseries. The objective of our multiyear

effort was to compare the efficacy of alternative disease management strategies

designed for six bare-root nurseries, with the standard fumigation procedures used by

the nurseries, as measured by seedling size and density and populations of soil-borne

pathogens. Our goal was to develop alternatives to fumigation specific to the particular

conditions, needs, and practices at each nursery that would reduce pathogen populations or limit disease and produce operational quantities of high-quality forest tree

seedlings. Portions of these results were reported earlier (Hildebrand et al. 1995,

Stone et al. 1997). Previously reported results were based on preliminary statistical

analyses, and some statistical significance reported here may differ.

Field trials were conducted at six U.S. Department of Agriculture, Forest Service nurseries: Bend Pine Nursery in Bend, Oregon; J. Herbert Stone Nursery in Central Point,

Oregon; Coeur d’Alene Nursery in Coeur d’Alene, Idaho; Lucky Peak Nursery near

Boise, Idaho; Humboldt Nursery near McKinleyville, California; and Placerville Nursery

near Camino, California. Study plots were set up generally in randomized complete

block designs or complete randomized designs in production beds, with four or five

replicates per treatment. Replicate treatment plots were at least 50 ft (15 m) long, with

Materials and

Methods
3

Bend Pine Nursery 

at least a 20-ft (6.1-m) buffer between plots. Treatments differed among nurseries

according to individual nursery pest problems, soil conditions, and management

concerns. Preplant soil treatments included cover cropping, bare fallowing, chemical

fumigation, incorporation of organic soil amendments, and timing and depth of sowing.

Chemical fumigants used for comparison were those conventionally used at each

nursery at the application rate and timing routinely employed. Methyl bromide fumigant

was MBC (67 percent methyl bromide and 33 percent chloropicrin) (MC-33, Terr-OGas®, Great Lakes Chemical, West Lafayette, IN).1 The MBC was injected into soil,

and the soil was covered by polyethylene sheeting for several days. Dazomet fumigant

was granular Basamid® (BASF AG, Limbergerhof, Germany). Dazomet was incorporated into the soil, then the soil was packed firmly with a roller and sufficient water

applied (usually 1 inch over 1 to 1.5 hours) to make a crust to seal in the fumigant.

Daily watering for 1 to 2 weeks kept the soil moist and maintained a 1⁄

2 - to 3⁄4 -inch

crust on the soil surface.

Conifer species tested represented a major crop species produced at each nursery. A

single seed lot for each species was sown into all treatment plots within a given nursery,

except as noted. All study plots were established in spring 1993 prior to soil treatments followed by sowing of conifer seed in spring 1994, and final measurements

were taken in fall 1995. Soil treatments were repeated in 1995 followed by sowing of

conifer seed in spring 1996 and final measurements in fall 1997.

The study area was a block 400 by 48 ft (122 by 14.6 m), divided into 32 plots, each

50 by 12 ft (15 by 3.7 m). Five replicates of each of five treatments were randomly

assigned to the plots, leaving seven unused plots. The entire study block received the

standard nursery soil amendment at 1.5-in (3.8-cm) depth or 200 yd3/acre (380 m3/ha)

of aged pine sawdust incorporated into the soil with 300 lb/acre (337 kg/ha) of ammonium nitrate (34-0-0) (S+N) at the beginning of summer in 1993 and 1995. Soils at the

nursery are sandy loams and loamy sands, organic matter content is about 2 percent,

and cation exchange capacities range from 10 to 12 meq/100 g.

Three treatments were bare fallow from June through September, two without tilling

(BF) and one with tilling (BFT) at 3-week intervals. One of the bare fallow without tilling treatments received a mulch of pine needles (collected locally the previous fall

from natural needle abscission), applied after sowing to cover the seed. The pine

mulch treatment was not repeated in the second trial (1995-97). The other bare fallow

without tilling treatment used Geotech® soil stabilizer (a hygroscopic spray-on gel

designed to hold soil in place) as mulch. The remaining two treatments had a pea

(Lathyrus hirsutus L.) cover crop (Pea CC) sown in May 1993 and 1995, and incorporated into the soil in August. One of the pea cover crop treatments was fumigated with

MBC, in September 1993 and 1995, at 350 lb/acre (393 kg/ha), after incorporation of

the pea cover crop. This treatment represented nursery standard practice, whereas

the pea cover crop treatment without fumigation provided a comparison.

1 The use of trade or firm names in this publication is for reader

information and does not imply endorsement by the U.S.

Department of Agriculture of any product or service.
4

Treatments were sawdust plus N with pea cover crop and MBC fumigation (“S+N, pea

CC, MBC”), sawdust plus N with bare fallow and tilling (“S+N, BFT”), sawdust plus N

with bare fallow (“S+N, BF”), sawdust plus N with bare fallow and pine needle mulch

(“S+N, BF, pine needle mulch”), and sawdust plus N with pea cover crop (“S+N, pea

CC”). One continuous seedling bed was sown through the center of each plot in four

linear strips in the study area in late April 1994 and 1996, with a single seed lot of

ponderosa pine (Pinus ponderosa Dougl. ex Laws.).

The study area was a 3- by- 9 plot block, with each of the 27 plots measuring 50 by

11 ft (15 by 3.4 m). Five replicates of each of five treatments were randomly assigned

to the plots, leaving two unused plots. Each plot had one bed of ponderosa pine and

one bed of Douglas-fir sown in continuous beds through the center of the plots. One

set of soil samples per replicate of each treatment provided preplant pathogen information for both Douglas-fir and ponderosa pine. Soils at the nursery are uniformly

sandy loams, organic matter content ranges from 3 to 5 percent, and cation exchange

capacities range from 7.5 to 12.5 meq/100 g.

The entire study block was kept bare fallow (BF) for the year before sowing. Treatments

consisted of soil amendments and modifications to bare fallow. Amendments of aged

conifer sawdust were applied at 1-in (2.45-cm) depth or 133 yd3/acre (250 m3/ha), and

were incorporated into the soil at the beginning of summer in 1993 and 1995. All but

one sawdust amendment treatment included additional ammonium nitrate (34-0-0) (N)

incorporated into the soil at 300 lb/acre (337 kg/ha) (S+N). In one sawdust treatment,

extra N was provided only after seedling emergence (S, delayed N). All but one treatment had tilling at 3-week intervals June through September (BFT). The treatment

representing nursery standard practice was sawdust plus N, periodic tilling, and fumigation with dazomet applied at 350 lb/acre (393 kg/ha) in September 1993 and 1995.

Treatments were sawdust plus N with tilling and fumigation (“S+N, BFT, dazomet”),

sawdust plus N with tilling (“S+N, BFT”), sawdust plus N without tilling (“S+N, BF”), no

sawdust with tilling (“BFT”), and sawdust with delayed N and tilling (“S, BFT, delayed

N”). Seedbeds were formed, and a single seed lot of Douglas-fir and another of ponderosa pine were sown in late April 1994 and 1996.

The study plots were each 50 ft by one bed width (4 ft) (15 by 1.2 m) except the fumigation treatment, which covered two bed widths. Five replicates of five treatments

were arranged in a randomized block design. All treatments included bare fallow with

periodic tilling (BFT) and hand weeding. Two organic amendment treatments were

incorporated into soil in late June 1993 and 1995. These were composted bark chips

(bark compost) and sewage sludge composted with wood waste (sludge), both applied

at a rate of approximately 29 yd3/acre (55 m3/ha). In the first trial, one treatment had a

layer of pine needles (collected locally the previous fall from natural needle abscission)

applied after sowing to cover the seed (pine needle mulch). The fumigation treatment

was dazomet applied at the standard rate of 350 lb/acre (393 kg/ha), in late September

1993 and 1995. Soils at the nursery are gravelly sandy loam, sandy loam, and silt

loam. Organic matter content ranges from 1.1 to 5.2 percent, and cation exchange

capacities range from 6.2 to 14.2 meq/100 g. Soil pH ranges from 5.8 to 7.5.

J. Herbert Stone

Nursery

Coeur d’Alene

Nursery
5

Lucky Peak Nursery

The five treatments were bare fallow with tilling (“BFT”), bark compost amendment

and bare fallow with tilling (“bark compost, BFT”), sewage sludge amendment and

bare fallow with tilling (“sludge, BFT”), bare fallow with tilling and pine needle mulch

(“BFT, pine needle mulch”), and bare fallow with tilling and dazomet fumigation (“BFT,

dazomet”). Seedbeds were formed and Rocky Mountain Douglas-fir (Pseudotsuga

menziesii var. glauca (Beissn.) Franco) sown along the center of each plot in early

May 1994 and 1996.

Two identical study areas were established in different parts of the nursery. Treatment

plots were each 50 ft by one bed width (15 by 1.2 m) except the fumigation treatment,

which covered two bed widths. Five replicates of five treatments were arranged in a

randomized block design. One study area was sown with a single seed lot of ponderosa

pine and the other with three seed lots of lodgepole pine (Pinus contorta Dougl. ex

Loud.). The seedbeds were formed and sown along the center of each plot in late May

1994 and 1996. In the 1995-97 trial, only one study area was treated, and sown with

ponderosa pine. Soils at the nursery are loam, loamy sand, sandy clay loam, and

sandy loam, with more clay occurring generally to the south and east. Organic matter

content ranges from 2.9 to 5.7 percent, and cation exchange capacities average 9.2 to

16.7 meq/100 g. Soil pH ranges from 5.2 to 6.

All treatments included bare fallow and hand weeding. One treatment was tilled at

monthly intervals between mid-June and late September. Two of the no-tilling treatments

had soil amendments incorporated in early June 1993 and 1995. One amendment

was composted waste material from the commercial production of mushrooms (compost), added at approximately 22 yd3/acre (42 m3/ha). The other amendment was

commercial sawdust containing supplemental nitrogen (S+N) added at approximately

22 yd3/acre (42 m3/ha), with 82 lb/acre (92 kg/ha) ammonium nitrate (34-0-0). The

third no-tilling treatment was the nursery standard fumigation with MBC applied at 350

lb/acre (393 kg/ha) in September 1993 and 1995. The five treatments were bare fallow

(“BF”); bare fallow with tilling (“BFT”); mushroom compost and bare fallow (“compost,

BF”); sawdust plus N and bare fallow (“S+N, BF”); and bare fallow and MBC (“BF,

MBC”).

Study plots were 30 by 4 ft (9 by 1.2 m), with treatments replicated four times and

arranged in a randomized block design. Seven treatments began with bare fallow with

tilling (BFT), then bed formation in October, and an overwinter soil mulch of rice straw,

sawdust, pine needles, hydromulch, or nothing. Weeds were controlled by flaming

during winter and hand weeding prior to conifer sowing in spring. Shasta red fir (Abies

magnifica A. Murr. var. shastensis Lemm.) seed was sown in mid-March (early) or

mid-April (late) 1994, and covered with soil, hydromulch, or sawdust. In the first trial

(1993-95), there was no fumigation treatment. Soils at the nursery are predominantly

loams, organic matter content ranges from 4 to 6 percent, and cation exchange

capacities range from 15 to 19 meq/100 g.

The seven treatments in the first trial were bare fallow with tilling and rice straw winter

mulch and late sowing with soil covering seed (“BFT, rice straw mulch, late sow, soil

over seed”); bare fallow with tilling and rice straw winter mulch and early sowing with

hydromulch covering seed (“BFT, rice straw mulch, early sow, hydromulch over seed”);

bare fallow with tilling and sawdust winter mulch and early sowing with hydromulch

Placerville Nursery
6

covering seed (“BFT, sawdust mulch, early sow, hydromulch over seed”); bare fallow

with tilling and sawdust winter mulch and early sowing with sawdust covering seed

(“BFT, sawdust mulch, early sow, sawdust over seed”); bare fallow with tilling and pine

needle winter mulch and early sowing with hydromulch covering seed (“BFT, pine

needle mulch, early sow, hydromulch over seed”); bare fallow with tilling and hydromulch winter mulch and early sowing with hydromulch over seed (“BFT, hydromulch,

early sow, hydromulch over seed”); and bare fallow with tilling and bare soil over winter

and early sowing with hydromulch covering seed (“BFT, bare, early sow, hydromulch

over seed”).

In the second trial (1995-97), eight treatments began with bare fallow with tilling, followed by bed formation and an overwinter soil mulch of sawdust, hydromulch, or vetch

(Vicia narbonensis L.) cover crop, with bed formation in spring. Two of these treatments included fumigation with MBC applied at 350 lb/acre (393 kg/ha) at the end of

summer before the vetch cover crop; the other six treatments had no fumigation.

Shasta red fir seed was sown in February (early) or mid-April (late) 1996, either on

the surface of the soil and covered with sawdust or under the surface covered with

soil in the conventional way. The past nursery standard treatment was late-summer

fumigation with MBC, followed by a vetch cover crop to hold the soil over winter, bed

formation in spring, and late sowing of conifer seed, covered with soil.

The eight treatments in the second trial were bare fallow with tilling and sawdust winter

mulch and early sowing with sawdust covering seed (“BFT, sawdust mulch, early sow,

sawdust over seed”); bare fallow with tilling and sawdust winter mulch and early sowing

with soil covering seed (“BFT, sawdust mulch, early sow, soil over seed”); bare fallow

with tilling and MBC fumigation and vetch winter cover crop and late sowing with sawdust covering seed (“BFT, MBC, vetch cover crop, late sow, sawdust over seed”); bare

fallow with tilling and MBC fumigation and vetch winter cover crop and late sowing

with soil covering seed (“BFT, MBC, vetch cover crop, late sow, soil over seed”); bare

fallow with tilling and hydromulch winter mulch and late sowing with sawdust over

seed (“BFT, hydromulch, late sow, sawdust over seed”); bare fallow with tilling and

hydromulch winter mulch and late sowing with soil over seed (“BFT, hydromulch, late

sow, soil over seed”); bare fallow with tilling and vetch winter cover crop and late sowing

with soil covering seed (“BFT, vetch cover crop, late sow, soil over seed”); and bare

fallow with tilling and vetch winter cover crop and late sowing with sawdust covering

seed (“BFT, vetch cover crop, late sow, sawdust over seed”).

Study plots were 24 by 4 ft (7 by 1.2 m), with five treatments replicated five times in a

randomized block design. In the first trial (1993-95), all treatments began with bare

fallow with tilling. Shasta red fir was sown in spring 1994. In three treatments, seed

was sown conventionally and covered with soil. Two treatments had seed sown shallow and covered with a mulch of composted redwood chips or hydromulch. Two treatments had fumigation, one with MBC and one with dazomet. Fumigants were applied

at 350 lb/acre (393 kg/ha) in fall. Soils at the nursery are loam and fine sandy loam,

organic matter content ranges from 3.2 to 8.4 percent, and cation exchange capacities

range from 10 to 20 meq/100 g.

Humboldt Nursery
7

Measurement of

Seedling Density and

Mortality

Treatments in the first trial were bare fallow with tilling and soil covering seed (“BFT,

soil over seed”); bare fallow with tilling and composted redwood mulch covering seed

(“BFT, redwood mulch over seed”); bare fallow with tilling and hydromulch covering

seed (“BFT, hydromulch over seed”); bare fallow with tilling and MBC fumigation and

soil covering seed (“BFT, MBC, soil over seed”); and bare fallow with tilling and

dazomet fumigation and soil covering seed (“BFT, dazomet, soil over seed”).

In the second trial (1995-97) at Humboldt Nursery, study plots were 4 by 50 ft (1.2 by

15 m), with five replicate plots for each of five treatments arranged in a randomized

block design. All treatments began with bare fallow and three included tilling. One of

the bare fallow without tilling treatments had glyphosate herbicide applied as needed.

One of the bare fallow with tilling treatments had fumigatiion with MBC; one had phosphate buffer seed treatment; and one had seed treatment with a biocontrol agent in

phosphate buffer. The biocontrol agent was an experimental mixture of three strains of

Pseudomonas chlororaphis (Guig. & Sauv.) Bergey, a nonpathogenic bacterium that

inhabits soil and the rhizosphere of conifers. Preliminary experiments at BC Research

Inc.2 had indicated that these bacterial strains might be useful as biocontrol agents

against the pathogenic fungus, Fusarium oxysporum Schlechtend.:Fr. Shasta red fir

seed was sown in spring 1996, and covered with soil in the conventional way.

Treatments in the second trial were bare fallow and glyphosate herbicide (“BF, herbicide”); bare fallow with tilling and phosphate buffer seed treatment (“BFT, phosphate

buffer”); bare fallow (“BF”); bare fallow with tilling and MBC fumigation (“BFT, MBC”);

and bare fallow with tilling and biocontrol seed treatment (“BFT, biocontrol”).

For the first growing season, seedling densities (seedlings/area) were recorded about

3 weeks after emergence, at midseason (about 8 weeks postemergence), and at the

end of the growing season (1+0). Three fixed plots 0.5 by 4 ft (0.15 by 1.2 m), oriented

across beds, were established in each replicate treatment plot at each nursery. Total

numbers of live seedlings, seedlings killed by disease (with symptoms of postemergence damping-off or Fusarium root disease), and seedlings killed by other causes

were recorded for each fixed plot at each measurement period. Cumulative percentage

of mortality from disease during the first growing season, beginning with emergence

and ending in mid-October (1+0), was used for statistical analyses. Final seedling

density was determined in mid-October at the end of the second growing season (2+0)

for three randomly located plots of the same size as above, in each treatment plot.

Seedling shoot length (height) and diameter were recorded for a sample of 50 trees

from each replicate treatment plot in each nursery at the end of the first growing season (1+0) and again at the end of the second growing season (2+0). Seedling height

and diameter were measured by using “Machine Vision,” a line-scanning image-analysis system developed by the University of Oklahoma for seedling inspection.

2 Axelrood, P.E. 1996. Personal communication. Plant pathologist,

Microbiology and Plant Pathology Group, B.C. Research Inc., 3650

Wesbrook Mall, Vancouver, BC V6S 2L2, Canada.

Measurement of

Seedling Size
8

Soil samples were collected three times at each nursery for determining population

levels of species of Fusarium and Pythium: (1) in spring at the time treatment plots

were established; (2) in October, about 3 weeks following fumigation treatments; and

(3) in spring of the following year immediately prior to sowing (preplant). Soil samples

were collected in polyethylene bags, transported to the laboratory in cooler boxes,

stored at 39.2°F (4 °C), and processed within 48 hours of receipt at the laboratory.

Soil samples were passed through a 0.24-in (6-mm) screen, and 3.5 oz (10 g) (fresh

weight) were added to flasks containing 3 fl oz (90 mL) of 0.1 percent water agar.

These samples were mixed and serially diluted (1:10, 1:100) in 0.1 percent water agar

for plating. A portion of each soil sample was oven dried for determining water content

and conversion of propagule counts on a dry-weight basis.

Four plates of each sample, with 0.017 fl oz (0.5 mL) diluted soil on each plate, were

prepared on two selective media: Komada’s medium (Komada 1975) and a modified

V-8 medium. Komada’s medium, modified with the amendment of 0.33 oz/qt (1 g/L) of

lithium chloride for suppression of Trichoderma spp. (Wildman 1991) was used for

enumeration of Fusarium species, primarily F. oxysporum. Dilution plates of Komada’s

medium were incubated under fluorescent light and read after 6 days. For enumeration

of Pythium spp., a modified V-8 medium was used, containing 6.8 fl oz (200 mL) clarified V-8 juice, 0.0035 oz (10 mg) rifampicin, 0.007 oz (20 mg) rose Bengal, 0.0875

oz (250 mg) ampicillin, 0.0035 oz (10 mg) pimaricin, and 7 oz agar (20 g) per 1.06 qt

(per 1 L). Plates were inoculated with soil dilutions as above, incubated in the dark at

room temperature, and read after 2 days. For both fungal assays, the average number of colonies on four plates multiplied by the dilution factor and corrected for water

content, yielded colony forming units (CFU) per gram of oven-dried soil.

Analyses of data were carried out by using Systat 8.0 (Systat 1998) or SAS statistical

software (SAS 1992). Analysis of variance (ANOVA) with Tukey’s multiple comparison

procedures were used for comparisons of seedling size factors among treatments at

each nursery. Seedling density and mortality data were logit transformed as recommended for proportional data containing zeros (Sabin and Stafford 1990) and analyzed by using ANOVA. Mean CFU for each replicate plot was analyzed by using the

Kruskal-Wallis procedure and the Mann-Whitney U statistic for comparisons between

treatments. Differences were declared statistically significant by using an α-level of

0.05.

For both trials of the study, average seedling densities and mortality were not significantly different between the fumigated (“S+N, Pea CC, MBC”) and the nonfumigated

bare fallow treatments (“S+N, BFT”; “S+N, BF”; and “S+N, BF, pine needle mulch”)

(table 1). The pea cover crop without fumigation treatment (“S+N, Pea CC”) resulted in

significantly lower densities and significantly higher mortality than all other treatments

in both trials.

In the 1993 trial, average ponderosa pine seedling diameter and shoot height were

significantly greater in the fumigated treatment (“S+N, Pea CC, MBC”) than the three

nonfumigated bare fallow treatments (“S+N, BFT”; “S+N, BF”; and “S+N, BF, pine

needle mulch”). In the 1995 trial, seedling heights and diameters were not significantly

different between the fumigated (“S+N, Pea CC, MBC”) and nonfumigated bare fallow

(“S+N, BFT” and “S+N, BF”) treatments. The bare fallow with pine needle mulch treatment (“S+N, BF, pine needle mulch”) was not repeated because it was operationally

difficult and apparently delayed seedling emergence.

Soil Sampling

Statistical Analyses

Results

Bend Pine Nursery
9

In both trials, all treatments resulted in seedlings with average heights well above the

minimally acceptable 3.9 in (10 cm) for 2+0 ponderosa pine (table 1). Average seedling

diameters in all treatments also were well above the minimally acceptable 0.16 in (4

mm). Average seedling diameter in the pea cover crop without fumigation treatment

(“S+N, Pea CC”) was significantly greater than in the other treatments, the result of

reduced competition owing to very low bed densities (one-third that of the other treatments; table 1).

The MBC fumigation left detectable levels of Fusarium in the soil. Average preplant

population levels of Fusarium in the nonfumigated bare fallow treatments (“S+N, BFT”;

“S+N, BF”; and “S+N, BF, pine needle mulch”) were not significantly different than the

fumigated treatment (“S+N, Pea CC, MBC”). Preplant Fusarium populations were significantly greater in the pea cover crop without fumigation treatment (“S+N, Pea CC”),

and seedling mortality was correspondingly much greater, for both trials of the study

(table 1). Preplant population levels of Pythium were similarly significantly higher in

the pea cover crop without fumigation (“S+N, Pea CC”) than in other treatments. In the

Table 1—Effect of soil treatments on 2+0 ponderosa pine seedling density,

diameter, height, mortality, and preplant levels of Fusarium and Pythium spp.,

at Bend Pine Nursery

Preplant Preplant 

Treatment a Density b Diameter Height Mortalityc Fusarium d Pythium d

No. per ft2 mm cm Percent - - - - - - CFU - - - - - -

1993-95:

S+N, Pea CC, 20.3 ae 6.8 b 25.4 a 5.5 a 170 a 2 a

MBC

S+N, BFT 21.3 a 6.2 a 19.1 b 11.4 a 620 a 14 b

S+N, BF 20.3 a 6.1 a 20.4 b 9.3 a 950 a 18 b

S+N, BF, pine 20.3 a 5.8 a 15.9 b na 220 a na

needle mulch

S+N, Pea CC 7.0 b 8.5 c 19.6 b 39.1 b 3,710 b 158 c

1995-97:

S+N, Pea CC, 15.1 x 6.2 x 19.3 x 6.6 x 1,200 x 16 x

MBC

S+N, BFT 13.5 x 6.0 x 19.8 x 8.4 x 510 x 23 x

S+N, BF 17.5 x 6.2 x 19.8 x 10.6 x 2,230 x 77 xy

S+N, Pea CC 6.0 y 7.8 y 18.6 x 44.0 y 5,090 y 129 y

na = data not available.

a BF = bare fallow, T = with tilling, S = sawdust soil amendment, N = ammonium nitrate, Pea CC = pea

cover crop incorporated as green manure, MBC = fumigation with 67 percent methyl bromide/33 percent

chloropicrin.

b 2+0 seedlings per square foot (per 0.09 m2).

c Percentage of seedlings killed by disease in the first growing season.

d Colony-forming units (CFU) per gram dry weight of soil.

e Means followed by the same letter are not significantly different at p ≤ 0.05: a-c for 1993-95 trial, x-y for

1995-97 trial.
10

1993 trial, preplant Pythium population levels were significantly lower in the fumigated

(“S+N, Pea CC, MBC”) than nonfumigated bare fallow (“S+N, BFT”; “S+N, BF”; and

“S+N, BF, pine needle mulch”) treatments, but these treatments were not significantly

different in the 1995 trial (table 1).

Average ponderosa pine density was significantly lower in the bare fallow without tilling treatment (“S+N, BF”) compared to the three nonfumigated bare fallow with tilling

treatments (“S+N, BFT”; “BFT”; and “S, BFT, delayed N”) in the 1993 trial (table 2).

The bare fallow without tilling treatment (“S+N, BF”) had heavy weed growth in the

1993 trial for both ponderosa pine and Douglas-fir. Ponderosa pine densities in all four

nonfumigated bare fallow treatments (“S+N, BFT”; “S+N, BF”, “BFT”; and “S, BFT,

delayed N”) were not significantly different from that in the dazomet fumigation treatment (“S+N, BFT, dazomet”) in the 1993 trial. In the 1995 trial, ponderosa pine density

was significantly lower in the bare fallow without sawdust (“BFT”) treatment than in all

other treatments. All the other nonfumigated bare fallow treatments resulted in ponderosa pine densities equivalent to that with dazomet fumigation, in the 1995 trial

(table 2).

J. Herbert Stone

Nursery

Table 2—Effect of soil treatments on 2+0 ponderosa pine seedling density,

diameter, height, mortality, and preplant levels of Fusarium and Pythium spp.,

at J. Herbert Stone Nursery 

Preplant Preplant

Treatmenta Densityb Diameter Height Mortalityc Fusariumd Pythiumd

No. per ft2 mm cm Percent - - - - - - CFU - - - - - -

1993-95:

S+N, BFT, 19.3 abe 8.5 a 31.5 a 9.1 a 130 a 19 a

dazomet

S+N, BFT 20.7 a 8.8 a 31.0 a 2.7 a 2,190 a 22 ab

S+N, BF 16.2 b 7.8 a 22.6 b 3.3 a 3,470 b 82 c

BFT 20.3 a 8.8 a 31.7 a 6.5 a 1,110 a 56 bc

S, BFT, 22.5 a 8.1 a 29.8 a 6.0 a 810 a 67 c

delayed N

1995-97:

S+N, BFT, 14.9 x 7.6 x 17.9 x 9.8 xy 870 x 4 x

dazomet

S+N, BFT 14.8 x 7.9 x 17.3 x 12.0 y 7,990 y 60 y

S+N, BF 15.1 x 7.1 x 16.2 x 9.2 xy 4,800 xy 45 y

BFT 13.1 y 7.9 x 16.7 x 7.0 x 4,300 xy 46 y

S, BFT, 15.2 x 8.0 x 17.9 x 7.4 x 2,650 xy 46 y

delayed N

a BF = bare fallow, T = with tilling, S = sawdust soil amendment, N = ammonium nitrate.

b 2+0 seedlings per square foot (per 0.09 m2).

c Percentage of seedlings killed by disease in the first growing season.

d Colony-forming units (CFU) per gram dry weight of soil.

e Means followed by the same letter are not significantly different at p ≤ 0.05: a-c for 1993-95 trial, x-y for

1995-97 trial.
11

For Douglas-fir in the 1993 trial, density was significantly higher in the delayed N

treatment (“S, BFT, delayed N”) than for all other treatments (table 3). The other three

nonfumigated bare fallow treatments (“S+N, BFT”; “S+N, BF”; and “BFT”) resulted in

Douglas-fir densities equivalent to that with dazomet fumigation (“S+N, BFT, dazomet”)

in the 1993 trial (table 3). For Douglas-fir in the 1995 trial, the only significant difference

was the low density in the no sawdust treatment (“BFT”) compared to the dazomet

treatment (“S+N, BFT, dazomet”). Douglas-fir densities in the other three nonfumigated bare fallow treatments (“S+N, BFT”; “S+N, BF”; and “S, BFT, delayed N”) were not

significantly different from that of the dazomet treatment (“S+N, BFT, dazomet”) in the

1995 trial (table 3).

Average diameters of both species in both trials in all treatments were well above the

minimum acceptable 0.16 in (4 mm) for ponderosa pine and 0.2 in (5 mm) for lowelevation Douglas-fir (tables 2 and 3). For ponderosa pine in both trials, treatments did

not result in significantly different diameters (table 2). Diameters were significantly different for Douglas-fir in the 1993 trial between the no sawdust treatment (“BFT”) and

the sawdust plus N with tilling treatment (“S+N, BFT”) (table 3). These differences

were apparently not related to density. In the 1995 trial for Douglas-fir, the only significant increase in diameter was in the no sawdust treatment (“BFT”), and was related to

significantly lower density (table 3).

Table 3—Effect of soil treatments on 2+0 Douglas-fir seedling density, diameter,

height, mortality, and preplant levels of Fusarium and Pythium spp., at J. Herbert

Stone Nursery 

Preplant Preplant

Treatmenta Densityb Diameter Height Mortalityc Fusariumd Pythiumd

No. per ft2 mm cm Percent - - - - - - CFU - - - - - -

1993-95:

S+N, BFT, 18.9 ae 7.4 ab 29.2 a 9.9 a 130 a 19 a

dazomet 

S+N, BFT 16.4 a 6.5 a 21.8 ab 13.1 a 2,190 a 22 ab

S+N, BF 14.4 a 6.6 ab 17.8 b 15.7 a 3,470 b 82 c

BFT 18.5 a 8.0 b 29.6 a 11.7 a 1,110 a 56 bc

S, BFT, 23.0 b 6.9 ab 27.4 ab 9.8 a 810 a 67 c

delayed N

1995-97:

S+N, BFT, 11.3 x 7.3 x 25.3 x 20.2 x 870 x 4 x

dazomet

S+N, BFT 9.7 xy 7.3 x 26.2 x 34.2 y 7,990 y 60 y

S+N, BF 10.4 xy 7.2 x 26.2 x 23.6 x 4,800 xy 45 y

BFT 6.8 y 9.0 y 30.7 x 44.0 y 4,300 xy 46 y

S, BFT, 10.1 xy 7.6 x 28.3 x 25.9 x 2,650 xy 46 y

delayed N

a BF = bare fallow, T = with tilling, S = sawdust soil amendment, N = ammonium nitrate.

b 2+0 seedlings per square foot (per 0.09 m2).

c Percentage of seedlings killed by disease in the first growing season.

d Colony-forming units (CFU) per gram dry weight of soil.

e Means followed by the same letter are not significantly different at p ≤ 0.05: a-c for 1993-95 trial, x-y for

1995-97 trial.
12

In the 1993 trial, average ponderosa pine heights exceeded the target 9.8 in (25 cm)

for all treatments except the bare fallow without tilling (“S+N, BF”), which had significantly shorter seedlings (table 2). In the 1995 trial, none of the treatments had a

significant effect on average ponderosa pine heights, which exceeded the minimum

acceptable 5.9 in (15 cm), but did not reach the target 9.8 in (25 cm). For both trials,

average Douglas-fir heights were mostly slightly shorter than the target 11.8 in (30

cm) for low-elevation Douglas-fir (table 3). In the 1993 trial, average Douglas-fir

heights were not significantly different between the dazomet (“S+N, BFT, dazomet”)

and the three nonfumigated bare fallow with tilling (“S+N, BFT”; “BFT”; and “S, BFT,

delayed N”) treatments (table 3). In the 1993 trial, average Douglas-fir height was significantly shorter in the bare fallow without tilling treatment (“S+N, BF”) compared to

the dazomet (“S+N, BFT, dazomet”) and the no sawdust (“BFT”) treatments. In the

1993 trial, average seedling height in the bare fallow without tilling treatment (“S+N,

BF”) did not meet the minimum acceptable 7.8 in (20 cm) for low-elevation Douglas-fir.

In the 1995 trial, none of the treatments had a significant effect on average Douglas-fir

height (table 3).

Dazomet fumigation left detectable preplant population levels of Fusarium and Pythium

in both trials of the study (tables 2 and 3). In the 1993 trial, preplant Fusarium populations were significantly greater in the bare fallow without tilling treatment (“S+N, BF”)

compared to all other treatments. Preplant Pythium populations in the 1993 trial were

significantly greater in the bare fallow without tilling (“S+N, BF”) and delayed N (“S,

BFT, delayed N”) treatments compared to the dazomet (“S+N, BFT, dazomet”) and the

nonfumigated sawdust plus N with tilling (“S+N, BFT”) treatments. Excessive weed

growth, mainly Portulaca oleracea L., in the bare fallow without tilling treatment (“S+N,

BF”) in the 1993 trial was associated with higher populations of both Fusarium and

Pythium. In the 1993 trial, none of the treatments had a significant effect on mortality

in ponderosa pine and Douglas-fir. In the 1995 trial, significantly higher population

levels of Fusarium occurred in the nonfumigated sawdust plus N with tilling treatment

(“S+N, BFT”) compared to the dazomet treatment (“S+N, BFT, dazomet”). In the 1995

trial, dazomet fumigation (“S+N, BFT, dazomet”) significantly reduced Pythium populations compared to all other treatments (tables 2 and 3). In the 1995 trial, mortality in

ponderosa pine was significantly greater in the nonfumigated sawdust plus N with

tilling treatment (“S+N, BFT”) compared with the no sawdust (“BFT”) and delayed N

(“S, BFT, delayed N”) treatments. In the 1995 trial, mortality in Douglas-fir was significantly greater in the nonfumigated sawdust plus N with tilling treatment (“S+N, BFT”)

and no sawdust (“BFT”) treatments compared to the dazomet (“S+N, BFT, dazomet”),

bare fallow without tilling (“BFT”), and delayed N (“S, BFT, delayed N”) treatments.

Mortality caused by disease was not consistently related to preplant population levels

of Fusarium or Pythium or with seedling density.

None of the treatments had a significant effect on average density of Douglas-fir

seedlings in either the 1993 or 1995 trials (table 4). In the 1993 trial, average diameters were not significantly different and met the minimum acceptable diameter of 0.2

in (5 mm) for interior 2+0 Douglas-fir. Average seedling heights in the 1993 trial were

significantly shorter in the bark compost treatment (“bark compost, BFT”). All treatments resulted in seedlings exceeding the minimum acceptable height of 5.9 in (15

cm) in the 1993 trial. In the 1995 trial, seedling diameters were equivalent statistically

Coeur d’Alene

Nursery
13

in the dazomet (“BFT, dazomet”) and the nonfumigated bare fallow without amendment treatment (“BFT”), but only the dazomet treatment resulted in average seedling

diameters meeting the minimum acceptable diameter. In the 1995 trial, seedling diameters and heights were significantly smaller for the bark compost (“bark compost,

BFT”) and sewage sludge (“sludge, BFT”) treatments and did not meet the minimum

acceptable diameter or height. The reduced sizes in these two treatments may have

been related to higher seedbed densities. Although densities were not significantly different by statistical tests, trends in the data indicate that the bark compost treatment

(“bark compost, BFT”) had an additional 2.4 (per 0.09 m2) square foot, and the sludge

treatment (“sludge, BFT”) had an additional 5.8 seedlings per square foot (per 0.09

m2) compared to the dazomet (“BFT, dazomet”) and bare fallow without amendment

treatment (“BFT”). In the 1995 trial, average seedling height was significantly greater

in the dazomet treatment (“BFT, dazomet”) than all other treatments.

Dazomet fumigation left detectable preplant population levels of Fusarium and Pythium

in both trials. In 1993, average preplant population levels of Fusarium were significantly greater in the sewage sludge treatment (“sludge, BFT”) than in the dazomet (“BFT,

dazomet”), bark compost (“bark compost, BFT”), and bare fallow alone (“BFT”) treatments. In 1995, preplant population levels of Fusarium were significantly higher in the

Table 4—Effect of soil treatments on 2+0 Douglas-fir seedling density, diameter,

height, loss, and preplant levels of Fusarium and Pythium spp., at Coeur

d’Alene Nursery

Preplant Preplant

Treatmenta Densityb Diameter Height Lossc Fusariumd Pythiumd

No. per ft2 mm cm Percent - - - - - - CFU - - - - - -

1993-95:

BFT, dazomet 28.4 a 6.0 a 24.1 ae 64 a 70 a 12 a

Bark compost, 26.7 a 5.0 a 16.1 b 62 a 220 a 26 b

BFT

BFT 25.7 a 6.0 a 20.0 a 53 a 170 a 31 b

Sludge, BFT 26.6 a na na 54 a 2,180 b 41 b

BFT, pine 26.1 a 6.0 a 20.0 a na 1,330 ab na

needle mulch

1995-97:

BFT, dazomet 26.2 x 5.0 x 21.0 x na 110 x 3 a

Bark compost, 28.7 x 4.0 y 14.0 y na 340 xy 35 b

BFT

BFT 26.3 x 4.5 x 16.7 z na 530 y 38 b

Sludge, BFT 32.1 x 4.0 y 12.6 y na 470 xy 38 b

na = data not available.

a BFT = bare fallow with tilling; bark compost and sewage sludge (sludge) were soil amendments.

b 2+0 seedlings per square foot (per 0.09 m2).

c Percentage of seedlings missing at end of first growing season compared to the number of emerged

seedlings.

d Colony-forming units (CFU) per gram dry weight of soil.

e Means followed by the same letter are not significantly different at p ≤ 0.05: a-b for 1993-95 trial, x-z for

1995-97 trial.
14

bare fallow alone treatment (“BFT”) compared to the dazomet treatment (“BFT,

dazomet”). Dazomet fumigation significantly reduced preplant population levels of

Pythium compared to the other treatments for both trials. However, these differences

in population levels of Fusarium and Pythium had no apparent effect on seedling densities (table 4).

Average ponderosa pine seedling density in the bare fallow with sawdust treatment

(“S+N, BF”) was significantly greater than the other four treatments for both trials

(table 5). In the 1993 trial, ponderosa pine densities were not significantly different

among the remaining four treatments. In the 1995 trial, ponderosa pine densities in

the bare fallow with tilling (“BFT”) and MBC (“BF, MBC”) treatments were significantly

greater than those in the bare fallow alone (“BF”) and mushroom compost (“compost,

BF”) treatments. For lodgepole pine in the 1993 trial, significantly greater densities

occurred in the bare fallow alone (“BF”) and MBC treatments (“BF, MBC”), compared

to the other three treatments (“BFT”; “compost, BF”; and “S+N, BF”) (table 6).

Ponderosa pine seedling diameters in all treatments for both trials exceeded the

minimum acceptable 0.16 in (4 mm) (table 5). In the 1993 trial, ponderosa pine diameters in the mushroom compost (“compost, BF”) and sawdust treatments (“S+N, BF”)

were significantly smaller than in the other three treatments (“BF”; “BFT”; and “BF,

MBC”). In the case of the sawdust treatment (“S+N, BF”), smaller diameter was related to

greater seedbed density. In the 1995 trial, ponderosa pine diameters in the bare fallow

Lucky Peak Nursery

Table 5—Effect of soil treatments on 2+0 ponderosa pine seedling density,

diameter, height, mortality, and preplant levels of Fusarium and Pythium spp., at

Lucky Peak Nursery

Preplant Preplant

Treatmenta Densityb Diameter Height Mortalityc Fusariumd Pythiumd

No. per ft2 mm cm Percent - - - - - - CFU - - - - - -

1993-95:

BFT 20.6 ae 6.5 b 25.4 b 13.9 ab 500 a 25 a

BF 17.2 a 7.0 b 27.0 b 17.1 b 240 ab 38 a

Compost, BF 18.5 a 6.0 a 20.5 a 11.3 ab 230 ab 24 a

S+N, BF 22.4 b 6.0 a 22.5 a 7.9 ab 210 ab 21 a

BF, MBC 20.6 a 6.6 b 27.1 b 3.7 a 80 b 7 b

1995-97:

BFT 30.6 x 6.6 x 24.5 x na 380 x 21 x

BF 25.6 y 7.1 y 24.9 x na 490 x 29 x

Compost, BF 27.5 y 6.1 z 22.1 x na 430 x 32 x

S+N, BF 33.5 z 6.2 z 23.3 x na 340 x 27 x

BF, MBC 30.9 x 6.7 x 26.8 y na 60 y 4 y

na = data not available.

a BF = bare fallow, T = with tilling, S = sawdust soil amendment, N = ammonium nitrate, compost = mushroom compost soil amendment, MBC = fumigation with 67 percent methyl bromide/33 percent chloropicrin.

b 2+0 seedlings per square foot (per 0.09 m2).

c Percentage of seedlings killed by disease in the first growing season.

d Colony-forming units (CFU) per gram dry weight of soil.

e Means followed by the same letter are not significantly different at p ≤ 0.05: a-b for 1993-95 trial, x-z for

1995-97 trial.
15

with tilling (“BFT”) and MBC treatments (“BF, MBC”) were significantly larger than in

the mushroom compost (“compost, BF”) and sawdust (“S+N, BF”) treatments. Again,

the smaller diameter in the sawdust treatment (“S+N, BF”) was related to greater

density. The significantly larger diameter in the bare fallow alone treatment (“BF”) was

related to lower seedbed density. For lodgepole pine in the 1993 trial, diameters were

not significantly different across all treatments, and exceeded the minimum acceptable

0.16 in (4 mm) (table 6).

Average ponderosa pine seedling heights were well above the minimum acceptable

3.9 in (10 cm) for all treatments in both trials (table 5). In the 1993 trial, average

ponderosa pine heights were similar in the two bare fallow alone (“BF” and “BFT”)

and fumigation (“BF, MBC”) treatments, and these were significantly greater than

either soil amendment treatment (“compost, BF” and “S+N, BF”). In the 1995 trial,

average ponderosa pine height in the MBC treatment (“BF, MBC”) was greater than in

the other four treatments. For lodgepole pine in the 1993 trial, average heights in the

MBC treatment (“BF, MBC”) were greater than those in the bare fallow and mushroom

compost treatment (“compost, BF”). For all other treatment comparisons, lodgepole

pine heights were not significantly different (table 6).

Methyl bromide fumigation left detectable preplant population levels of Fusarium and

Pythium (tables 5 and 6). In the 1993 trial for the ponderosa pine block, MBC fumigation (“BF, MBC”) reduced preplant population levels of Fusarium compared to the bare

fallow with tilling treatment (“BFT”). In the 1995 trial, MBC reduced preplant population

levels of Fusarium compared to all other treatments. In the lodgepole pine block in the

1993 trial, preplant population levels of Fusarium were not significantly different for all

treatments. In the ponderosa pine block for both trials, MBC reduced preplant population levels of Pythium compared to all other treatments. Mortality was significantly less

Table 6—Effect of soil treatments on 2+0 lodgepole pine seedling diameter and

height, 1+0 seedling density, mortality, and preplant levels of Fusarium spp., at

Lucky Peak Nursery, 1993-95 trial

Preplant 

Treatmenta Density b Diameter Height Mortalityc Fusarium d

No. per ft2 mm cm Percent CFU

BFT 16.4 ae 4.7 a 16.8 ab 18.3 ab 460 a

BF 19.8 b 4.5 a 16.4 ab 16.0 ab 510 a

Compost, BF 13.5 a 4.4 a 13.4 b 29.7 b 650 a

S+N, BF 16.0 a 4.6 a 15.9 ab 21.0 ab 1,020 a

BF, MBC 19.4 b 4.8 a 18.9 a 11.0 a 110 a

a BF = bare fallow, T = with tilling, S = sawdust soil amendment, N = ammonium nitrate, compost = mushroom compost soil amendment, MBC = fumigation with 67 percent methyl bromide/33 percent chloropicrin.

b 1+0 seedlings per square foot (per 0.09 m2).

c Percentage of seedlings killed by disease in the first growing season.

d Colony-forming units (CFU) per gram dry weight of soil.

e Means followed by the same letter are not significantly different at p ≤ 0.05.
16

for the MBC treatment (“BF, MBC”) compared with the bare fallow alone treatment

(“BF”) in the ponderosa pine, and compared with the mushroom compost treatment

(“compost, BF”) in the lodgepole pine. Preplant population levels of Fusarium and

Pythium were not consistently related to mortality or seedling density.

For the 1993 trial in Shasta red fir, the conventional late sowing with soil covering the

seed (“BFT, rice straw mulch, late sow, soil over seed”) resulted in significantly lower

seedling density and greater mortality caused by disease, compared to all other treatments, which had seed sown early and covered with a nonsoil mulch (table 7). Seedling

diameters and heights were significantly greater in the same late sowing treatment,

compared to the treatment with sawdust winter mulch and hydromulch seed covering

(“BFT, sawdust mulch, early sow, hydromulch over seed”), and the treatment with no

winter mulch and hydromulch seed covering (“BFT, bare, early sow, hydromulch over

seed”). This increase in seedling size was apparently related to the lower density in

the late sowing treatment. None of the other treatments had significant effects on

seedling size.

Placerville Nursery

Table 7—Effect of soil treatments on 2+0 Shasta red fir diameter, height, and

1+0 density, mortality, and preplant population levels of Fusarium spp., at

Placerville Nursery, 1993-95 trial

Preplant

Treatmenta Densityb Diameter Height MortalitycFusariumd

No. per ft2 mm cm Percent CFU

BFT, rice straw mulch, 18.5 ae 5.1 a 16.5 a 41 a 5,290 a

late sow, soil 

over seed 

BFT, rice straw mulch, 29.9 b 5.0 ab 15.6 ab 12 b 4,460 a

early sow, 

hydromulch over seed

BFT, sawdust mulch, 26.7 b 4.6 b 14.4 b 15 b 3,820 a

early sow, 

hydromulch over seed 

BFT, sawdust mulch, 27.8 b 4.8 ab 15.5 ab 15 b 3,240 a

early sow, 

sawdust over seed 

BFT, pine needle mulch, 26.4 b 5.0 ab 15.4 ab 11 b 4,710 a

early sow, 

hydromulch over seed 

BFT, hydromulch, early 25.9 b 4.9 ab 15.7 ab 13 b 5,410 a

sow, hydromulch 

over seed 

BFT, bare, early sow, 24.1 b 4.6 b 14.4 b 16 b 3,230 a

hydromulch over seed

a BFT = bare fallow with tilling; mulches were used to cover the soil over winter.

b 1+0 Seedlings per square foot (per 0.09 m2).

c Percentage of seedlings killed by disease in the first growing season.

d Colony-forming units (CFU) per gram dry weight of soil.

e Means followed by the same letter are not significantly different at p ≤ 0.05.
17

In both the 1993 and 1995 trials, all treatments (with the exception of the crop failures

with vetch cover crop without fumigation in the second trial) yielded seedlings with

average diameters exceeding the acceptable minimum 0.16 in (4 mm), and average

heights exceeding the acceptable minimum 5.1 in (13 cm) for Shasta red fir (tables 7

and 8).

In the 1995 trial, there were significant treatment effects on density, but differences in

seedling sizes were apparently density-dependent effects (the greater the density, the

smaller the seedlings) (table 8). The treatment with sawdust winter mulch, early sowing,

and sawdust covering the seed (“BFT, sawdust mulch, early sow, sawdust over seed”)

had significantly greater density than all other treatments, including those with MBC

fumigation. The treatment with the next highest density was sawdust winter mulch,

early sowing, and soil covering the seed (“BFT, sawdust mulch, early sow, soil over

seed”). This treatment was significantly different from the remaining treatments, including those with MBC fumigation. The vetch cover crop treatments without fumigation

(“BFT, vetch cover crop, late sow, sawdust over seed” and “BFT, vetch cover crop, late

Table 8—Effect of soil treatments on 2+0 Shasta red fir seedling density, diameter, height, loss, and preplant population levels of Fusarium and Pythium spp. at Placerville Nursery, 1995-97 trial

Preplant Preplant 

Treatmenta Density b Diameter Height Lossc Fusarium d Pythiumd

No. per ft2 mm cm No. per ft2 - - - - - - CFU - - - - - -

BFT, sawdust mulch, early sow, 39.7 ae 4.7 c 13.2 c 31.8 a 3,860 ab 49 a

sawdust over seed

BFT, sawdust mulch, early sow, 25.2 b 5.0 c 14.1 c 39.3 a 2,650 bc 55 a

soil over seed

BFT, MBC, vetch cover crop, late 17.9 c 5.8 ab 15.8 b 17.2 b 3,810 ab 71 a

sow, sawdust over seed

BFT, MBC, vetch cover crop, late 11.2 c 5.8 ab 15.1 b 33.8 a 930 c 72 a

sow, soil over seed

BFT, hydromulch, late sow, 16.3 c 5.7 b 16.7 a 17.6 b 650 c 67 a

sawdust over seed

BFT, hydromulch, late sow, 4.4 d 6.0 a 15.9 ab 50.0 c 5,770 a 64 a

soil over seed

BFT, vetch cover crop, late na na na na 690 c 71 a

sow, soil over seed

BFT, vetch cover crop, late na na na na 990 c 57 a

sow, sawdust over seed

na = data not available owing to crop failure.

a BFT = bare fallow with tilling; mulches and cover crop were used to cover the soil over winter; MBC = fumigation with 67 percent methyl bromide/33 percent chloropicrin.

b 2+0 seedlings per square foot (per 0.09 m2).

c Difference between maximum and final 2+0 density in number of seedlings per square foot.

d Colony-forming units (CFU) per gram dry weight of soil.

e Means followed by the same letter are not significantly different at p ≤ 0.05.
18

sow, soil over seed”) were crop failures, with seedling emergence too low to count.

The treatment with hydromulch winter mulch, late sowing, and soil covering the seed

(“BFT, hydromulch, late sow, soil over seed”) had significantly lower density than all

other treatments.

In the 1993 trial, preplant population levels of Fusarium spp. were not significantly different among all treatments (table 7). In the 1995 trial, preplant population levels of

Pythium spp. were not significantly different among all treatments (table 8). In the

1995 trial, preplant population levels of Fusarium spp. were significantly lower in four

treatments, including the crop failures (“BFT, MBC, vetch cover crop, late sow, soil

over seed”; “BFT, hydromulch, late sow, sawdust over seed”; “BFT, vetch cover crop,

late sow, soil over seed”; and “BFT, vetch cover crop, late sow, sawdust over seed”),

compared to two treatments, including the one with the highest seedling density

(“BFT, sawdust mulch, early sow, sawdust over seed” and “BFT, MBC, vetch cover

crop, late sow, sawdust over seed”) (table 8). Preplant population levels of species of

Pythium and Fusarium did not correlate with seedling density or loss after emergence.

Shasta red fir seedling density, diameter, and height were not significantly different in

any of the treatments after the first growing season in the 1993 trial (table 9). Emergence

was uneven for all treatments, with many patches without seedlings, resulting in high

variation between measurement plots within treatments. Although not statistically significant, trends in the data suggest that the bare fallow with tilling and hydromulch

treatment (“BFT, hydromulch over seed”) resulted in an average of two seedlings per

square foot (per 0.09 m2) more than the bare fallow with tilling (“BFT, soil over seed”)

and the bare fallow with tilling and composted redwood mulch covering seed (“BFT,

redwood mulch over seed”) treatments (table 9). Differences between treatments in

Table 9—Effect of soil treatments on 1+0 Shasta red fir density, diameter, and

height, mortality, and preplant population level of Fusarium spp., at Humboldt

Nursery, 1993-95 trial 

Preplant

Treatmenta Densityb Diameter Height Mortalityc Fusariumd

No. per ft2 mm cm Percent CFU

BFT, soil over seed 19.3 ae 2.2 a 4.0 a 7.3 a 530 a

BFT, redwood mulch 19.6 a 2.3 a 4.9 a 5.0 a 1,180 a

over seed

BFT, hydromulch 21.6 a 2.1 a 4.0 a 4.7 a 850 a

over seed

BFT, MBC, soil 20.1 a 2.2 a 3.8 a 0. a 110 a

over seed

BFT, dazomet, soil 20.1 a 2.4 a 4.2 a 0.6 a 30 a

over seed 

a BFT = bare fallow with tilling, MBC = fumigation with 67 percent methyl bromide/33 percent chloropicrin, 

redwood mulch = composted redwood chips, redwood mulch and hydromulch used to cover seed.

b 1+0 seedlings per square foot (per 0.09 m2).

c Percentage of seedlings killed by disease in the first growing season.

d Colony-forming units (CFU) per gram dry weight of soil.

e Means followed by the same letter are not significantly different at p ≤ 0.05.

Humboldt Nursery
19

population levels of Fusarium spp. and in seedling losses also were not significant.

Data for the 2+0 seedlings were not available. Whether treatments resulted in 2+0

seedlings meeting the minimum acceptable diameters and heights was not recorded.

In the 1995 trial at Humboldt Nursery, treatments had no significant effect on density

of 2+0 Shasta red fir seedlings (table 10). However, trends in the data suggest that

density with the MBC treatment (“BFT, MBC”) was two seedlings per square foot (per

0.09 m2) fewer than the density in the bare fallow and herbicide (“BF, herbicide”) and

the bare fallow with tilling and biocontrol (“BFT, biocontrol”) treatments (table 10).

Seed treated with phosphate buffer, including the biocontrol seed treatment, apparently flowed through the seeder more smoothly, and slightly more seed was applied per

unit area, as evidenced by the seeder running out of seed several feet short of the

expected end of the seeding run for both treatments (“BFT, phosphate buffer” and

“BFT, biocontrol”). Differences in seedling diameter and height were statistically significant. Biologically significant differences are indicated in that bare fallow with tilling and

phosphate buffer seed treatment (“BFT, phosphate buffer”) and bare fallow alone

(“BF”) treatments resulted in diameters 0.5 mm less than that after MBC fumigation

(“BFT, MBC”), but this may have been density related. Similarly, some heights differed

by at least 2 cm, with the MBC treatment (“BFT, MBC”) resulting in taller seedlings

than the bare fallow with tilling and phosphate buffer seed treatment (“BFT, phosphate

buffer”). However, trends in the data suggest the MBC treatment resulted in the lowest

density (although not significant statistically), and the larger morphology may be a

density effect. All treatments resulted in seedlings meeting the minimum acceptable

diameter of 0.16 in (4 mm) for Shasta red fir. However, all treatments failed to produce

seedlings with the minimum acceptable height of 5.1 in (13 cm). Fumigation with MBC

(“BFT, MBC”) reduced preplant population levels of Fusarium spp. to undetectable levels.

Preplant population levels of species of Pythium and Fusarium were significantly

lower in the fumigation treatment (“BFT, MBC”) than in all other treatments, but did not

correlate with seedling density or seedling loss.

Table 10—Effect of soil treatments on 2+0 Shasta red fir density, diameter,

height, first year loss, and preplant population levels of Fusarium spp., and

Pythium spp. at Humboldt Nursery, 1995-97 trial

Preplant Preplant

Treatmenta Density b Diameter Height Lossc Fusarium d Pythium d

No. per ft2 mm cm No. per ft2 - - - - - - CFU - - - - - -

BF, herbicide 7.2 ae 4.4 ab 11.1 ab 4.0 a 1,330 a 286 a

BFT, phosphate 6.6 a 4.0 c 9.3 c 1.8 a 1,430 a 272 a

buffer

BF 6.9 a 4.1 bc 10.4 abc 2.2 a 1,300 a 232 a

BFT, MBC 5.2 a 4.6 a 11.4 a 4.1 a 0 b 3 b

BFT, biocontrol 7.3 a 4.2 bc 10.7 ab 3.1 a 1,330 a 323 a

a BF = bare fallow, T = with tilling, MBC = fumigation with 67 percent methyl bromide/33 percent 

chloropicrin; phosphate buffer and biocontrol were seed treatments.

b 2+0 seedlings per square foot (per 0.09 m2).

c Difference in numbers of seedlings per square foot emerged and final stand.

d Colony-forming units (CFU) per gram dry weight of soil.

e Means followed by the same letter are not significantly different at p ≤ 0.05.
20

Seedling diameter (and proportionate root development) is the most important measure

of seedling quality and outplanting performance specified by reforestation customers

in the West. Seedbed density determines the cost per unit area of growing space, and

a difference of two seedlings per square foot (per 0.09 m2) can mean a difference in

production of millions of seedlings. For these reasons, seedbed density and seedling

diameter are the two measures of seedling production quality that provide the best

basis for comparison between standard soil fumigation and nonfumigation alternatives.

Seedling diameter is inversely related to seedbed density; i.e., when all other conditions are equal, higher seedbed density produces smaller seedlings. Seedbed density

reflects a combination of factors including seed quality, sowing density, losses owing

to disease, and nondisease mortality. In our study, mortality losses owing to postemergence disease were determined, whereas losses owing to preemergence mortality

were not determined. Final seedbed densities reflect a combination of the sowing

density, the undetermined preemergence mortality, postemergence mortality, and

other incidental losses.

Seedling diameters, and to a lesser extent, heights, reflected density-dependent effects

in some trials. Where densities differ, interpretation of differences in seedling size can

be confounded. For example, at Bend Pine Nursery in both trials, very low ponderosa

pine density in the pea cover crop without fumigation treatment (“S+N, Pea CC”) was

associated with significantly larger seedling diameter (table 1). Similarly, at J. Herbert

Stone Nursery in the 1995 trial, very low Douglas-fir density in the bare fallow with

tilling/no-sawdust treatment (“BFT”) was associated with significantly larger seedling

diameter (table 3). Conversely, at Lucky Peak Nursery in both trials, significantly

greater ponderosa pine density in the bare fallow/sawdust treatment (“S+N, BF”) was

associated with significantly smaller seedling diameter (table 5). Similarly, at Placerville

Nursery in the 1995 trial, significantly greater densities in the bare fallow with tilling/

sawdust mulch/early sow treatments (“BFT, sawdust mulch, early sow, sawdust over

seed” and “BFT, sawdust mulch, early sow, soil over seed”) were associated with

significantly smaller seedling diameters (table 8).

Growers who routinely fumigate have tailored watering, fertilizing, and other cultural

practices to fumigation as a key component of the cultural system. While alternative

systems are under development, seedlings grown under alternative systems may be

smaller or less uniform than those grown in fumigated soil. Modifications in irrigation,

fertilization, and other cultural practices, however, should improve seedling production

and quality, while reducing or eliminating the need for soil fumigation.

Strategies for preventing infection of seedlings by soil-borne pathogens include

reducing pathogen populations and providing barriers to infection (Agrios 1969). In

our tests, treatments that reduced pathogen populations included bare fallowing with

or without tilling, and incorporation of slowly decomposing organic soil amendments.

Bare fallowing depletes the food base for facultatively saprobic pathogens (Bloomberg

1965, 1985). Periodic tilling may mechanically damage pathogen propagules as well

as bring propagules to the soil surface where they may be exposed to desiccation and

lethal temperatures.

Discussion
Weeds can provide a food base for pathogens, shade the soil surface, and may

diminish the beneficial effects of bare fallowing, as demonstrated in the 1993-95 trials

at J. Herbert Stone Nursery. Severe weed infestation before and after sowing in the

bare fallow without tilling treatment (“S+N, BF”) corresponded to significantly higher

levels of soil-borne Fusarium and significantly reduced ponderosa pine densities compared to all other treatments, with a similar trend for Douglas-fir (tables 2 and 3). The

apparent low mortality of ponderosa pine in this treatment may have reflected the difficulty of detecting seedlings emerging in plots with a dense weed cover. Mortality

of Douglas-fir also may have been underestimated in this treatment because of the

difficulty of finding dead emerged seedlings in the dense weed cover. The trend

toward smaller seedlings in both species in this treatment in the 1993 trial compared

to the other treatments, also was probably related to the weed infestation, and may

reflect combined competition with the weeds and injurious but nonlethal root

colonization by Fusarium or other pathogens.

For some nurseries, conventional practices can aggravate disease problems and may

make chemical fumigation necessary. Incorporation of a cover crop as a green

manure before sowing conifer seed results in increased levels of soil Fusarium spp.,

Pythium spp., and other micro-organisms (Hansen et al. 1990). High soil populations

of Fusarium and Pythium can lead to severe first-year seedling mortality (Hansen et

al. 1990, James et al. 1996, Stone and Hansen 1994). Our tests showed this at Bend

Pine Nursery, in the pea cover crop without fumigation treatment (“S+N, pea CC”)

(table 1). More seedlings meeting size specifications can be produced from bare fallow treatment than from treatments using cover crops, whether fumigated or not

(Hansen et al. 1990).

In our tests, we found that population levels of Fusarium and Pythium spp. were not

consistently related to first-year mortality or seedbed density. This is not surprising

considering the diversity of populations of these potential pathogens often characteristic

of forest nursery soils (Bloomberg 1976, Bloomberg and Lock 1972). Soil populations

of F. oxysporum, the major species consistently isolated from the test nurseries, often

yield both pathogenic and saprobic isolates (Gordon and Martyn 1997, Gordon and

Okamoto 1992, James et al. 2000) that are morphologically similar. Therefore, our

Fusarium assays yielded populations of both pathogens and nonpathogens, and the

proportion of populations capable of causing disease was unknown.

Slowly decomposing organic soil amendments, such as aged sawdust or compost,

may tend to favor the growth of competitive soil saprobes to the detriment of soil

pathogens, which use simple organic substrates. Amendments with high carbon to

N ratios may be difficult for facultatively saprobic pathogens to use as a food base,

which would promote other soil saprobes, enabling them to outcompete pathogens. A

low proportion of N will eventually stunt tree seedlings, and they require supplemental

N. Delay in N application until well after seedling germination may help limit the

growth of pathogens until after seedlings are vigorous enough to withstand infection.

Care must be taken to provide N early enough to prevent stunting.

Sawdust soil amendments had a beneficial effect in some of our trials. As a soil

amendment, sawdust and ammonium nitrate fertilizer are commonly incorporated at

the same time, but when N application was delayed until after seedling germination

(“S, BFT, delayed N”), higher seedling densities tended to result at J. Herbert Stone

21
Nursery (tables 2 and 3). Treatment without sawdust amendment (“BFT”) resulted in

significantly lower ponderosa pine and Douglas-fir densities at J. Herbert Stone Nursery

in the 1995 trial (tables 2 and 3). Sawdust amendment (“S+N, BF”) resulted in higher

seedbed densities of ponderosa pine in both the 1993 and 1995 trials at Lucky Peak

Nursery (table 5), but not for lodgepole pine. Incorporation of other organic amendments had variable results. For example, at Coeur d’Alene Nursery, addition of bark

compost (“bark compost, BFT”) or sewage sludge (“sludge, BFT”) tended to increase

Douglas-fir density only in the 1995 trial, and at Lucky Peak Nursery, addition of

mushroom compost (“compost, BF”) did not increase density for ponderosa pine or

lodgepole pine compared to the unamended treatments.

Treatments that provide barriers to infection include early sowing, and shallow sowing

with mulching. Early sowing provides a temporal barrier by allowing conifer seed to

germinate, and the radicle to become suberized, before soil temperatures reach optimum for pathogens. Covering the seed with mulch, rather than soil, provides a spatial

barrier that partially separates the germinating seedling from soil pathogens. Slowly

decomposing mulch, like sawdust, also may provide a biological barrier, where competitive saprobes displace pathogens. For example, covering seed with sawdust or

hydromulch rather than soil resulted in increased density of Shasta red fir at Placerville

Nursery (tables 7 and 8).

For nurseries in this study that do not routinely use cover crops, fumigation appeared

to contribute little toward increased seedling survival or size. For example, at J. Herbert

Stone Nursery, dazomet fumigation (“S+N, BFT, dazomet”) did not increase seedling

density, height, or diameter for Douglas-fir or ponderosa pine compared to nonfumigated bare fallow with tilling and sawdust amendment treatments (“S+N, BFT” and “S,

BFT, delayed N”) (tables 2 and 3). At Coeur d’Alene Nursery, dazomet fumigation

resulted only in a consistent increase in Douglas-fir seedling height, compared to nonfumigated bare fallow treatments (table 4). At Lucky Peak Nursery, bare fallow/sawdust (“S+N, BF”) and bare fallow with tilling (“BFT”) treatments resulted in similar or

greater density and diameter of ponderosa pine compared to MBC fumigation treatment (“BF, MBC”) (table 5). For lodgepole pine at Lucky Peak Nursery, bare fallowing

(“BF”) resulted in similar seedling density and diameter as the MBC treatment (“BF,

MBC”) (table 6). At Humboldt Nursery, fumigation with dazomet or MBC (“BFT,

dazomet” or “BFT, MBC”) did not significantly increase Shasta red fir seedling density

in both the 1993 and 1995 trials (tables 9 and 10). In contrast, at Placerville Nursery,

summer bare fallowing with tilling followed by vetch cover crop without MBC fumigation

(“BFT, vetch cover crop, late sow, soil over seed” and “BFT, vetch cover crop, late sow,

sawdust over seed”) resulted in failure of the Shasta red fir crop owing to disease.

However, nonfumigated alternatives without cover crop at Placerville Nursery (summer

bare fallowing with tilling, followed by sawdust as winter mulch and early sowing of

Shasta red fir seed, “BFT, sawdust mulch, early sow, sawdust over seed” and “BFT,

sawdust mulch, early sow, soil over seed”) resulted in greater seedbed densities than

the vetch cover crop with MBC treatments (“BFT, MBC, vetch cover crop, late sow,

sawdust over seed” and “BFT, MBC, vetch cover crop, late sow, soil over seed”) (table 8).

Our tests indicated that at many nurseries, modification of cultural practices can greatly

reduce the need for chemical fumigation. At every nursery in our tests, some combination of bare fallowing and other cultural practices produced equivalent or greater

seedling numbers and diameters compared to fumigation. Similar results have been

22
23

reported for two Southern nurseries (Barnard et al. 1997) and in a previous study at

Bend Pine nursery (Stone and Hansen 1994). The best treatments were different at

each nursery, and sometimes varied between trials and species at the same nursery.

Including bare fallowing in the rotation of crops can reduce the need for fumigation,

although many industrial growers use fumigants periodically to help avoid unexpected

losses owing to disease. Given the ever-shrinking list of registered fungicides and herbicides, and the high value and limited source of genetically improved stock, a nursery

cannot afford unexpected disease development.3

In nursery operations where enough space is available to make bare fallowing feasible, the advantages include immediate implementation, lack of regulatory restrictions,

less expense compared to routine chemical fumigation, and freedom from related

human health and environmental concerns associated with fumigation. Unfortunately,

alternatives to preplant soil fumigation may alleviate one problem and aggravate

another, e.g., bare fallowing may increase soil erosion by wind and water. In many

cases, compensating strategies are available, e.g., soil coverings that stabilize the soil

surface and abate the effects of wind.

Fine-tuning of alternative practices may be necessary. For example, at Placerville

Nursery, we had initial success with overwinter soil mulches and early sowing without

the need for chemical fumigation to produce high-quality seedlings. However, after a

few years, the nursery suffered excessive losses caused by Macrophomina phaseolina,

a pathogen previously controlled by methyl bromide/chloropicrin (Frankel et al. 1999).

Appropriate cultural practices are under continual development to address the specific

needs of individual nurseries. Long-term reductions in dependence on chemical fumigation depend on development of cultural alternatives that favor microbial antagonists

of pathogens and promote diverse soil microbial communities.

In our study, bare-fallowing with tilling, bare fallow without tilling and with weed

control, early sowing, sawdust with N soil amendment, sawdust soil amendment with

N application delayed until after conifer germination, and nonsoil seed coverings were

treatments that produced conifer seedlings with similar or better densities and diameters compared to chemical fumigation. The same combinations of treatments were not

the most effective among nurseries, and even among crops at the same nursery.

Clearly, there is no one alternative to chemical fumigation, and each nursery needs to

adapt cultural practices that are most effective for the given conditions and crop

requirements.

3 Littke, W. 2003. Personal communication. Forest pathologist,

Nursery and Forest Regeneration Research and Development,

Weyerhaeuser Company, WTC 1A3, 32901 Weyerhaeuser Way

South, Federal Way, WA 98001.
24

The Special Technology Development Program of the U.S. Department of Agriculture,

Forest Service, Forest Health Protection, funded this project, R6-93-01, Alternatives

to Fumigation, Phase 1, during fiscal years 1993 through 1998. The project was

completed in fiscal year 1999. We are grateful to the participating nurseries for their

cooperation.

We thank Dr. Paige Axelrood, plant pathologist, Microbiology and Plant Pathology, BC

Research Inc., for providing the bacterial mixture for the biocontrol seed treatment for

the 1995 trial at Humboldt Nursery.

We also thank our reviewers whose suggestions greatly improved this manuscript:

Willis Littke, forest pathologist, Nursery and Forest Regeneration Research and

Development, Weyerhaeuser Corporation; Michelle Cram, plant pathologist, Southern

Region, Forest Health Protection, USDA Forest Service; Walt Thies, research plant

pathologist, Corvallis Forestry Sciences Laboratory, Pacific Northwest Research

Station, USDA Forest Service; and Timothy Max, Station statistician, Portland Forestry

Sciences Laboratory, Pacific Northwest Research Station, USDA Forest Service.

To convert: Multiply by: To find:

Inches (in) 25.4 Millimeters 

Inches (in) 2.54 Centimeters 

Feet (ft) .3048 Meters 

Acres .405 Hectares 

Square feet (ft2) .0929 Square meters

Pounds per acre (lb/acre) 1.12 Kilograms per hectare

Cubic yards per acre (yd3/acre) 1.89 Cubic meters per hectare

Millimeters (mm) .0394 Inches

Centimeters (cm) .394 Inches

Milligrams (mg) .000035 Ounces (oz)

Grams (g) .0352 Ounces (oz)

Milliliters (mL) .03378 Fluid ounces (fl oz)

Liters (L) 1.057 Quart (qt)

Milliequivalents per 100 grams .284 Milliequivalents per ounce

(meq/100 g)

Degrees Celsius (°C) 1.8 and add 32 Degrees Fahrenheit (°F)

Agrios, G.N. 1969. Control of plant diseases. In: Agrios, G.N. Plant pathology. New

York: Academic Press. Chapter 8.

Axelrood, P.E.; Neumann, M.; Trotter, D. [et al.] 1995. Seed-borne Fusarium on

Douglas-fir: pathogenicity and seed stratification method to decrease Fusarium

contamination. New Forests. 9: 35-51.

Barnard, E.L.; Kannwischer-Mitchell, M.E.; Mitchell, D.J.; Fraedrich, S.W. 1997.

Development and field performance of slash and loblolly pine seedlings produced

in fumigated nursery seedbeds and seedbeds amended with organic residues. In:

James, R.L., ed. Proceedings, third meeting of IUFRO working party S7.03-04, diseases and insects in forest nurseries. Forest Health Protection Report 97-4. Coeur

d’Alene, ID: U.S. Department of Agriculture, Forest Service, Northern Region: 86-91.

Acknowledgments

Metric and English

Equivalents

Literature Cited
Bloomberg, W.J. 1965. The effect of chemical sterilization on the fungus population of

soil in relation to root disease of Douglas-fir seedlings. Forestry Chronicle. 41: 182-187.

Bloomberg, W.J. 1976. Distribution and pathogenicity of Fusarium oxysporum in a

forest nursery soil. Phytopathology. 66: 1090-1092.

Bloomberg, W.J. 1985. The epidemiology of forest nursery diseases. Annual Review

of Phytopathology. 23: 83-96.

Bloomberg, W.J.; Lock, W. 1972. Strain differences in Fusarium oxysporum causing

diseases of Douglas-fir seedlings. Phytopathology. 62: 481-485.

Boyd, R.J. 1971. Effects of soil fumigation on production of conifer nursery stock at

two northern Rocky Mountain nurseries. Res. Pap. INT-91. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment

Station. 19 p.

Cordell, C.E. 1983. Effective soil fumigation. In: Proceedings, 1982 Southern nursery

conferences. R8-TP-4. Atlanta, GA: U.S. Department of Agriculture, Forest Service,

Southern Region: 196-201.

Cordell, C.E.; Kelley, W.D.; Smith, R.S. 1989. Integrated nursery pest management.

In: Cordell, C.E.; Anderson, R.L.; Hoffard, W.H. [et al.], tech. coords. Forest nursery

pests. Agric. Handb. 680. Washington, DC: U.S. Department of Agriculture, Forest

Service: 5-13.

Danielson, R.M.; Davey, C.B. 1969. Microbial recolonization of a fumigated nursery

soil. Forest Science. 15(4): 368-380.

Frankel, S.J.; Stone, J.K.; Cram, M.M. [et al.]. 1999. Progress towards alternatives

to methyl bromide fumigation in bare-root forest nurseries in the United States. In:

Methyl Bromide Alternatives Newsletter. Baltimore, MD: U.S. Department of

Agriculture, Agricultural Research Service. 5(3): 10-12.

Gandy, D.G.; Chanter, D.O. 1976. Some effects of time, temperature of treatment,

and fumigant concentration on the fungicidal properties of methyl bromide. Anals

of Applied Biology. 82: 279-290.

Gordon, T.R.; Martyn, R.D. 1997. The evolutionary biology of Fusarium oxysporum.

Annual Review of Phytopathology. 35: 111-128.

Gordon, T.R.; Okamoto, D. 1992. Population structure and the relationship between

pathogenic and nonpathogenic strains of Fusarium oxysporum. Phytopathology. 82:

73-77.

Hamm, P.B. 1990. Fusarium hypocotyl rot. In: Hamm, P.B.; Campbell, S.J.; Hansen,

E.M., eds. Growing healthy seedlings: identification and management of pests in

Northwest forest nurseries. Spec. Publ. 19. Corvallis, OR: College of Forestry,

Oregon State University: 6-7.

Hansen, E.M.; Myrold, D.D.; Hamm, P.B. 1990. Effects of soil fumigation and cover

crops on potential pathogens, microbial activity, nitrogen availability, and seedling

quality in conifer nurseries. Phytopathology. 80: 698-704.

25
26

Hildebrand, D.M.; Stone, J.K.; James, R.L. [et al.]. 1995. Alternatives to chemical

fumigation technology development project: preliminary results. In: Landis, T.D.;

Cregg, B., tech. coords. National proceedings, Forest and Conservation Nursery

Associations—1995. Gen. Tech. Rep. PNW-GTR-365. Portland, OR: U.S. Department

of Agriculture, Forest Service, Pacific Northwest Research Station: 15-22.

James, R.L.; Beall, K. 1999. An evaluation of the effects of dazomet on soil-borne

diseases and conifer seedling production, USDA Forest Service Lucky Peak

Nursery, Boise, Idaho. Report 99-9. Coeur d’Alene, ID: U.S. Department of

Agriculture, Forest Service, Northern Region, Forest Health Protection. 15 p.

James, R.L.; Dumroese, R.K.; Wenny, D.L. 1991. Fusarium diseases of conifer

seedlings. In: Sutherland, J.R.; Glover, S.G., eds. Proceedings, first meeting of

IUFRO Working Party S2.07-09 (diseases and insects in forest nurseries). Info.

Rep. BC-X-331. Victoria, BC: Forestry Canada, Pacific and Yukon Region: 181-190.

James, R.L.; Page-Dumroese, D.S.; Kimball, S.K.; Omi, S. 1996. Effects of Brassica

cover crop, organic amendment, fallowing, and soil fumigation on production of

bare-root Douglas-fir seedlings, USDA Forest Service Nursery, Coeur d’Alene,

Idaho. Report 96-5. Coeur d’Alene, ID: U.S. Department of Agriculture, Forest

Service, Northern Region, Forest Health Protection. 16 p.

James, R.L.; Perez, R.; Dumroese, R.K.; Wenny, D.L. 2000. Virulence of Fusarium

oxysporum on Dougals-fir germinants: comparison of isolates from nursery soil

and roots of healthy and diseased seedlings. In: Lilja, A.; Sutherland, J.R., eds.

Proceedings, fourth meeting of IUFRO working party 7.03.04 (diseases and

insects in forest nurseries). Res. Pap. 781. Vantaa, Finland: Finnish Forest Research

Institute: 49-64.

Kelpsas, B.R.; Campbell, S.J. 1994. Influence of mechanical incorporation method

on dazomet distribution in conifer nursery soil. Tree Planters’ Notes. 45(2): 53-57.

Komada, H. 1975. Development of a selective medium for quantitative isolation of

Fusarium oxysporum from natural soil. Review of Plant Protection Research. 8:

114-125.

Munnecke, D.E.; Van Gundy, S.D. 1979. Movement of fumigants in soil, dosage

responses and differential effects. Annual Review of Phytopathology. 17: 405-429.

Sabin, T.E.; Stafford, S.G. 1990. Assessing the need for transformation of response

variables. Spec. Publ. 20. Corvallis, OR: College of Forestry, Oregon State

University. 31 p.

SAS Institute. 1992. The SAS system for Windows, version 3.10. Cary, NC: SAS

Institute, Inc.

Smith, R.S., Jr.; Fraedrich, S.W. 1993. Back to the future—pest management without

methyl bromide. Tree Planters’ Notes. 44(3): 87-90.

Stone, J.K.; Hansen, E. 1994. Green manure effects on soil-borne pathogens. In:

Landis, T.D., tech. coord. Proceedings, Northeastern and Intermountain Forest

and Conservation Nursery Association meeting. Gen. Tech. Rep. RMRS-243. Fort

Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain

Research Station: 57-64.
Stone, J.K.; Hildebrand, D.M.; James, R.L.; Frankel, S.J. 1997. Alternatives to

chemical fumigation in bare-root forest nurseries: effects on pathogen levels, and

seedling density, mortality and quality. In: James, R.L., ed. Proceedings, third meeting of IUFRO working party S7.03-04 (diseases and insects in forest nurseries).

Forest Health Protection Report 97-4. Coeur d’Alene, ID: U.S. Department of

Agriculture, Forest Service, Northern Region: 59-69.

Systat. 1998. Systat version 8.0. Evanston, IL: SPSS Inc.

Tanaka, Y.; Russell, K.W.; Linderman, R.G. 1986. Fumigation effect on soil-borne

pathogens, mycorrhizae, and growth of Douglas-fir seedlings. In: Proceedings,

meeting of combined Western Forest Nursery Council and Intermountain Nursery

Association. Gen. Tech. Rep. RM-137. Fort Collins, CO: U.S. Department of

Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station:

147-152.

U.S. Department of Agriculture [USDA]. 1999. Administration extends deadline on

methyl bromide ban to 2005. In: Methyl Bromide Alternatives Newsletter. Baltimore,

MD: Agricultural Research Service. 5(1): 1. http://www.ars.usda.gov/is/np/mba/

mebrhp.htm. (December 2003).

U.S. Department of Agriculture [USDA]. 2000. What if? The strawberry industry

without methyl bromide. In Methyl Bromide Alternatives Newsletter. Baltimore, MD:

Agricultural Research Service. 6(3): 1-3. http://www.ars.usda.gov/is/np/mba/

mebrhp.htm. (December 2003).

Vaartaja, O. 1967. Re-infestation of sterilized nursery seedbeds by fungi. Canadian

Journal of Microbiology. 13: 771-776.

Wildman, H. 1991. Lithium chloride as a selective inhibitor of Trichoderma species on

soil isolation plates. Mycological Research. 95: 1364-1368.

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The Forest Service of the U.S. Department of Agriculture is

dedicated to the principle of multiple use management of the

Nation’s forest resources for sustained yields of wood, water,

forage, wildlife, and recreation. Through forestry research,

cooperation with the States and private forest owners, and

management of the National Forests and National Grasslands,

it strives–as directed by Congress–to provide increasingly

greater Service to a growing Nation.

The United States Department of Agriculture (USDA) prohibits

discrimination in all its programs and activities on the basis of

race, color, national origin, gender, religion, age, disability,

political beliefs, sexual orientation, or marital or family status.

(Not all prohibited bases apply to all programs.) Persons with

disabilities who require alternative means for communication

of program information (Braille, large print, audiotape, etc.)

should contact USDA’s TARGET Center at (202) 720-2600

(voice and TDD).

To file a complaint of discrimination, write USDA, Director,

Office of Civil Rights, Room 326-W, Whitten Building, 14th

and Independence Avenue, SW, Washington, DC 20250-9410

or call (202) 720-5964 (voice and TDD). USDA is an equal

opportunity provider and employer.

Pacific Northwest Research Station

Web site http://www.fs.fed.us/pnw

Telephone (503) 808-2592

Publication requests (503) 808-2138

FAX (503) 808-2130

E-mail pnw_pnwpubs@fs.fed.us

Mailing address Publications Distribution

Pacific Northwest Research Station

P.O. Box 3890

Portland, OR 97208-3890
U.S. Department of Agriculture

Pacific Northwest Research Station

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P.O. Box 3890

Portland, OR 97208-3890

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