\

OIT mum

And Their (]ontrol

Agriculture Handbook No. 343

Agricultural Research Service

UNITED STATES DEPARTMENT OF AGRICULTURE

In cooperation with

Iowa Agriculture and Home Economics Experiment Station

Contents

Page

Fungus diseases 1

Crown rust 1

Stem rust 4

Smuts 4

Septoria blight 5

Helminthosporium diseases 5

Helminthosporium leaf blotch 5

Helminthosporium culm rot 6

Victoria blight 6

Fusarium blight 6

Root rot 6

Minor fungus diseases 7

Downy mildew 7

Powdery mildew 7

Ergot 7

Anthracnose 7

Snow mold 7

Bacterial diseases 8

Halo blight 8

Bacterial stripe blight 8

Virus diseases 8

Soilbome oat mosaic 8

Barley yellow dwarf 9

Blue dwarf 9

Nonparasitic diseases 9

Blast 9

Nonparasitic leaf spot 10

Gray speck 10

Minor nonparasitic diseases 10

Nutritional diseases 10

Miscellaneous causes of damage 10

Washington, D.C. Issued November 1968

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402 — Price 35 cents

iii
CAUTION

If pesticides are handled or applied improperly, or if unused

parts are disposed of improperly, they may be injurious to

humans, domestic animals, desirable plants, pollinating insects,

fish, or other wildlife, and may contaminate water supplies. Use

pesticides only when needed and handle them with care. Follow

the directions and heed all precautions on the container label.

FOLLOW THE LABKU

U.S. DirAITMCNT OF AMICUIIUIIC
OAT DISEASES AND THEIR CONTROL

By M. D. SIMONS and H. C. MURPHY, plant pathologists,

Crops Research DivisioUy Agricultural Research Service

Oats rank as one of the major crops grown

in the United States. Among the cereal crops,

they rank next to corn and wheat in importance.

Oats are subject to a very large number of

diseases, a good many of which can cause severe damage. Individual oatfields may be severely damaged or even totally destroyed.

Diseases are a major limiting factor in oat

production. Averaged over a period of years,

oat diseases annually account for millions of

dollars worth of damage through their effect

on yield and quality of grain. They also play

a similar, if not more significant, role in the

production of oat forage.

The average annual percentage loss from

each of the major oat diseases from 1951

through 1960 are shown in table 1. Data were

obtained from 23 oat-producing States. The

percentage was based on estimates of the potential total oat production for the United

States if diseases had been absent. The causal

organisms or other agents and the principal

control measures for each disease are also

shown in table 1.

FUNGUS DISEASES

Crown Rust

Year in and year out, in the principal oatgrowing regions of the world, crown rust is

one of the most destructive of all oat diseases.

Crown rust occurs almost everywhere that oats

are grown but is most serious in humid areas.

Crown rust infection results in reduced yields,

lower test weight, and increased lodging. The

severity of this disease varies greatly from

year to year, ranging from negligible to total

destruction of certain fields. In years when

crown rust is severe, losses over large areas

have amounted to 30 percent or more of the

potential crop.

In the major oat-growing regions of the

United States, the fungus causing crown rust

may follow either of two distinct life cycles.

In either case the uredial or repeating stage

ordinarily first appears on oats at least several weeks before ripening. The uredia appear

as bright, orange-yellow, round to oblong pustules (pi. 1, A), The bright color and the absence of bits of loose epidermis around the

pustules distinguish crown rust from stem rust

of oats. The urediospores produced in the

uredia are adapted to dissemination by the

wind and are blown from infected to healthy

plants, producing a new uredial generation

every 8 to 10 days, depending on the weather.

This cycle can be maintained indefinitely as

long as growing oat plants are available. In

the Deep South the disease is maintained

throughout the winter in this manner on

winter-grown oats. The rust then moves north

as the oats begin to grow there in the spring.

An alternative life cycle, which is the "complete" life cycle, occurs only in climates having cold winters and where the alternate host

buckthorn (Rhamnus) is present (fig. 1). As

the oat plants begin to ripen, teUa appear on

them. The telia are black or dark brown and

sometimes form in rings around the uredia

(pi. 1, A). The telia remain covered indefinitely

by the epidermis or skin of the plant. The

teliospores are adapted to surviving through

the cold, northern winters, and in the spring

they germinate and form small spores. These

small spores cannot infect oats, but are carried

by the wind to the young leaves of buckthorn

plants, which they do infect. The fungus infection on the buckthorn gives rise to aecia,

usually on the undersides of the leaves. The

aecia, often descriptively called cluster cups,

are orange-yellow, usually round or somewhat

irregular, slightly raised structures that are

composed of small individual aecial cups (pi. 1,

J5). As the aeciospores mature, they are blown

to oat plants where they cause infection that

gives rise to uredia. This completes the cycle.

The crown rust fungus is composed of pathogenic strains or races, all of which look alike

even under the microscope, and are distin-
2 AGRICULTURE HANDBOOK 343, U.S. DEPARTMENT OF AGRICULTURE

TABLE 1.—Common diseases of oats, the organisms or other agents causing them, principal control

measures, and estimated average annual loss to the United States oat crop

Disease Causal organisms or other agents Principal control measures Loss '

Crown rust

Stem rust

Smuts

Septoria blight

Helminthosporium

leaf blotch.

Helminthosporium

culm rot.

Victoria blight

Fusarium blight __

Root rot

Downy mildew _

Powdery mildew .

Ergot

Anthracnose

Snow mold

Halo blight

Bacterial stripe blight

Soilborne oat mosaic _

Barley yellow dwarf _

Blue dwarf

Blast

Nonparasitic leaf spot.

Gray speck

Puccinia corónala

Puccinia graminis

Ustilago avenae and Ustilago

kollert,

Septoria avenae

Helminthosporium avenae

Helminthosporium sativum —.

Helminthosporium victoriae _

Various species of Fusarium

Various species of Pythium,

Fusarium, Helminthosporium,

and other fungi.

Sclerospora macrospora

Erysiphe graminis

Claviceps purpurea

Colle totrichuTn graminicola

Fusarium nivale and Typhula

iotana,

Pseudom^onas coronafaciens

Pseudomonas striafaciens

Soilborne virus

Aphid-transmitted virus

Leafhopper-transmitted virus _

Various unfavorable growing

conditions.

Unfavorable weather conditions

Soil deficient in available

manganese.

Resistant varieties, buckthorn eradication,

early planting.

Resistant varieties, barberry eradication,

early planting.

Resistant varieties, seed treatment

Good cultural practices, moderately resistant varieties.

Good cultural practices, seed treatment,

moderately resistant varieties.

Avoid susceptible varieties

Resistant varieties

Good cultural practices, seed treatment,

resistant varieties.

Good cultural practices, seed treatment

None ordinarily needed

do

do

do

do -

do

do

Resistant varieties

Resistant varieties, aphid control

None ordinarily needed

Resistant varieties

do

Application of manganese salts to soil or

plants, tolerant varieties.

Percent

3.7

2.3

.6

1.8

.7

.2

.4

.9

2.4

TT +

TT

T-

.3

.4

.5

3.8

.3

2.5

.1

T +

trace.

guished by their ability or inability to attack

certain varieties of oats. Since about 1930, oat

pathologists and breeders have concentrated

on breeding for resistance to specified races.

During the 1930's races 1 and 6 were the most

common, and the varieties Bond and Victoria,

both of which were highly resistant to these

races, were used to develop the many resistant

varieties released during the 1940's.

Victoria-derived varieties were well protected from crown rust but were eliminated

from the principal oat-growing States by another disease, Victoria blight. Bond-derived

varieties, such as Clinton, then took over and
OAT DISEASES AND THEIR CONTROL

performed very well until newer and previously rare races of crown rust, principally

race 45, built up to a point where the resistance

of the Bond varieties was virtually useless. Efforts were then directed towards producing

FIGURE 1.—Life cycle of crown rust of oats. Overwintering black spores (teliospores) (1) germinate in the

spring and produce several very small spores

(basidiospores) (2). If these spores lodge on the

leaves of the buckthorn, they will germinate there.

The resulting infection of the buckthorn (3) produces spermagonia and cluster cups (aecia) (4).

Cluster-cup spores (aeciospores) are blown to

nearby oatfields, where they germinate (5) and

infect the oats, causing summer-spore pustules

(uredia) (6) to develop on the leaves and leaf

sheaths. These pustules bear the summer spores

(7), which reinfect the crop, producing more of

their kind (8) until the infected plants begin to

ripen, when the black spores (teliospores) (9) (10)

appear, mature, and spend the winter on residue of

infected plants, where they germinate the following

spring. In the winter-oat States summer spores

may infect the fall-sown oat crop, and the rust may

overwinter there and continue its existence without the intervention of the cluster-cup stage.

new resistant varieties using Landhafer and

Santa Fe, which were resistant to race 45 as

well as to the older races.

Bond-derived varieties had scarcely been

perfected when new races of crown rust, such

as 264 and 290, which could attack them, were

discovered in North America. These races are

widely distributed and now represent a major

threat to currently grown oat varieties. Newer

varieties utilizing the resistance of Trispernia,

Florad, Florida 500, P.I. 174545, and other cultivated varieties are now being produced. Already, rare races are known that parasitize

some of these, and on the basis of past experience, it seems likely that all will eventually

become susceptible. Thus, we can see that

developing crown rust-resistant varieties is a

never-ending battle involving the plant breeder's ability to develop new resistant varieties,

and the crown rust organism's ability to produce new races that can attack them.

Research is now underway to study the utilization of the many types of very high resistance known to occur in wild oat species from

the Mediterranean region. Cultivated varieties

with such types of resistance may have a

longer useful life than resistant varieties of

the past. Another approach involves the production of varieties that are uniform in appearance but are a mixture of similar lines

that diifer only in the type of crown rust resistance they carry. Attention is also being

given to "tolerance,'' which is the ability of

certain susceptible varieties to tolerate rust

infection with relatively little effect on yield

or grain quality.

The three principal control measures for

crown rust are the use of resistant varieties,

eradication of the alternate host, and early

planting. Varietal resistance, an excellent control when available, has been discussed above.

In the colder sections of the country, destruction of buckthorn bushes eliminates an important source of early-season infection in

oatfields. The buckthorn also serves as an efficient breeding ground for new races of the

fungus, and eradication of buckthorn will increase the effectiveness of resistant varieties

by slowing down the appearance and spread

of new, virulent races. In the spring oat areas,

early planting is advisable since it enables the

oat crop to mature before the crown rust fungus has had time to build up to damaging

proportions. For the same reason, early maturing oat varieties generally suffer less rust

damage than later maturing varieties.

Experimental work has shown that fungicides control crown rust, but the cost is ordinarily prohibitive.
AGRICULTURE HANDBOOK 343, U.S. DEPARTMENT OF AGRICULTURE

Stem Rust

Stem rust is of major importance on oats

and occurs almost everywhere that oats are

grown. Like crown rust, it is more serious in

humid areas and can cause severe damage

when conditions are favorable. It is in fact

quite capable of causing damage of the same

magnitude as that caused by crown rust. Widespread losses due to stem rust, however, have

occurred less frequently over the years. Damage is expressed primarily in terms of reduced

yields and lower bushel weight, accompanied

by lodging.

The life cycle and spore stages of the stem

rust fungus are very similar to those of crown

rust, except that the common barberry (Berberis) is the alternate host. The uredia and

telia occur on the stems, leaf sheaths and

blades, and panicles of the oat plant. The pustules of the uredial stage are large, usually

oblong, and dark, reddish brown (pi. 1, C).

The epidermis of the host is ruptured and

often pushed up around the edges and through

the center of the pustules so that it is easily

visible. The dark, characteristic color of the

pustules distinguishes stem rust readily from

the light, orange-yellow crown rust. As the

host plant approaches maturity, telia begin to

form in and around the uredia, particularly

on the stems and sheaths (pi. 1, C). The teha

are black, usually oblong, and exposed by rupture of the epidermis. The general appearance

of the aecia of the stem rust fungus on barberry is similar to that of the aecia of the

crown rust fungus on buckthorn. Stem rust,

too, can overwinter in the Deep South on winter-grown oats.

The stem rust fungus is composed of pathogenic strains or races, but these have not been

nearly as numerous as those of the crown rust

fungus. They are, nevertheless, very important

and must be taken into account when any

phase of resistance is considered. New and

more virulent races of stem rust have recently

appeared in the United States, accompanied

by some striking changes in race prevalence

and distribution.

Control measures recommended for stem

rust parallel those for crown rust. The most

important single control measure is to plant

adapted resistant varieties. Early planting also

results in less damage from stem rust and is,

of course, beneficial from the standpoint of

general growth of the crop. An intensive campaign of barberry eradication has been carried out for quite a number of years, and few

barberries are left in the major oat-producing

regions of the United States. Wherever they

remain, they should be destroyed. Fungicides

will control stem rust, but as with crown rust,

the cost is ordinarily prohibitive.

Smuts

Loose smut and covered smut are two of the

most common and widespread diseases of oats.

Before the development of resistant varieties,

smuts ranked next to the rusts in importance.

Unlike the rusts, they can cause serious losses

in the drier areas of the country. In years

when smut has been severe, over one-half of

the panicles of oats in individual fields have

been smutted, and losses over larger areas

have averaged as much as 20 percent.

The two smut diseases have similar life histories, are controlled by similar methods, and

are difficult to distinguish under some conditions (pi. 1, D), For these reasons, they will

be considered together.

The sori, or spore balls, of the loose smut

fungus form in the oat panicle, usually destroying most or all of the seed and hulls. The

sori appear as dark-brown to black, loose, powdery spore masses. Wind and rain usually dislodge most of the spores soon after they appear,

and infected panicles are often inconspicuous

in the field.

The sori of the covered smut fungus do not

ordinarily destroy the outer hulls, and the

spore masses remain more or less intact inside

the hulls until threshing time. Types of sori

appearing to be intermediate between loose

and covered smut are not uncommon.

The disease cycle of loose smut begins when

the spores are blown from infected panicles to

the developing grain on other plants. This may

be any time between the time the healthy panicles are flowering until the time the grain is

ready for harvest. The spores of the covered

smut fungus, and probably those of the loose

smut fungus also, are transferred from infected

panicles to noninfested seed when the spore

balls are broken and scattered during threshing. The spores remain dormant on the seed,

either outside or inside the hulls, until the seed

is planted. The spores germinate as the seed

germinates, and the very young seedling is infected as it emerges from the seed. The fungus

grows within the plant and keeps pace with

the growing plant, but it causes little or no

visible stunting or other symptoms until the

plant heads. The spore masses are produced in

the young panicle before heading, and the cycle

is completed as the smutted panicle emerges

from the boot.

The smut fungi, like the rust fungi, are
OAT DISEASES AND THEIR CONTROL

composed of pathogenic strains or races. However, plant pathologists and breeders have

been relatively successful in developing oat

varieties resistant to all common races of both

smuts. Control by the use of resistant varieties

is generally regarded as quite satisfactory. In

addition to the use of resistant varieties, both

smuts can be controlled by seed treatment.

Several new systemic fungicides in preliminary

investigations have controlled loose and covered smuts. However, these fungicides are not

registered and cannot be recommended before

registration. Most of the organic-mercury

seed-treatment materials, which are available

on the market under numerous trade names,

usually control smut if applied strictly according to the manufacturers' directions.

CAUTION: Grain treated with organicmercury materials is extremely hazardous to

humans and livestock, and safety precautions

recommended by the manufacturer should be

followed carefully. Seed treatments may prevent the appearance and buildup of new smut

races capable of attacking resistant varieties

and also control certain other diseases.

Septoria Blight

Certain species of the Septoria fungus have

long been known to parasitize oats, but it was

not until the late 1940's that one of these was

recognized as the cause of a major oat disease. In some years, the disease has caused

appreciable losses in the North Central States.

Any of the aerial parts of the oat plant may

be infected. On the leaves, lesions are circular

to elongate, light yellow to dirty white in the

center, surrounded by a band of dull brown,

which is in turn surrounded by yellowish tissue blending into green (pi. 2, A). The center

of the lesion may be dotted with scattered

small, dark, spore-producing bodies called pycnidia. Infection of the culms sometimes causes

severe lodging. Lesions are greyish brown to

shiny black, generally beneath the leaf sheath.

The hulls of the grain and sometimes even the

groats may become infected and turn black.

The fungus overwinters on stubble and oat

straw left in the field. In the spring spores are

produced on this material and are blown by

the wind to the green grain. The initial infections (micropycnidia) are mainly on the leaves.

When these infections or micropycnidia produce spores, they are washed by the rain or

dew down the leaf blade to the sheath, giving

rise to sheath and stem infection. Damage may

be severe if this occurs while the stem is still

soft and green. Damage is slight if infection

occurs at later stages of development. The

spores may also reach the panicle by splashing

rain or by the wind rubbing plants together.

The disease does not appear to be ordinarily

transmitted by the seed.

Various varieties of oats show different reactions and different amounts of infection by

septoria blight. Developing highly resistant

varieties has proved to be difficult. Some varieties, however, are decidedly less susceptible

than others, and the choice of such moderately

resistant varieties is recommended as a control measure. Crop rotation should also help

to prevent the disease from building up to serious proportions. Seed treatment will probably

have little effect. Fungicides applied as a foliar

spray will control the disease, but the cost is

ordinarily prohibitive.

Helminthosporium Diseases

Three species of the Helminthosporium fungus cause economically significant diseases of

oats in the United States. These diseases in certain years have been very important and have

caused widespread damage.

Helminthosporium Leaf Blotch

Helminthosporium leaf blotch is one of the

most serious oat diseases in Southern United

States. In the North Central States it is common but not usually serious. In the seedborne

phase of the disease, the seedlings emerge with

a twisted or contorted appearance, and narrow

brown stripes occur on the first, second, and

third leaves. The fungus may also overwinter

on crop residues of the previous year. Secondary infection of older leaves is initiated by

windborne spores produced on diseased seedlings or from spores produced on crop residues

of the previous year. On the older leaves the

lesions start as small, brown necks, which develop into more or less well-defined longitudinal strips of dead tissue (pi. 2, B). In severe

attacks a high percentage of the leaf is destroyed.

Oats resistant to helminthosporium leaf

blotch are rare, but some varieties are less susceptible than others; and selection of these

moderately resistant varieties, when available,

is suggested as a control measure. Because the

disease is carried over the winter on the seed

and on oat stubble and straw left in the field,

seed treatment can be expected to help but will

not give complete control. Crop rotation should

be beneficial in reducing the amount of inoculum present to start the disease in the spring.
6 AGRICULTURE HANDBOOK 343, U.S. DEPARTMENT OF AGRICULTURE

Helminthosporium Culm Rot

Helminthosporium culm (stem) rot of oats

is apparently most serious in the Southeastern

States. Roots and crowns of infected plants are

also rotted (pi. 2, C). This disease is not perfectly understood, but certain varieties are obviously more susceptible than others. Avoiding

these varieties is suggested as a control

measure.

Victoria Blight

In 1945, a new and devastating helminthosporium disease appeared on varieties of oats

that possessed the Victoria type of crown rust

resistance. This soon proved to be one of the

most destructive plant diseases ever known,

reducing yields of susceptible varieties in the

North Central States by as much as 50 percent.

It is apparently much less severe in the Southern and Western States.

Infected plants show longitudinal bronze or

lead-colored striping of the leaf blades and

sheaths, a basal stem and root rot, and an

abundant, black sporulation of the fungus on

the nodes of mature plants (pi. 2, D). Infected

stems often break over near the ground line.

The disease is caused by a fungus that produces a toxin or poison that spreads throughout the plant.

All varieties of oats having the Victoria

type of crown rust resistance are susceptible

to Victoria blight; whereas all other varieties

are immune or highly resistant. Consequently,

the use of resistant varieties is a practical,

effective control measure. The fungus is seedborne and also lives over winter on crop residue and in the soil. Consequently, crop rotation

and seed treatment can be expected to help but

will not give complete control.

Fusarium Blight

Many species of the Fusarium fungus attack

oats in the United States. The damage caused

by these fungi is more severe in the North

Central States and other northern areas than in

the South. Reasonably accurate estimates of

actual amounts of damage are difficult to make,

but fusarium blight very likely reduces yields

appreciably; and isolated instances of severe

damage have been reported.

Affected plants occur singly or in patches of

variable size. The development of this disease

may involve invasion and killing of the young

plants either before or after emergence. If the

young plants are not killed outright, infection

may later reach the crown, resulting in small

plants with few tillers. Head blight is not as

common on oats as on other small grains; however, diseased kernels occasionally become sufficiently concentrated to be a menance to livestock, as is scabby barley. Infected oat spikelets

are light and chaffy and either light pink or

buff.

Oat varieties differ in reaction to the Fusarium fungi, and the use of the more resistant

varieties is suggested as a control measure. The

fungi can overwinter on the seed, crop residue,

or organic matter in the soil. Consequently,

seed treatment and crop rotation can be expected to help but will not give complete control. Early planting, which allows the plants

to develop at temperatures too low for the best

growth of these fungi, is probably beneficial.

Root Rot

Several of the fungi already discussed, such

as species of Helminthosporium and Fusarium,

attack the roots of oats as well as other parts

of the plant. Other fungi, the most important

of which are probably species of Pythium, attack mainly the roots and newly germinated

seedling plants (pi. 3, C), Because of the nature of these diseases, an accurate estimate of

damage is difficult to make. However, the root

disease complex is one of the more important

diseases of oats in the humid areas.

Pythium fungi may attack the seed so rapidly that the embryo is killed before germination, or the young seedling may be killed before

emergence. Plants attacked, but not killed, are

yellow and stunted. As the season advances,

they gradually turn green but never fully recover. On the roots, water-soaked translucent

areas, which later turn reddish brown, are

evident.

In general, any factor retarding the growth

of the plant and directly affecting the fungus

makes injury more pronounced. Conversely,

plants grown under optimum conditions of

temperature, water supply, and fertility are

not apt to be seriously affected by root-rotting

fungi. Consequently, the most important control measure is the application of good cultural

practices. Although these fungi usually live in

the soil, seed treatment with volatile organic

fungicides can be expected to help but will not

give complete control. Damage has been shown

to be greater to plants grown from seed stored

for several years than to plants grown from

newer seed. Plants grown from small seeds are

also attacked more severely than plants grown

from large seeds. No highly resistant oat varieties are known, but some are more resistant

than others.
OAT DISEASES AND THEIR CONTROL

Minor Fungus Diseases

A very large number of additional fungi are

known to parasitize oats. The majority of these

ordinarily cause little or no actual loss in yield

or quality of grain. Nevertheless, some are

occasionally important in restricted areas of

the United States or in other countries. For

these reasons a few of the potentially more

important diseases caused by these fungi will

be discussed briefly.

Downy Mildew

In the South downy mildew has been known

to cause severe damage in individual flooded

fields. Infected plants tend to be stiff and upright. The upper leaves may be curled about

the panicles, and the panicles are often curled

and twisted, sometimes so severely that the

panicle is reduced to a cluster of frayed and

tangled spikelets (fig. 2). Curved, elongate

structures also occur in the panicle. The thickwalled spores of the fungus are well adapted

to persisting in the soil, and the disease is carried from one locality to another in sporebearing plant residue, in seed grain, and by

wind and water. Control measures are not ordinarily needed. Avoiding areas likely to be

flooded is helpful.

Powdery Mildew

Powdery mildew is not uncommon on oats.

It has been reported as a serious disease, reducing forage yields of susceptible varieties

FIGURE 2.—Downy mildew on oat panicles. Panicle on

extreme right is healthy. (Photograph courtesy of

Iowa State University of Science and Technology,

Department of Botany and Plant Pathology.)

only in a very few instances in the Southern

States. The fungus is an external parasite

appearing first as patches of white fluffy

growth on the lower leaves and leaf sheaths

(pi. 3, A). As the disease progresses, the patches

become powdery, turn gray or brown, and may

eventually cover extended areas of oat tissue.

If control measures ever become necessary, resistant varieties probably could be found and

used.

Ergot

Ergot has never been regarded as a serious

disease of oats in the United States, but individual panicles showing infection are not rare.

The disease is readily identified by the large,

purple-black, horn-shaped fungal bodies called

sclerotia, which appear in place of the seed

(fig. 3). These sclerotia, like ergot sclerotia of

other cereals and grasses, must be regarded

as poisonous to humans and livestock. Control

measures are not ordinarily needed.

Anthracnose

Anthracnose is not ordinarily considered a

serious disease of oats but is widespread, particularly in the Southern States where it is

occasionally responsible for isolated instances

of appreciable damage. The disease appears as

brown, oval to elongate lesions, dotted with the

dark-brown sporulating bodies of the fungus

on the leaves and sheaths. Infection may occur

in the stem and roots, and infection of the

panicles results in light-weight and shriveled

grain. Late in the season severely infected

plants are characterized by their small size

and paucity of tillers. Should control measures

be warranted, resistant varieties probably

could be found and used. The disease does not

appear to be seedborne and probably overwinters on the residue of infected plants in the

field. Consequently, crop rotation and good cultural practices can be expected to help but will

not give complete control.

Snow Mold

Snow mold has been recognized as a disease

of potential importance on winter oats, as the

crop is moved northward through the development of more winter-hardy varieties. The fungus is conspicuous as white superficial mycelium on the leaf and crown tissues of winter

oats as snow melts in the spring. The resulting

dead plants are matted tightly to the ground

with a whitish-gray, bleached appearance (pi.

3, 5). The disease usually occurs in local areas,

or spots, with healthy plants adjoining. Differ-
AGRICULTURE HANDBOOK 343, U.S. DEPARTMENT OF AGRICULTURE

FIGURE 3.—Ergot on oats.

leaves later turn brown and die back from the

tip. Where highly susceptible varieties are involved, the dead leaf tips may include several

inches of the leaf not occupied by lesions, indicating considerable additional indirect damage.

A seed-rotting phase of the disease also occurs.

The bacteria overwinter on the seed and

plant residue in the field. Plant residue is the

most important source of inoculum in the

spring. The disease is favored by cool, wet

weather and is checked by warm, dry weather.

Different varieties of oats show differing degrees of resistance, and where control is necessary, the use of resistant varieties and seed

treatment is recommended.

Bacterial Stripe Blight

Bacterial stripe blight of oats is widespread

over the United States, but is seldom as

prevalent or noticeable as halo blight. Lesions

on the leaves first appear as sunken, watersoaked dots, which may join to form long,

water-soaked stripes or blotches (pi. 4, B).

These stripes may show narrow, yellowish

margins. A bacterial exúdate is formed, which

dries down to thin, white or colorless scales.

The lesions occur mainly on the leaf blade but

may occasionally be found on other parts of

the plant. Halolike borders do not develop

around these lesions. The life cycle of bacterial

stripe blight is similar to that of halo blight.

Control measures are not ordinarily needed.

enees in varietal reaction to snow mold of oats

have been reported. Should control measures

be warranted, resistant varieties are available.

BACTERIAL DISEASES

Halo Blight

Halo blight is the most common bacterial disease of oats, occurring to some extent each

year in the major oat-growing regions of the

country. Although there have been occasional

reports of appreciable damage, average losses

from this disease are very small.

Halo blight appears in the Northern States

early in the season, usually earlier than any of

the diseases caused by leaf-spotting fungi. It is

evident by lesions mainly on leaves, although

they are also found on other parts of the plant.

The lesions, first light green and later yellow

(pi. 4, A), are small oval chlorotic spots surrounded by a halolike border. Severely infected

VIRUS DISEASES

Several virus diseases of oats are known. At

least two, soilborne oat mosaic and barley yellow dwarf, have been known to cause serious

losses in the United States and should be ranked

as major oat diseases.

Soilborne Oat Mosaic

This severe mosaic disease of oats occurs in

the Southeastern States. It is caused by a virus

that comprises at least two distinct strains. In

years favoring the disease the virus can reduce

yield as much as 50 percent in some susceptible

varieties.

In its most destructive form, soilborne oat

mosaic causes infected plants to grow in small

rosettes. Less severe symptoms include lightgreen to yellow dashes and streaks paralleling

the axis of the leaf, and sometimes a necrotic

mottling (pi. 5, D). Other strains of the virus

may induce the appearance of eyespot lesions
OAT DISEASES AND THEIR CONTROL

that are spindle shaped with light-green to

ashen-gray borders and green centers.

The virus is soilborne and overseasons in

the soil. Symptom expression is strongly favored by cold temperatures.

Many highly resistant varieties are known,

and the use of these varieties will effectively

control the disease.

Barley Yellow Dwarf

Barley yellow dwarf virus (BYDV) was first

recognized as a serious disease of oats in the

late 1940's. Since that time, several outbreaks

have resulted in severe losses in yield and quality. BYDV is usually present in at least trace

amounts throughout most of the oat-growing

regions of the United States and in recent

years has been the most important of all oat

diseases.

The disease first appears as yellowish-green

blotches on the older leaves, principally near

the tips. These areas may then take on a reddish cast, which on different varieties and under different conditions, may range from yellow (pi. 5, A) through intermediate stages to

brilliant scarlet. When compared with normal

plants, infected plants are dwarfed, mature

early, produce seed low in bushel weight, and

have more blast (pi. 5, B). (See next column

for discussion of blast.)

BYDV is transmitted by a number of species

of aphids. Different strains of the virus exist,

and certain species of aphids transmit given

strains of the virus more readily than others.

The complete life cycle of BYDV is not known

as this is written (1968), but it is known that

many wild grasses susceptible to the virus may

play a part in its overwintering.

Oat varieties highly resistant to BYDV are

not known, but some are much more resistant

than others. Several new resistant varieties

are now being developed. Consequently, the use

of the most resistant varieties available and

the control of the insect vectors are suggested

as control measures.

Blue Dwarf

The virus disease of oats called blue dwarf

is transmitted by a leafhopper. Individual

plants may be severely infected, but ordinarily

only a small part of the plants in a field will

be damaged. Infected plants are dwarfed, uniformly dark bluish green, and long hved (pi.

5, C). The leaves are shortened, upright, and

rigid. Control measures do not appear to be

necessary.

NONPARASITIC DISEASES

Several nonparasitic diseases of oats are

known. The most common are blast, nonparasitic leaf spot, and gray speck caused by a

manganese deficiency. A number of other nonparasitic diseases occur less frequently.

Blast

Blast of oats refers to a type of sterility

appearing at the time of heading in a form

of white, empty hulls, usually near the base

of the panicle (fig. 4).

Blast appears to be associated with extremes

of such factors as temperature, moisture, light,

and nutrition that occur at certain critical periods during development of the plant. The

amount of blast is usually greater in lateseeded material than in early-seeded.

As different varieties of oats vary considerably in susceptibility to blast, the use of the

FIGURE 4.—Left, Oat panicle affected by blast; right,

healthy panicle.
10 AGRICULTURE HANDBOOK 343, U.S. DEPARTMENT OF AGRICULTURE

more resistant varieties, where available, is indicated as one control measure. In general, the

better adapted varieties will tend to be more

resistant. Good cultural practices can be expected to help but will not give complete control in reducing damage from blast.

Nonparasitic Leaf Spot

Damage by nonparasitic leaf spot often is

very conspicuous on oat leaves. It can reduce

yields of susceptible varieties under certain

conditions. The injury appears, in some cases

at least, to be associated with an inability to

recover from water soaking due to high humidity. Rapid drying of the foliage, along with

an abrupt change from cool-moist to warmdry environment, accentuates the expression of

symptoms 'on susceptible varieties.

The symptoms of nonparasitic leaf spot vary

on different oat varieties and are influenced

by environmental conditions. Typically, symptoms are spots, mainly on the leaves. These

spots range from ashy gray to straw-colored

brown (pi. 4, C) and from irregular to circular and elongate. Distinctly concentric areas

of different shades of color may be present.

Most commercially grown oat varieties have

good resistance to nonparasitic leaf spot, and

avoiding susceptible varieties controls nonparasitic leaf spot satisfactorily.

Gray Speck

The gray speck disease of oats, caused by a

deficiency in available manganese in the soil,

is not common in the major oat-growing regions in the United States. However, it is a

problem in alkaline-organic and other lowmanganese soils.

The symptoms of gray speck first appear as

light-green to gray-brown spots and streaks,

mainly on the leaves (pi. 4, Z)). The plants may

be greatly reduced in height and yield, and

the entire plant may become yellow where the

disease is severe.

Gray speck can be controlled by the application of manganese salts to the soil or by spraying the plants with 1 percent manganese sulfate. Certain varieties of oats are much more

tolerant of manganese deficiency than others,

and the use of such varieties should, in most

cases, control gray speck.

Minor Nonparasitic Diseases

Nutritional Diseases

Deficiencies of various other elements in the

soil, including nitrogen, copper, boron, zinc,

nickel, calcium, molybdenum, iron, and cobalt,

also cause diseases of oats. Ordinarily, such

diseases are serious only under special soil conditions. Excessive amounts of these elements

also may cause diseases.

Miscellaneous Causes of Damage

Herbicides, such as 2,4-D, and high and low

extremes of environmental factors, such as

temperature, soil moisture, or ozone, may also

result in abnormal plant development. Hail

(pi. 3, J5), soKd sheets of ice in fields, hghtning, and other unfavorable conditions in

the physical environment can damage oats. The

symptoms of all these nonparasitic diseases

may be sometimes confused with those caused

by various pathogens.
AGRICULTURE HANDBOOK 343, U.S. DEPARTMENT OF AGRICULTURE

^

11

PLATE 1

A, Left and center, telia of crown rust on oat leaves; right, uredia. B, Aecia of crown rust on buckthorn leaf. C,

Left, telia of stem rust on oat leaves; right, uredia. D, Left, healthy oat panicle; center, covered smut on oat

panicle; right, loose smut.
12

AGRICULTURE HANDBOOK 343, U.S. DEPARTMENT OF AGRICULTURE PLATE 2

A, Septoria blight on oat leaves and culms. B, Helminthosporium leaf blotch on oat leaves. C, Left, healthy oat

culms; right, helminthosporium culm rot on oat culms. D, Victoria blight on oat plants.
AGRICULTURE HANDBOOK 343, U.S. DEPARTMENT OF AGRICULTURE

13

PLATE 3

A, Powdery mildew on oat leaf. B, Left, snow mold on oat plants; right, healthy plants. C, Left, pythium root

rot on oat seedlings ; right, healthy seedling. D, Hail damage on oat culms.
14

AGRICULTURE HANDBOOK 343, U.S. DEPARTMENT OF AGRICULTURE PLATE 4

A, Halo blight on oat leaves. B, Bacterial stripe blight on oat leaves. C, Nonparasitic leaf spot on oat leaves. D,

Gray speck on oat leaf.
AGRICULTURE HANDBOOK 343, U.S. DEPARTMENT OF AGRICULTURE

15

PLATE 5

A, Barley yellow dwarf on young oat plants. B, Left, healthy oat plants; right, barley yellow dwarf on adult plants.

C, Left, blue dwarf on adult oat plants ; right, healthy plants. D, Soilborne mosaic on oat leaves.

A U.S. GOVERNMENT PRINTING OFFICE : 1968 O—278-B42