e,x SOYBEAN DISEASES

Agriculture Handbook No. 302

Agricultural Research Service

UNITED STATES DEPARTMENT OF AGRICULTURE

NAL DIGITIZING PROJECT

MBP0000256
Contents

Page

Bacterial diseases — 1

Bacterial blight 1

Bacterial pustule —- 2

Wildfire ...- 2

Bacterial wilt — : 3

Rhizobium-induced

chlorosis -—— 3

Fungus diseases — 5

Diseases affecting leaves —. 5

Brown spot — 5

Downy mildew 5

Target spot —— -- 6

Frogeye leaf spot - 8

Phyllosticta leaf spot —... 8

Powdery mildew 9

Black patch 10

Alternaría leaf spot 10

Diseases affecting roots

and stems .... 11

Brown stem rot .-..—.. 11

Stem canker -— 11

Phytophthora rot 11

Pythium rot — -. 12

Rhizoctonia rot 13

Fusarium root rot 13

Anthracnose .- 14

Phymatotrichum root rot 23

Charcoal rot 23

xSclerotial blight 23

Stem rot ..-..„. 25

Page

Diseases affecting seed — 25

Purple seed stain ...25

Soybean smut ..- 26

Pod and stem blight 26

Yeast spot 28

Virus diseases ..— 28

Soybean mosaic 28

Bud blight 28

Yellow mosaic 30

Diseases caused by

nematodes 30

Root-knot nematode 31

Soybean cyst nematode .... 33

Sting nematode 34

Other nematodes 35

Relation of nematode injury to other diseases.. 35

Nutritional diseases 36

Potassium deficiency .-.— 36

Nitrogen deficiency 36

Iron deficiency 37

Manganese deficiency 37

Molybdenum deficiency.... 37

Miscellaneous causes of

damage 37

Herbicide injury .. 38

Arsenic injury 38

Root damage and

drought 38

Lightning injury 38

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.

This handbook supersedes Circular 931, "Diseases of Soybeans."

Washington, D. C. , ' Issued July, 1966

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

Washington, D. C. 20402 Price 30 cents
SOYBEAN DISEASES

By J. M. DUNLEAVY, plant pathologist, Crops Research Division, Agricultural Research Service, and professor, Dept. Botany and Plant Pathology, Iowa Agricultural and Home Economics Experiment Station; and D. W. CHAMBERLAIN

and J. P. Ross, plant pathologists. Crops Research Division, Agricultural Research Service

Annual losses from all soybean

diseases have been estimated at

12 percent of the total crop. The

severity of disease and amount of

loss depend upon environmental

conditions, such as temperature

and humidity. Disease-free soybean fields in the United States

are the exception rather than the

rule, and it is not unusual to

find fields with several diseases

present. All diseases do not occur

every year in a locality. A disease

may be very destructive one year

and not apparent the next.

All soybean diseases reduce

yields. The amount of loss depends on the type of disease, the

severity of the disease on each

plant, and the number of plants

infected. Yield reductions are not

usually severe enough to be observed directly by the producer.

Bor example, severe bud blight

leaves no doubt in the producer's

mind that yield is being greatlyreduced; but" bacterial blight,

downy mildew, and brown stem

rot, which reduce yields as much

as 20 percent, are not obvious on

the basis of plant symptoms alone.

Some soybean diseases are now

effectively controlled by use

of resistant varieties. Other diseases, such as soybean wilt, bud

blight, and brown stem rot, warrant control but cannot now be

controlled easily and effectively.

Soybean diseases are caused by

bacteria, fungi, viruses, and nematodes that depend on the plant

for their nutrition. The constant

feeding of these organisms on soybeans weakens the plants so that

they do not grow properly and fail

to produce maximum yields. This

handbook presents information on

diseases, various nutritional disorders, and miscellaneous causes

of damage to soybean plants.

BACTERIAL DISEASES

Bacteria are very small, singlecelled organisms that can be seen

only through a microscope. They

sometimes grow so well on plant

tissue that they can be observed

as a thick, cream-colored mass

oozing from infected tissue. Bacteria invade plants through natural openings or wounds. They

divide and redivide so rapidly

that great pressure is developed

between cell walls, which soon

buckle. The bacteria then enter

the cells and destroy them.

Bacterial Blight

Bacterial blight, caused by the

bacterium Pseudomonas glycinea

Coerper, is one of the most widespread diseases of soybe.ans.

Bacteria splashed from soil or

nearby diseased plants invade the

leaves through stomata, the minute breathing pores on the leaf

surface. The bacteria multiply

rapidly and produce small, angular, wet spots (pi. 1, A) in about 7

days. The spots turn yellow and

1
then brown as thé- tissue dies.

Many small infections sometimes

run together; the resulting large

dead areas may fall out. Diseased

leaves may be severely torn by high

winds during rainstorms. Heavy

infection may cause dropping of

lower leaves. Bacterial blight is

likely to be most severe during

periods of cool weather and frequent rain or dew. It is usually

observed early in the season before

plants flower. Under conditions

ideal for the development of the

disease, heavy infections of bacterial blight are known to reduce

yields as much as 22 percent.

Although these conditions are not

often encountered in commercial

fields, severe losses from bacterial blight can result when they

do occur.

The bacteria that cause blight

are seedborne. They survive in

dead leaves from one growing season to the next. Planting seed

from severely diseased plants or

replanting seed in infested fields

most commonly allows this disease

to become established. If the environment is unfavorable for disease development, these practices

do not always result in a diseased

crop.

Bacterial Pustule

Bacterial pustule, caused by the

bacterium Xanthomonas phaseoli

(E. F. Sm.) Dows. var. sojensis

(Hedges) Starr & Burkh., is present to some extent in most soybean-growing areas. In the

North its prevalence and severity

seem, to vary considerably with

the season, but in the South it is

more uniformly severe when susceptible varieties are grown. Infection occurs in the same way as

that described for bacterial blight.

Spots produced by the bacteria are

usually confined to the leaves. At

first, they appear as small, yellowish-green areas with reddishbrown centers, more conspicuous

on the upper surface of the l^af.

A small, raised pustule usually

develops at the center of the lesion,

especially on the lower leaf surface (pi. 1, B). This is the stage

at which the disease is most

readily distinguished from bacterial blight. The pustule and

the absence of a wet appearance

before the spot turns yellow distinguish bacterial pustule from

bacterial blight. Many small infections sometimes run together;

the resulting large dead areas

sometimes fall out, giving the leaf

a ragged appearance. Bacterial

pustule losses as high as 12 percent

have been measured.

The disease-producing bacteria

are seedborne and overwinter in

diseased leaves. The varieties

Bragg, Clark 63, Hampton, Hardee, Hill, Hood, Lee, and Scott are

highly resistant.

Wildfire

Wildfire, caused by the bacterium Pseudomonas tabaci (Wolf &

Foster) F. L. Stevens, occurs only

where bacterial pustule is found.

The wildfire bacteria alone do not

produce a disease in soybeans.

They invade only the parts of the

leaf already parasitized by the

pustule bacteria. Light - brown,

dead areas of various sizes, surrounded by broad, yellow halos,

appear on the leaves (pi. 2, A).

At times the dead spots are dark

brown to black, with indistinct

halos. In damp weather the spots

enlarge and run together. Such -

severe infection can cause considerable loss of leaves.

The bacteria are seedborne.

They may live for 3 to 4 months in f

dead leaves at the soil surface but \

are killed in a few weeks when

covered with soil. This disease

can be easily controlled by plant-;

ing varieties resistant to bacterial

pustule. Where resistant varie-\

ties are unavailable, wildfire çan(

be controlled by using seed from]
fields in which the disease was

not present and by practicing

crop rotation.

Bacterial Wilt

Bacterial wilt has long been an

important disease of garden beans

but only in recent years has it been

recognized as a disease of soybeans. It is widely distributed

throughout the soybean-growing

regions of the United States. The

disease is caused by a bacterium

related to Corynebacterium flaccumfaciens (Hedges) Dows. and

is seedborne. The bacteria occur

both on and within the diseased

seed. Bacteria within the seed

cannot be reached by treating the

seed with disinfectants or with

chemicals without injuring the

seed. Diseased seeds sometimes

appear normal or sometimes show

light discoloration, shriveling, or

malformation (ñg. 1). Severely

diseased seed may be flat and irregular in outline, with seed coats

covered with a fuzzy fungus

growth. Such seed is usually dead

and only spreads the bacteria when

planted with disease-free seed.

Seedlings severely diseased by

bacterial wilt are greatly stunted,

and growing points can be killed.

Main stems are frequently forked

just above the cotyledons. Leaves

are small and stems are thin and

weak. Less severely diseased

plants sometimes show only a

depression in growth or slightly

misshapen leaves. These plants

usually have a fair to good pod set

with only occasional unfilled pods.

If, however, the weather becomes

dry, many pods do not fill, the

leaves become yellow, and the

plants mature as much as 2 weeks

early. Such weakened plants are

subject to infection by many other

disease-producing organisms.

Pod and stem blight, charcoal rot,

and fusarium blight are the most

frequent secondary diseases ; stem

canker, anthracnose, alternaría

leaf spot, and phyllosticta leaf spot

are observed less frequently.

Infected plants sometimes show

netlike yellow blotches on the

leaves (pi. 2, B), Severely infected plants may wilt and die

at any time during the season.

Depending upon the degree of

severity and time of infection,

stems may remain green and

leaves may adhere to stems after

pods mature. The veins along the

upper edge of the pods may turn

brown, and the pods may split as

the seeds enlarge. Infection of

green seed and pods sometimes

results in brown, circular spots on

the pod walls opposite the seed.

Most commercial seed is contaminated. Seed of some varieties appears more severely contaminated than others. Whether

this is a measure of varietal susceptibility is not known. No resistant varieties are known, but

there is a considerable range in

susceptibility. Varieties Lee and

Adams appear less susceptible

than other varieties.

Soybeans from severely diseased fields should never be used

for seed. Poor soil conditions,

low temperatures, and heavy rains

soon after planting increase the

severity of infection. There is

no effective method of controlling

wilt. A search for wilt-resistant

soybeans, followed by development

of resistant varieties, seems to

offer the only hope of controlling

this disease.

RfifzobfUfn-lnduced Chlorosis

Chlorosis or yellowing of the

upper leaves of young, actively

growing soybean plants has been

observed under some conditions.

This chlorosis results from the

interaction between a specific variety of soybeans and a specific

strain of Rhizobium (nodulating)

bacteria.

Under field conditions, seldom

do more than two or three leaves
on an individual plant develop

chlorosis (pi. 3, A), and these

leaves develop a normal green

color in 10 to 14 days. Chlorosis

occurs during rapid plant growth

when the plant is highly dependent

upon nodule nitrogen. The variety Lee frequently shows chlorosis when grown on low nitrogen

soils in the South, whereas the

equally susceptible variety Hawkeye seldom shows the condition

when grown on the higher nitrogen soils common to the northcentral area.

Field studies have demonstrated

no yield reduction from chlorosis.

/

/

FIGURE 1. — Various degrees of seed distortion by the wilt bacteria (left to right,

top to bottom). Seed at upper left is healthy; seed at lower right is flattened

and dead.
FUNGUS DISEASES

Fungi, commonly called molds,

are small, filamentous, branched

organisms. Some fungi live

almost exclusively on decaying

organic matter and only occasionally invade living plant tissue with

their threadlike structure. Others seem to thrive as w^ell in living

plant tissue as in decaying plant

debris. Still another group of

fungi can survive only in living

plant tissue. Fungi enter plants

either through wounds or natural

openings or by direct penetration.

Diseases Affecting Leaves

Brown Spot

Brown spot, caused by the

fungus Septoria glycines Hemmi,

was formerly regarded as a disease of minor importance but has

increased in prevalence in the Midwest in recent years. It appears

early in the growing season on the

primary leaves of young soybean

plants as angular, reddish-brown

lesions that vary from the size of

pinpoints to one-fifth inch wide

(pi. 3, B), As the plants grow,

the fungus spores produced on

the primary leaves spread and

infect the trifoliolate leaves, stems,

and pods. Heavily infected leaves

gradually turn yellow and fall prematurely. Defoliation progresses

from the base toward the top of

the plant. In severe cases the

lower half of the stems may be

bare of leaves before maturity.

The fungus grows through the

pod wall into the seed and is

seedborne. Infected areas on the

cotyledons furnish spores for

subsequent infection of primary leaves. Soybeans from

severely diseased fields should not

be used for seed because seed treatment does not satisfactorily control brown spot. The fungus lives

through the winter on diseased

stems and leaves left in the field.

Greatest damage occurs where

soybeans are planted every year in

the same field. When this practice is followed, severe brown spot

infection starts early in the season

and one-half or more of the foliage

may be lost before maturity. Crop

rotation with at least 1 year between successive soybean crops

should be used to reduce disease

severity. None of the commercially grown soybean varieties are

resistant to brown spot.

Downy Mildew

Downy mildew, caused by the

fungus Peronospora manshurica

(Naum.) Syd. ex Gaum., is one

of the most common soybean diseases in the United States. It is

characterized in its early stages by

indefinite, yellowish-green areas

on the upper surface of the leaves.

In severe cases entire leaflets are

discolored. As infection progresses, the diseased areas become

grayish brown to dark brown and

are surrounded by yellowish-green

margins (fig. 2). Severely infected leaves fall prematurely. A

gray fungus growth, sometimes

called mildew, develops on the

lower side of these diseased areas.

Spores produced by the fungus

spread the disease from plant to

plant. In addition to these externally borne summer spores, thickwalled resting oospores develop

within the leaf tissues. These

spores overwinter in the fallen

leaves and probably provide a

source of infection for the next

season's crop.

The fungus grows within the

plant, invades the pods, and covers

some seeds with a white crust of

oospores (fig. 3). When such

seeds are planted, a small percentage of the seedlings are infected.

The first leaves to unfold on these

seedlings sometimes are covered

with mildew growth and provide

centers of infection in the new
wmm

fir-^^I^^W^i

FIGURE 2. — Downy mildew lesions on upper surface of soybean leaflets : A, Early

stage of disease; Bj later stage.

season's crop. This type of infection also occurs when disease-free

seed is planted in fields that contained downy mildew infected soybeans the previous year.

Downy mildew occurs in all

soybean-growing areas. Under

average conditions in Iowa it

can reduce production 8 percent.

Frequent dews cause more severe

disease development and greatly

increase the percentage of seed

infested with oospores. Such

seeds are smaller and this phase

of the disease has been known

to reduce yield an additional 6

percent.

Recent research has shown

that 26 races of the fungus

exist. These races are widely distributed throughout the soybeanproducing sections of the country

and usually only one or two races

occur in a field. Soybean plants

resistant to all known races of the

fungus have been found and improved; resistant varieties are

being developed.

Target Spot

Target spot, caused by the fungus Corynespora cassiicola (Berk.

& Curt.) Wei, is primarily a disease of soybean leaves; however,

the fungus occasionally causes

spotting of the petioles, stems,

pods, and seeds.

On the leaves the spots are reddish brown, circular to irregular,

and vary from pinpoint size when

immature to one-half inch or more

in diameter when mature. The

larger spots sometimes form concentric rings of dead tissue, and

the name "target spot" was suggested by this zonation (fig. 4).

The spots of dead tissue are frequently surrounded by a dullgreen or yellowish-green halo, thus

resembling the wildfire disease;
FIGURE 3. — Soybean seeds encrusted with oospores of downy mildew fungus.

however, the wildfire spots are

light brown or dark brown to

black. Narrow, elongated spots

are present along the veins on the

upper leaf surface on highly susceptible varieties. The disease

results in premature defoliation.

Spots on petioles and stems are

dark brown and vary from mere

specks to elongated spindle-shaped

lesions. Pod spots are generally

circular, about one-sixteenth inch

in diameter, with slightly depressed, purple-black centers with

brown margins. The fungus

penetrates the pod wall in some

cases and causes a small, blackishbrown spot on the seed.

Plant pathologists in Nebraska

have reported that the fungus

causes dark, reddish-brown

lesions, which turn dark violet

brown with age, on the roots and

stems of soybean seedlings.

Lesions are particularly evident

during cool, moist weather

early in the growing season.

Temperatures between 60° and

70° F. are optimum for disease

development. Target spot is not

considered a major problem in

Nebraska since warm weather arrests disease development.

The target spot fungus also attacks cowpeas, cotton, and sesame.

The foliage disease was first reported in the United States in 1945

and is generally distributed

throughout the Southern States.

So far, it has not been reported

from the Corn Belt States.

Soybean yield losses caused by

target spot were studied in Mississippi by growing susceptible and
'. •> . .•.- 'I

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FIGURE 4. — Concentric rings forming target spot lesion on leaflet.

resistant lines. Yield losses

ranged from 18 to 32 percent on

susceptible lines. Nearly all varieties adopted for production in the

Southern States are resistant.

Frogeye Leaf Spot

Frogeye leaf spot, primarily a

disease of soybean leaves, is caused

by Cercospora sojina Hara.

When the fungus infects the leaf

tissues, it causes an eyespot type

of lesion composed of a gray or

light-tan central area with a narrov^^, reddish-brown border (fig.

5). Heavily spotted leaves usually fall prematurely. Infection

can also occur on stems and pods

late in the growing season. The

fungus sometimes enters the seed

from infected pods.

The causal fungus overwinters

in the seed and on leaves and stems

from diseased plants. It may be

introduced into fields through the

planting of infected seed. For

this reason, seed for planting

should not be saved from a crop

severely infected with frogeye.

Although frogeye has been primarily a disease of the southern

soybean-producing areas, it was

prevalent in the southern half of

Indiana up to 1950. At that time,

the resistant varieties Lincoln and

Wabash replaced the susceptible

varieties Gibson and Patoka. In

1959, however, frogeye infection

was found in Indiana on Lincoln

and Wabash. This led to the discovery that there are two races of

the fungus that causes frogeye,

and Indiana workers have found

that Kent and Lee are resistant to

both races.

Phyllosticta Leaf Spot

Phyllosticta leaf spot, caused by

the fungus Phyllosticta sojicola

Massai., does not occur so regular-
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Á- -":. y- /-s fei*'"*'"4s .-''¿^tl

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, 4 Ç -

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5

FIGURE 5. — Frogeye lesions: A, Lower surface of soybean leaflet; B, upper surface.

ly as the bacterial leaf spots but

sometimes causes severe damage

in localized areas. The spots may

be round to oval or indefinite in

shape. On young plants, the spots

form from the margin of the leaf

inward. When first formed, the

spots are dull gray ; they later turn

gray to tan with a dark-brown border (fig. 6). In general, the disease does not persist long after the

third- or fourth-leaf stage of

growth of the soybean plant.

However, the disease has caused

severe defoliation in certain areas,

such as the Eastern Shore of

Maryland and southeastern Missouri.

Lesions on the pods were observed in Maryland (1951), and

infected seeds were found beneath

the lesions.

Control measures for this disease have not been worked out.

Resistant varieties are not available.

Powdery Mildew

Powdery mildew of soybeans is

caused by one or more species of

fungi. Early accounts of the disease name Erysiphe polygoni DC.

as the causal agent. In more recent investigations Microsphaera

diffusa Cke. & Pk. has been reported to cause powdery mildew.

In the greenhouse the causal fungus spreads rapidly on susceptible

varieties. In the field it does not

appear to be adapted to prolonged

survival ; here it occurs only sporadically. In its early stages,

powdery mildew may be recognized by the presence of small

colonies of thin, light-gray or

white fungus spreading rapidly

on the upper surface of the leaf.

Reddening of the underlying leaf

tissue sometimes is evident. In

time, the whitened areas of fungus enlarge but seldom coalesce to

cover all the leaf surface. Many
^^*^*>#*-'*"''., . *--*^

;-. "in. *.,5Bf

Hï*Li^ifc^;f:';' \

fes?"']

FIGURE 6. — Phyllosticta leaf spot lesions on primary leaves of young soybeans.

spores are produced, and the infected areas take on a white powdery appearance as though coated

withflour. Heavily infected

leaves soon turn brown and drop

off.

Black Patch

On soybean leaves the black

patch funguSj identified as Rhizoctonia leguminicola Gough & Elliott, causes small, circular to irregular spots that are bun" at the

center and have narrow, purplish

borders. The spots resemble

frogeye leaf spot lesions, but the

black patch disease can be recognized by the dark-brown to black

aerial fungus strands on the surface of the spots. The disease is

of minor economic imporiance, and

so far it has been observed on soybeans in Georgia and Iowa only.

The fungus attacks clovers and

other forage legumes rather commonly. In no instance has the

fungus been observed to produce

fruiting structures on its host or

in culture.

Alternaría Leaf Spot

Alternarla leaf spot, caused by

a fungus of the genus Alternaria,

is a common disease of soybeans in

many parts of the United States.

It is especially prevalent in the

Midwest. Although young plants

are occasionally infected, alternaria leaf spot is generally a leaf disease of plants nearing maturity

and occurs too late to cause serious damage. The spots are brown

with concentric rings, and vary

from i/i. to 1 inch in diameter.

The spots often coalesce to produce

large dead areas on the leaf blade.

Alternarla leaf spot resembles target spot, but it lacks the reddishbrown color and the yellowishgreen halo that characterize target spot.

Small, brick-red spots on soybean leaves have been investigated

at the Arizona Agricultural Experiment Station. These spots,

caused primarily by sunburn or

aphid injury, were found to enlarge and turn brown, and were

frequently covered with a sooty10
black fungus growth. The fungus, described as Alternaría

atrans Gibson, was considered to

be a weak parasite.

Diseases Affecting Roots

and Stems

Brown Stem Rot

Brown stem rot is caused by

a soil-inhabiting fungus, Cephalosporium gregatum Allington &

Chamberlain, which enters the

soybean plant through the root and

lowerstem. Infected plants show

no outward symptoms during the

major part of the growing season.

After mid-July, however, if the

stem is split lengthwise, a brown

discoloration can be seen inside the

root and lower stem (pi. 4). The

browning progresses upward in

the stem as the season advances.

In most seasons, the internal

browning is the only symptom by

which diseased plants can be

recognized.

Leaf symptoms do not always

occur but when they do, the leaf

tissues between the veins turn

brown and dry rapidly, usually

about 3 weeks before maturity.

The leaf tissues near the veins

remain green for a few days, and

then the entire leaf withers (pi.

5). A severely diseased field

turns brown instead of turning

yellow green as does a normally

maturing field. Severely diseased plants may lodge.

Brown stem rot was first discovered in central Illinois in 1944.

The disease occurs commonly

through the North Central States

and Canada, and was reported

from North Carolina and Virginia

in 1963.

Yield reduction due to brown

stem rot depends upon the severity

of infection in a field. When all

plants are infected, the loss may

amount to 25 percent. Fewer infected plants result in lower losses.

Yield losses caused by brown stem

rot are not usually recognized by

the grower because the symptoms

of the disease (internal browning)

are hidden ; hence, the grower may

attribute the loss to some other

cause.

There are no resistant varieties.

The only control measure known

at present is a rotation in which

soybeans are grown in a field only

once in 3 or 4 years.

Stem Canker

Stem canker, caused by the fungus Diaporthe phaseolorum (Cke.

& Ell.) Sacc. var. caulivora Athow

& Cald., kills plants from midseason to maturity. The disease

is most common in the Midwest.

Dead plants with dried leaves still

attached may be the first indication of its presence. A brown,

slightly sunken lesion girdles the

stem, usually at the base of a

branch or leaf petiole (pi. 6).

These begin as small, confined

areas on the stems. They are at

first very superficial, and only the

epidermis is involved. These

spots enlarge and spread rapidly

until the stem is girdled and the

upper part of the plant is killed.

In the fall the stem canker fungus produces sexual fruiting

bodies called perithecia on dead

stems in the field. The spores produced in these bodies spread the

disease the following growing season. The stem canker fungus is

seedborne. Seedlings can be infected when disease-free seed is

sown in fields containing diseased

stubble. The variety Hawkeye is

more susceptible than other varieties grown in the Midwest.

Phytophthora Rot

Phytophthora rot, caused by the

fungus Phytophthora megasperma Drechs. var. sojae A. A.

Hildeb., is one of the more recently

discovered diseases of soybeans.

It attacks the plant at any stage in

11
its development. Under certain

conditions, it causes preemergence

damping-off of the germinating

seed, postemergence killing of

seedlings, or a more gradual killing or reduction in vigor of plants

throughout the season. Young

plants, however, are most susceptible to killing.

When preemergence dampingoff occurs, gaps of various lengths

may appear in rows and the stand

of plants may be so sparse that

replanting is necessary. If seedlings are infected, the fungus produces a soft rot of the tender stem

that kills the plants rapidly. In

older plants, the first symptoms

are yellowing and wilting of the

leaves, frequently accompanied by

a dark-brown lesion on the stem

(fig. 7). The lesion may extend

below the soil line, or it may be several inches above the soil line and

extend upward into the branches.

Leaves and stems are infected

when splashed with contaminated

soil. The lesion on the taproot is

usually dark brown and the branch

roots may be destroyed. Sometimes the brown discoloration is

confined to the root tissue just beneath the bark.

FIGURE 7. — Rot of lower part of soybean stems caused by Phytophthora

fungus.

Phytophthora rot is especially

noticeable in low, poorly drained

areas of a field, but in wet seasons

it also appears on higher ground.

It is more severe on heavy clay

soils than on the lighter soils.

The disease occurs over most of

the soybean-producing areas of the

United States and Canada.

The variety Harosoy is very susceptible to this disease and is

usually more severely damaged by

the disease than some of the other

varieties, such as Adams and

Hawkeye. Blackhawk, Monroe,

Crest, Ross, Henry, Harosoy 63,

Hawkeye 63, Clark 63, Lindarifi

63, Chippewa 64, and Madison are

highly resistant.

Pythium Rot

^ Pythium rot, caused by the fungi Pythium ultimum Trow and

Pythium debaryanum Hesse, can

occur on soybeans at any time

from the seedling stage to midseason. When soybean seeds are

planted in cold soil, the Pythium

fungus can cause seed decay or kill

the young seedlings before they

emerge from the soil. It also

causes damping-off of seedlings

after they have emerged. The

seedlings that emerge often have

well-developed cotyledons but a

dead growing point. This condition is known as baldhead.

In the early stages of infection

of older plants, the first symptom

is a water-soaked appearance of

the outer tissues of the stem above

the soil line. Later, these tissues

turn brown and slough off. This

gives the stem a shredded appearance. Wilting of the plant follows. The outer tissues of the

large roots are soft, brown, and

moist and slough off readily.

Pythium rot, like some of the

other rots of the root and basal

stem, is favored by cold, wet soil

early in the growing season. Seed

treatment does not control the disease in heavily infested soils but

12
does give some protection against

seed decay. The Pythium fungi

attack a wide variety of crop

plants over most of the United

States and Canada, and rotation is

therefore of httle value. There

are no resistant varieties.

Rhizoctonia Rot

Rhizoctonia rot is caused by Rhizoctonia solani Kuehn, a widespread, soil-inhabiting fungus,

which causes damping-off, root rot,

and basal stem rot in a number of

crop plants. Infection begins

very early in the season. The

young soybean plants develop a

reddish-brown decay of the outer

layer of the root and basal stem

(fig. 8). Frequently this decay

girdles the stem and the plant dies.

The lower part of the taproot with

its secondary root system is sometimes destroyed. As the upper 2

or 3 inches of soil dry out, the af1 i A, I

i ■■«

1 ^ V.

f \

1 "%

r w 1

\

A 1

\

%

/'

j

'T

' f

FIGURE 8. — Girdling, near the soil line,

of stems of young soybean plants,

caused by Rhizoctonia fungus.

fected plants wilt and die for lack

of water and nutrients.

Dead plants sometimes occur in

areas 4 to 10 feet in diameter distributed irregularly over the field.

Frequently the dead plants appear

in scattered sections of rows distributed over an entire field. The

disease is not consistently severe

on soybeans, but in wet years it

may cause up to 10-percent loss in

stand in some North Central

States. Seed treatment is of no

value in controlling the rot caused

by the Rhizoctonia fungus, and

there are no resistant soybean

varieties.

In certain areas of the South,

particularly in Louisiana and

North Carolina, the Rhizoctonia

fungi sometimes spot and blight

the leaves. The spots are irregular in shape and are buff to dark

brown. This type of infection develops best under hot, humid

conditions.

Fusarium Root Rot

Fusarium root rot of soybeans

occurs in Iowa, Minnesota, and

Missouri and occasionally on sandy

soils in the Southern States. Several phases of the disease have

been described and have been attributed to the different species of

Fusarium.

The root rot phase is caused by

the fungus, Fusarium orthoceras

Appel & Wr. The fungus is usually confined to roots or the lower

portions of stems (fig. 9). If the

stem is invaded, it is usually

through the pith. The fungus can

infect nearly mature pods and is

seedborne. Wilting sometimes

occurs from severely rotted roots

when soil moisture is inadequate.

Under such conditions, diseasefree plants grow normally but diseased plants wilt and die. This

phase of the disease is most frequently observed on seedlings or

young plants, and entire fields can

be affected.

13
y*ft

0

\f

i

FIGURE 9. — Pusarlum root rot of soybeans: Left, healthy plant; right,

plant with roots almost completely

destroyed.

The true wilt phase of the disease causes wilting of older plants

and is much less common than the

root rot phase. Wilt of soybeans

has never caused such extensive

losses as have wilts of cotton, cowpea, and watermelon. Infected

soybean plants do not wilt so easily

as infected plants of these other

crops and may only develop yellow

leaves that fall prematurely.

When the stem base and taproot

of an infected plant are split longitudinally, a brown or black discoloration of the vascular tissues is

evident. Fusarium oxysporum

Schlecht, f. tracheiphilum (E. F.

Sm.) Snyd. & Hans, and F. udum

Butl. f. crotalariae Padwick, the

crotalaria wilt fungus, have been

shown to cause a wilt of soybeans

in South Carolina.

Occasionally, inspection of infected plants reveals rotted tap14

roots, and the plants exist only on

water and nutrients supplied by a

few lateral roots that have developed from the base of the stem.

Seedlings infected with F. orthoceras and showing signs of wilting

or death of lower leaves should

not be cultivated until adequate

soil moisture is available. When

the plants are cultivated, soil

should be ridged around the base

of the plants. This will promote

development of roots from the

stem base. These roots are not

easily infected by the fungus, and

they help the plants to recover

rapidly.

_ Poor stands resulting from this

disease are usually associated with

heavy rains, soil compaction, and

soil flooding just after planting.

Anthracnose

Anthracnose of soybeans is

caused by two species of fungi,

each producing similar symptoms.

Glomerella glycines (Hori) Lehman & Wolf was ñrst reported as

the causal agent of anthracnose of

soybeans in America. The imperfect fungus Colletotrichum truncatum (Schw.) Andrus & W. D.

Moore, which causes stem anthracnose of lima bean, is now probably

the most common cause of anthracnose of soybeans. Although

it is not a major disease of soybeans, anthracnose is found in all

major soybean-producing areas of

the United States and has been

reported from China. Soybean

plants in all stages of growth are

subject to anthracnose. When infected seeds are planted, many of

the germinating seeds are killed

before emergence. Dark-brown,

sunken cankers are often seen on

the cotyledons of young seedlings.

The causal fungus may completely

destroy one or both cotyledons or

grow from them into the tender

tissues of the young stems. There

it produces numerous, small, shallow, elongated, reddish-brown
A. Bacterial blight spots appear wet in early stages of development and are flat;

B. raised pustules surrounded by rings of yellow tissue on the lowei- side ol'

soybean leaves are symptoms that characterize bacterial pustule.

15
Afíricultmo Handlxïok 'i02, U. S. Departincnl of A^riculUiri'

A, Left, soybean leaf ciamagcd by wildfire bacteria displays broad, yellow halos

ai'ound dead areas of leaf; right, healthy soybean leaf: ß. progressive development of bacterial wilt on leaves; leaf at uppei' left is healthy.

16
AKricullurc llaiKÜxKik ."02. U. .S. I Joparlnu'nl i)£ .Agricullurcj ri.-\Ti: ;î

RliiiDhiimi-induced chlorosis of soybean plants: B. blown

on primary leaves of young soybeans.

;pot lesions

17
Agriculture Hunilljuok 302, U. S. Department of Agriculture PLATE 4

Split soybean stems: Left and renter, internal discoloration of brown

stem rot; right, healthy stem. Scale is metric.

18
Afíi-iiuIhiiX' Hanilliodk :;!)-'. U. S. I)c])arlni<'nt ..í A¡;ricvilluio l'i.Aïi: r,

Leal' blight sometimes results from plugging' of vessels in stem by brown

stem lo! fungus.

19
Agriculture Handbook 302, U. S. Department ot Agriculture Pl.ATK G

Stem cankei' lesion girdling soybean stem.

20
Agriculture HandtMXík 302, U. S. Department of Agriculture PLATE 7

Soybean seeds with purple stain.

21
ABiicullnrc' Handliook 302. U. R. DeliHitinenl of Agriculture

A, Soybean leaflets showing ( left to right i severe, niodei ate, and light symptoms

of potassium deficiency; B. new leaves of soybean plants with symptoms of iron

deficiency.

22
lesions or large, deep-seated, darkbrown lesions that usually kill the

young plant. The causal fungus

is capable of growing within stem,

leaf, and pod tissues of soybean

plants without giving external evidence of its presence until some

favorable condition of the plant

induces sporulation of the fungus

on the surface of the invaded

tissue.

On older plants, anthracnose

reaches its most destructive stage

of development during rainy periods in late summer. Lower

branches and shaded leaves are

killed by the fungus. Likewise,

young pods may be attacked. Indefinite brown areas develop and

coalesce to cover the entire surface

of the diseased stems and pods, and

the causal fungus forms numerous,

black fruiting bodies (acervuli) on

the diseased areas. As these

structures develop, several short,

dark spines (setae) emerge from

each acervulus, giving a dark,

stubble-bearded appearance to the

diseased area. Seeds in diseased

pods may be shriveled and moldy.

On the other hand, infected seeds

may exhibit no outward sign of

disease.

Both species of fungi survive

from one growing season to the

next in diseased stems left in the

field. Both overwinter on infected

seeds and are spread by planting

diseased seeds in new localities.

Treatment of anthracnoseinfected seed with fungicides

greatly improves the stand, but it

does not eliminate the disease.

Phymatotrichum Root Rot

Phymatotrichiim root rot of soybeans is caused by the same fungus, Phymatotrichum omnivorum

(Shear) Dug. that attacks cotton.

It is prevalent in heavy alkaline

soils in parts of Texas. The fungus attacks soybeans in these

areas, causing the plants to wilt

rapidly and die in patches of considerable size. If the dead plants

are pulled from the soil, one can

observe the decayed bark of the

roots. Frequently, brownish

woolly strands of fungus growth

can be seen on the roots. The fungus may persist in the soil for

many years in its resting, or sclerotial, stage.

Charcoal Rot

Charcoal rot, caused by the fungus Macrophomina phaseoli

(Maub.) Ashby, is a disease of the

roots and stem base of soybeans.

When the bark is peeled from diseased areas, small, black propagating bodies (sclerotia) of the fungus may ,be seen (fig. 10, A).

These are frequently abundant

enough to cause the tissue beneath

the bark to appear grayish black,

and this color has suggested the

name ''charcoal rot'* for this

disease.

The charcoal rot fungus is a

weak parasite of soybeans and

causes disease only when plant

growth is retarded by hot, dry

weather, poor soil, or other unfavorable conditions. The fungus

is widely distributed in soils in the

warmer sections of the United

States and attacks many other cultivated plants and weeds, as well

as soybeans.

Sclerotial Blight

Sclerotial blight, caused by the

fungus Sclerotium rolfsii Sacc,

like charcoal rot is characterized

by a rot at the base of the plant

stem, but differs from charcoal rot

in that these fungus sclerotia are

much larger and rounder than

those of the charcoal rot fungus

and are brown instead of black.

Furthermore, they are produced

on a cottony, mycelial growth on

the outside of the stem, rather than

under the bark (fig. 10, B). Infected plants die prematurely,

sometimes before the seeds form.

23
The disease is found in sandy soils

in the South where high summer

temperatures occur; the name

"southern blight" is sometimes applied to this disease. In some

fields plant losses may be as high

as 25 to 30 percent, but it is more

common to see small, scattered

areas of dead plants among the

healthv ones.

The sclerotial blight fungus attacks a wide variety of plants,

including practically all the summei' legumes adapted to the South.

Attempts at the Georgia Agricultural Experiment Station to select

soybean plants resistant to sclerotial blight have been unsuccessful.

Good control of sclerotial blight

of peanuts has been obtained by

FIGURE 10. — A, Soybean stem base and root with exposed small, black sclerotia

of the charcoal rot fungus; B, soybean stem with a heavy surface growth of the

sclerotial blight fungus.

24
plowing deeply enough to bury

plant debris under 4 inches of soil.

This disease control practice

should also prove valuable in soybean production. Crop rotation

with nonsusceptible crops, such as

corn or cotton, also has been successful in controlling this disease.

In rare instances in North Carolina this fungus has been observed

to cause circular brown or strawcolored spots on the leaves. These

spots usually have concentric

markings and narrow, darkcolored borders of necrotic tissue.

A clump of white mycelium or a

small, spherical, brown sclerotium

is often present at the center of the

lesion. The size of the spot may

vary from less than one-fourth to

more than one-half inch in diameter. Leaf infections occur when

wind and rain splash soil containing the causal fungus onto the

leaves.

Stem Rot

Stem rot, caused by the fungus

Sclerotinia sclerotiorum (Lib.)

D. By., begins on soybean stems

near the soil level. The fungus

first forms a white cottony growth

on the stem of the infected plant.

Then large, hard, black bodies

called sclerotia are formed on or

inside the stem (fig. 11, A). The

fungus overwinters in the sclerotia. In spring the sclerotia germinate and the fungus causes new

infections. Infected plants are

killed by the girdling action of

the fungus.

The disease has been reported

from Illinois, Iowa, Maryland,

New Hampshire, and New York,

and Ontario, Canada. It appears

infrequently and is considered a

minor disease, but it has occasionally been locally destructive. It

is most frequently observed after

prolonged wet periods.

The greatest hazard connected

with stem rot is encountered in

foreign trade, where no tolerance

forsclerotia is allowed. The large

sclerotia are not separated from

the seed in combines; consequently, a small center of infection

in a field can contaminate a large

volume of seed during harvesting

operations. At a foreign port of

entry, the finding of even a few

sclerotia in a shipment of soybeans

intended for human consumption

can result in the rejection of the

entire shipment.

Soybeans contaminated with

sclerotia should not be used for

seed.

Diseases Affecting Seed

Purple Seed Stain

Purple seed stain was reported

on soybeans in the United States

for the first time in 1927 when

an Indiana Agricultural Experiment Station report called attention to the objectionable discoloration. The disease has since been

observed in most States where soybeans are grown.

As far as is known, this disease

is caused by the fungus Cercospora

kikuchii (T. Matsu. & Tomoyasu)

Gardner. Investigations in Mississippi, however, have demonstrated that 10 other species of

Cercospora isolated from other

hosts are capable of causing purple

discoloration of soybean seed when

the fungi are artificially injected

into developing pods. Whether

these 10 other species of Cercospora are capable of causing purple stain of soybean seeds in the

field remains to be determined.

The causal fungus probably overwinters on diseased leaves and

stems as well as in infected seed.

The amount of purple discoloration in seed lots is important from

the grading standpoint, and the

value of seed may be lowered considerably by excessive amounts of

purpling. Weather conditions at

the time of flowering and plant

maturity apparently have a pronounced influence on the percentage of discolored seeds that

25
develop, since this varies widely

from year to year with a given

variety. Under conditions favorable for the disease, 50 percent or

more of the seeds of certain varieties may be discolored.

Although the symptoms of

purple stain are most conspicuous

on the seed, the causal fungus also

attacks the leaves, stems, and pods

on susceptible varieties. The foliage symptoms become most evident during the latter part of

the growing season as the plants

approach maturity. Individual

lesions are angular, reddishbrown spots, approximately onesixteenth inch in diameter. They

frequently coalesce to produce

large areas of dead tissue. The

discoloration on the seed varies

from pink or light purple to dark

purple, and ranges in size from a

small spot to the entire area of

the seed coat (pi. 7). Cracks

often occur in the discolored

areas, giving the seed a rough,

dull appearance.

When diseased seeds are

planted, the fungus grows from

the seed coats into the cotyledons

and from these into the stem of a

small percentage of the seedlings.

The fungus produces spores

abundantly on the diseased seedlings, and the wind-blown and

rain-splashed spores lodge on the

leaves of adjacent plants. Leaf

spots develop and the fungus produces a second crop of spores that

cause infections on other leaves,

stems, and pods.

Varietal differences in susceptibility have been observed, and

varieties Hill and Lee with moderate resistance have been released. Seed treatment aids in

preventing loss of seedlings, but it

will not assure freedom from purple stain in the seed crop. Dusting with copper dusts during the

growing season has given partial,

but inadequate, control of purple

seed stain in tests conducted at

the North Carolina Agricultural

Experiment Station.

Soybean Smut

Soybean smut has been found

only in several locations in Missouri. The fungus Melanopsichium missouriense Whitehead &

Thirum causes pods and seeds to

be transformed into small, dark

brownish-black galls resembling

pieces of charcoal (fig. 11, B).

From one to three galls per thousand seeds have been found in

seed lots. The galls contain many

minute spores that allow the

fungus to overwinter and infect

plants the following summer.

Very little information is available on soybean smut. Seed containing galls should not be planted.

Removal of galls by hand sorting

seed is of little value because

many spores sift through the

cracked surfaces of galls and contaminate seed.

Pod and Stem Blight

Pod and stem blight, caused by

the fungus Diaporthe phaseolorum

(Cke. & Ell.) Sacc. var. sojae

(Lehman) Wehm., is chieñy a

disease of the pods and stems

of plants nearing maturity. The

fungus penetrates the pod walls

and grows on the seed coats. It

is usually not present on seed in

sufficient quantity to be observed ;

but if plants are weakened by other

diseases or unfavorable environmental conditions, the fungus can

completely cover some seed, causing the seed coats to appear white.

Such severely diseased seed is

usually weak before invasion of

the fungus and does not germinate

after infection.

The disease can be identified

by the numerous, small, black

fruiting bodies called pycnidia

that appear on the pods and stems

of infected plants. The pycnidia

are scattered on the pods, but

they are usually arranged in linear

26
FIGURE 11. — A, Black sclerotia (top) produced by stem rot fungus compared to

soybeans (bottom) ; B. dark-brown smut galls (top) produced by soybean smut

fungus compared to soybeans ( bottom ).

27
rows on stems. Pod and stem

blight fungus is well distributed

over the soybean-growing areas of

the United States.

High humidity and rain favor

the formation and spread of spores

from pycnidia. The disease is

seedborne, and the fungus can

also overwinter on diseased stems

in fields. Because the fungus is

widespread on other crop plants,

crop rotation is probably of little

value. Canadian workers found

that seed treatment greatly

increased germination rates in

seed affected by pod and stem

blight, although the treatment did

not completely control the disease.

Yeast Spot

Yeast spot is caused by one

of the yeast fungi Nematospora

coryli Pegl. The spores of the

fungus are carried on the mouth

parts of the green stink bug and

are forced into the developing

seeds during the feeding activity

of this insect. Seeds infected

early may not mature or they may

remain small and shriveled. On

the developing seeds the infected

tissue is slightly depressed and

varies from yellowish or light

cream to brown. There may be

little or a considerable volume of

dead tissue extending well into

the infected seeds. Externally,

pods usually show small, pinpoint,

discolored areas. Seed shriveling

caused by stink bug injury and

yeast spot can frequently be differentiated from shriveling caused by

soybean wilt and other diseases by

locating the small discolored puncture wounds on seeds.

The disease has been reported

only from the South.

VIRUS DISEASES

Viruses are minute, submicroscopic particles that enter plants

through leaf wounds or are

injected by insects or nematodes

feeding on internal tissues. Virus

particles multiply within the host

plant and produce disease symptoms. Many different types of

viruses can infect soybeans ; however, only three are found frequently.

Soybean Mosaic

Soybean mosaic occurs to a limited extent in all soybean-producing areas of the United States.

Virus-infected plants are stunted

and leaves are narrower and

deeper green than normal. Leaf

margins are turned down and

blades are severely ruffled along

the edge of the main veins (fig. 12,

A and B) ; however leaf distortion may disappear in some varieties with the onset of high

summer temperatures. Infected

plants produce distorted pods and

fewer seeds than normal plants.

Varieties differ widely in their

range of susceptibility to soybean

mosaic. In general, commercial

soybean varieties are less susceptible than vegetable varieties, but

no immune varieties are known.

Soybean mosaic virus can be

transmitted in 100 percent of the

seed of very susceptible varieties,

but the percentage is much lower

in most improved varieties. Diseased plants should be rogued

from fields where soybeans are

grown for seed production. Seeds

from infected plants occasionally

show mottling and should never

be saved for planting. Several

species of aphids spread the virus

from plant to plant in the field.

Leaf symptoms resembling soybean mosaic are frequently caused

by 2,4-D (fig. 12, D and E) and

related herbicides.

Bud Blight

Bud blight, caused by the

tobacco ringspot virus, occurs

mainly throughout the Midwes28
tern States and in southern Canada ; however, it has been found

in most soybean-producing ai'eas.

In recent years, most severe

outbreaks have occurred in Indiana where the disease was first

reported. Many strains of the

virus exist and some cause only

severely dwarfed, barren plants.

Sj'mptoms of bud blight vary

with the stage of development

at which plants become infected.

When plants are infected before

flowering, the stem tip bud turns

brown, curves downward markedly, and becomes dry and brittle

FIGURE 12. -Soybeanin leaflets leaflets showing: showing: A and B. Mosaic .symptoms; C, normal

leaflet; D and E, injury caused by 2,4-D.

29
(fig. 13). The youngest leaf often

develops a rusty flecking. The

plant is dwarfed and produces

virtually no seed. Sometimes the

inside of the stem below the

blighted terminal bud is discolored, most often at the nodes.

When infected during flowering,

plants produce mainly small undeveloped pods. Infection after

flowering results in poorly filled

pods that have a conspicuous dark

blotching. Many of these pods

fall to the ground. Infected plants

usually remain green after normal plants have matured, and thus

are easily found in the fall.

If plants are infected before

flowering, the virus is transmitted

to seed ; but if plants are infected

after flowering, the virus is not

transmitted to seed. Bud blight

is without question the most severe

virus disease of soybeans. Fields

have been a complete loss when

bud blight infection occurred in

the early stage of development of

the soybean plants.

The disease usually appears

first at the border of a field and

progresses inward. This suggests an insect carrier of the virus,

but none has been found. There

is recent evidence that tobacco

ringspot virus is soilborne and

that a low percentage of plants

may be infected when grown in

virus-contaminated soil. Some

soybean varieties appear resistant to the disease but additional

research is required. There is no

effective control for bud blight.

Yellow Mosaic

Yellow mosaic of soybeans is

caused by Phaseolus virus 2,

the same virus that causes yellow mosaic of garden beans. The

FIGURE 13. — Curved stem tip of young

soybean plant infected with virus that

causes bud blight.

younger leaves of infected plants

show a yellow mottling, or sometimes an indefinte yellow band

along the major veins. Rusty

necrotic spots develop in the yellow areas as the leaves mature.

Infected plants are not noticeably stunted.

The virus is not seedborne and

does not reduce yields severely.

The disease is widely distributed

in the Midwest. It is unusual

to find more than 1 percent of

the plants infected in a field;

however, fields with more than

75-percent infection have been

observed in Iowa.

DISEASES CAUSED BY NEMATODES

Nematodes are tiny animals,

generally eellike, but the adult

female of certain kinds are swollen, being pear or lemon shaped.

The larger forms are barely

visible without magnification.

30
Nematodes of soybeans persist

from season to season in soil and

are distributed by movement of

soil from one field to another.

The type and severity of symptoms resulting from nematode

injury are inñuenced by many

factors, the size of the nematode

population at planting time being

an important one. If the population is high, severe injury is

more likely to occur than if the

population is low. The distribution of nematodes throughout a

field is almost always variable;

hence, plants in some areas of the

field may show nematode damage

while plants in other areas appear

normal.

Root-Knot Nematode

Root knot is caused by nematodes of the genus Meloidogyne.

All species of this genus known

to occur in the United States are

parasitic on soybeans. The most

common species is probably M,

incognita incognita (Kofoid &

White, 1919) Chitwood, 1949.

The most characteristic symptom of root knot is the root galls.

These are roughly spherical or

elongate and may vary from slight

root swellings to galls as large as

2 inches in diameter. The roots

are almost club shaped when infection is severe. Root galls

caused by the species M. hapla

Chitwood, 1949 remain relatively

small, and clusters of short roots

often emerge just above the galls

and partly cover theiç. Soybean

roots severely infected by this

species are often densely matted

as a result of extensive root

growth above the galls.

Symptoms on aboveground

parts of affected plants are a variation in color from light green to

yellow and a marginal firing of

the leaves. All degrees of stunting may occur. Plants with

galled roots wilt before normal

plants during dry weather and

may die a week or more before the

normal date of maturity. Yields

of seed are sometimes greatly reduced by root knot. Most varieties of soybean appear to be

fairly tolerant of infection by the

root-knot nematodes ; consequently, the condition of aboveground plant parts is not always

a good indication of the extent of

disease on the roots. Plants having a liberal supply of soil nutrients and water may show little or

no aboveground symptoms, even

though roots are heavily galled.

The nematodes survive the winter as eggs, especially in cold

areas. In the spring the eggs

hatch and larvae enter the root

tips. The nematodes migrate

through the roots and take permanent positions in cortical tissue

adjacent to the vascular cylinder

and begin feeding on cells that

become special nurse cells. Adjacent cells are stimulated to

greatly increased growth. Thus,

knotlike swellings, commonly

called galls, appear in the roots

(fig. 14). Nematode galls cannot

be removed without breaking the

root and can be distinguished

by this test from the beneficial,

nitrogen-fixing nodules that are

weakly attached to the root and

are easily broken away (fig. 15, ß).

Female nematodes in a favorable host produce large numbers

of eggs from which larvae develop

and migrate to new positions in

the root, thus producing more or

bigger galls. This life cycle may

be repeated about every 30 days

in roots growing in warm soil,

thus rapidly increasing the nematode population and causing malformation of roots of susceptible

varieties.

Control of root knot on soybeans is best obtained through the

use of resistant varieties. Hill,

Hardee, Jackson, and Bragg, varieties adapted for the southern

soybean-growing areas, have

31
fairly good resistance to M. incognita incognita. Delmar, adapted

for growing in Delaware and

Maryland, is also resistant to this

species. Resistant varieties are

occasionally found to be susceptible, probably because of the existence of different strains of the

nematodes. When resistant varieties adapted for a given location

are not available, root knot can be

controlled by reducing the nemaFiGURE 14. — Nematode galls (root knot) on soybean roots.

32
tode population through use of

rotation with immune or highly

resistant crops, such as corn (in

the case of M. hapla) or oats and

certain grasses (in the case of

other species).

Soybean Cyst Nematode

The soybean cyst nematode,

Heterodera glycines Ichinohe,

1952, was discovered in the United

States in 1954 near Wilmington,

N. C, and since then has been

found in Virginia and in Mississippi River Delta areas of Arkansas, Illinois, Kentucky, Mississippi, Missouri, and Tennessee.

Fields infested with this nematode have been placed under

quarantine.

Symptoms of the soybean cyst

nematode disease vary, depending

on the size of the nematode population, soil type and fertility, and

weather conditions. Soybean

plants growing in severely infested sandy soil start to turn yellow about 1 month after seedling

emergence, remain severely

stunted throughout the growing

season, and yield virtually no soybeans. Infested soybean fields

may have yellow, stunted plants

in roughly circular areas ranging

from a few feet to 50 or more feet

in diameter; however, in some

locations this yellowing symptom

is absent. Symptoms on plants

growing in infested fields with

heavier soil types generally are

not so severe as those that develop

on plants growing in lighter soils.

Soybean roots heavily infected

with cyst nematodes are characteristically dark and often lack

nitrogen-fixing nodules (fig. 15,

A and B).

Life cycles of the soybean cyst

and root-knot nematodes are similar in many respects; however,

many eggs of the cyst nematode

remain within the swollen lemonshaped body of the female, which

acts as a protective envelope for

the eggs. After hatching within

the cyst, larvae emerge and seek

FIGURE 15. — A, Soybean cyst nematode infected soybean roots; B, healthy roots

bearing numerous beneficial nodules.

33
out soybean roots; after invasion

and migration they take up a feeding position. Subsequently the

female develops a lemon-shaped

body that is attached to the root

and can be seen upon close observation (fig. 16). These females

develop into cysts. The time required for a complete life cycle is

approximately 3 weeks at a soil

temperature of 75° F. Soil temperatures above 90° tend to suppress development of the larvae.

All soybean varieties currently

grown in the United States are

susceptible to the soybean cyst

nematode ; however, resistant soybean lines have been found, and

breeding programs are in progress to transfer resistance to commercially adapted varieties.

Control of the soybean cyst

nematode at present (1965) is

limited to crop rotation with nonsusceptible crops. Soybean, common bean, three kinds of lespedeza

(annual, common, and sericea),

common and hairy vetch, mung

bean, adzuki bean, and white lupine are crops known to be susceptible to the cyst nematode.

Hemp sesbania (Sesbania exaltata (Raf.) Cory) and henbit

deadnettle (Lamium amplexicaule

L.) are weed hosts. In crop rotation experiments for cyst nematode control in North Carolina, 3-

to 4-year rotations were necessary to obtain normal soybean

yields; in Tennessee, however,

normal soybean yields were obtained when soybeans were grown

in a 2-year rotation with cotton.

Soil fumigation controls the

cyst nematode, but this method of

control is not economically feasible at present.

Sting Nematode

The sting nematode, Belonolaimus longicaudatus Rau, 1958, is

one of the most devastating nematodes that attack soybeans. Soybeans injured by this nematode

have failed to grow beyond the

seedling stage. The sting nematode is generally limited to sandy

soils of the southeastern and

southern Coastal Plain.

Aboveground signs of sting

nematode damage on soybeans are

stunting and dull, light-green foliage. Seedlings often wilt during

hot weather and eventually die.

FIGURE 16. — Lemon-shaped encysted female nematodes attached to soybean roots.

34
The root abnormalities are apparently caused by inhibition of roottip elongation and result in stubby

clusters of coarse, short roots

(fig. 17). Frequently a branch

root will attain considerable length

but will develop few if any side

rootlets. Roots of affected plants

at first have small, distinct lesions ;

later, the roots darken. When

afl'ected plants are dug, soil tends

to cling to the roots and is difficult

to remove without washing.

The sting nematode is one of

the larger plant-parasitic nematodes and may attain a length of

one-eighth inch. For the most

part the nematode feeds in the

root-tip region and along the sides

of young roots without penetrating or becoming attached.

Control of this nematode by

crop rotation is difficult because

of the large number of plant species that serve as hosts. Tobacco

and watermelon have shown good

resistance.

Control by soil fumigation with

l,2-dibromo-3-chloropropane is

very effective, and population

buildup of nematodes surviving

fumigation is slow. However,

this method of control is not economically feasible at present.

Other Nematodes

In addition to those mentioned

in detail in this section, numerous

other nematodes parasitize soybean roots. Populations of pin

(Paratylenchus), spiral {Helicotylenchus), stunt (Tylenchorhynchus), and root-lesion (Pratylenchus) nematodes increased in

soybean fields in Illinois ; and

stubby root {Trichodorus), lance

{Hoplolaimiis), and ring (Criconemoides) nematodes have been

associated with plant injury in

the South. Although little is

known of the extent of damage

caused by these nematodes to soybeans, undoubtedly some reduce

yields significantly.

FIGURE 17. — Roots of soybean seedling

severely injured by sting nematodes.

Relation of Nematode Injury

to Other Diseases

Nematode injury often places

plants under a stress and thereby

increases susceptibility of the

plant to diseases caused by other

plant pathogens. Woi-k carried

out in greenhouses in Minnesota

indicates that damping-off of soybeans caused by Rhizoctonia solani is greatly increased by rootknot nematodes, Meloidogyne

jaranica javanica (Traub, 1885)

Chitwood, 1949 or M. hapla.

In North Carolina susceptibility of soybeans to fusarium

wilt was greatly increased when

plants grew in soil infested by

soybean cyst nematodes and the

wilt Fusarium species of fungus.

Plants growing in plots treated

with a nematocide remained free

of the wilt, whereas large numbers of plants of the variety Jackson died in plots that received no

nematocide application.

35
Although the relation of parasitic nematodes to other diseases

of soybeans has only recently been

recognized, research investigations in the future will probably

reveal that the intensity of certain other soybean diseases is increased by nematode injury.

NUTRITIONAL DISEASES

Chlorosis of soybean leaves may

be caused by a deficiency of nutrients, as v^ell as by parasitic microorganisms. It is important to

recognize these nutritional diseases because they are much more

easily corrected than diseases

caused by parasites. Potassium,

nitrogen, and phosphorus are the

elements most frequently deficient

in agricultural soils. Application

of commercial fertilizer remedies

shortages of any of these elements. Acid soils frequently

require calcium for good soy})ean

growth. Application of agricultural lime corrects this deficiency.

The soil test is an effective tool

for determining whether soil acidity, phosphorus, or potassium

might be limiting production.

Iron, manganese, molybdenum,

boron, zinc, and copper are required for soybean growth but

only in minute quantities and are

called microelements. The microelements are usually present in

adequate quantities to permit satisfactory crop production, but

soils sometimes lack one or more

of these elements and soybeans

grown on such soils develop characteristic symptoms.

Potassium Deficiency

Chlorosis and firing of soybean

leaves caused by insufficient potassium occur rather frequently.

The first indication of this deficiency is an irregular yellow mottling around the edges of the leaflets. The chlorotic areas soon

merge to form a continuous yellow border. This is followed by

marginal browning that may

36

spread to include half or more of

the area of the leaflet (pi. 8, A).

Only the center and base of the

leaflet may remain green when

deficiency is severe. The dead

tissue usually falls out, giving the

leaflets a ragged appearance. Old

leaves also show symptoms. This

distinguishes the disease from

iron and manganese deficiency in

which the symptoms first occur on

the youngest leaflets as they are

unfolding. Potash deficiency is

most commonly observed on sandy

soils or on sandy ridges of fields.

Nitrogen Deficiency

Nitrogen deficiency is indicated

by a gradual paling of the green

color over the entire leaf blade,

with only a slight yellowish tinge.

The symptom is most commonly

the result of the absence of Rhizobium japonicum (Kirch.) Buch.,

the bacterium that causes root

nodulation and that fixes atmospheric nitrogen for plant use. If

soils are low in nitrogen and the

nodule-forming bacteria are not

present, nitrogen deficiency will

result. Because of poor bacterial

development in highly acid soils,

nodules on soybean roots are usually few or entirely absent. Application of lime reduces soil

acidity and promotes better nodulation, thus correcting nitrogen

deficiency in acid soils low in

nitrogen.

Formerly, nitrogen-deficiency

symptoms were seen frequently

when noninoculated soybean seed

was sown. They are rarely seen

now because the soybean rootnodule bacteria are present in
most of the soils in the major

producing areas. Seed should be

inoculated whenever soybeans are

grown in fields where they have

not been grown previously.

Soybeans parasitized by soybean cyst nematodes or affected

by fusarium root rot frequently

have very few nodules on their

roots. If such plants are grown

in soil low in nitrogen, nitrogendeficiency symptoms will be evident. Nitrogen deficiency should

not be confused with Rhizobiuminduced chlorosis. (See p. 3.)

Iron Deficiency

Iron deficiency is indicated by a

general yellowing or chlorosis of

the new growth of soybean plants

(pi. 8, B). The veins of the affected leaves, however, remain

green. In the most severe cases,

the leaves show many brown spots

of dead tissue that are more numerous near the leaf margin.

Ultimately the dead tissue drops

out, leaving only the leaf veins.

Plant growth is depressed in proportion to the severity of the deficiency. The deficiency is observed most commonly on soils

high in lime where it has reduced

yields as much as 37 percent.

Manganese Deficiency

Manganese-deficient soybean

plants have a light-green to white

mottling between leaf veins. The

degree of leaf discoloration is proportional to the severity of the

deficiency. Symptoms range from

pale-green leaves with slight deficiency to white with severe deficiency. The veins remain green

until chlorosis nears the white

stage, at which time the veins

become yellow and the leaves fall

from the plant. If moisture is

excessive, the leaves frequently

are covered with many small

brown spots. The deficiency seldom affects entire fields and

usually develops in an irregular

pattern.

Manganese deficiency occurs in

only a few soils. It has been

observed most frequently on dark,

poorly drained soils limed to near

the neutral level in northwestern

Indiana, in the old lakebed area of

Indiana and Ohio, and in the high

lime soils of the Coastal Plain in

the Southeastern United States.

Manganese deficiency has reduced

yields as much as 75 percent.

Molybdenum Deficiency

Soils deficient in molybdenum

are rare in the United States.

The chief symptom of molybdenum deficiency in soybean plants

is a slight depression in growth.

Yields increased substantially

when molybdenum was applied to

the soil in Georgia, Mississippi,

and Wisconsin. Yields also increased in Indiana and Missouri

after this microelement was

applied. Only very small quantities of molybdenum are required

to correct deficiencies. Great care

should be used in applying molybdenum to soil, because if it is

added in excessive amounts, plants

may accumulate levels that are

toxic to animals.

MISCELLANEOUS CAUSES OF DAMAGE

Plant damage from causes other

than disease is frequently confused with symptoms of diseases

caused by bacteria, fungi, viruses,

nematodes, or nutritional deficiencies. The following kinds

of soybean injury are those most

frequently confused with disease

symptoms.

37
Herbicide Injury

Indiscriminate use of 2,4-D

frequently damages soybeans.

Spraying activities on windy

days can result in injury to

plants several hundred feet

away. Severely affected plants

develop malformed leaves that are

tightly puckered and narrow.

Terminal leaves are most severely

malformed. Small, round, raised,

light-colored areas develop along

the stems. In most cases, however, injury is not so severe and

leaves merely become elongated

with ruffled margins (fig. 12, D

and E). This type of injury is

temporary and foliage subsequently developed appears normal.

Soybeans injured by 2,4-D look

very much like soybeans infected

by soybean mosaic virus (fig. 12,

A and B). The herbicide spray

usually affects all soybean plants

in the area of the field. In areas

affected with soybean mosaic,

healthy and diseased plants usually grow interspersed. Many

broadleaf weeds in a 2,4-Daflfected area show the same type

of symptoms as soybeans, whereas

with soybean mosaic, weeds are

unaffected.

Herbicides, such as atrazine,

diuron, and simazine, sometimes

remain active in soil and cause

yellowing of soybean plants the

year after application. Plants

begin to show the yellowing soon

after emergence. If there is considerable herbicide residue, the

plants will also be dwarfed and

eventually killed. Soybeans that

are yellow but only slightly stunted

usually make a slow recovery as

the roots reach deeper, less contaminated soil. The amount of

residue depends on many factors.

Although use of a field treated the

previous year will not necessarily

result in injury to soybeans, the

practice should be avoided if

possible.

38

Arsenic Injury

Soybean leaves are very sensitive to chemicals containing

arsenic. This type of damage

results in a brown leaf spot with

concentric rings. Arsenic injury

has become rare with use of improved insecticides in preference

to calcium arsenate.

Root Damage and Drought

After any type of root damage

to soybean plants, even a slight

drought can be serious. In addition to causes already discussed,

cultivation injury, excessive fertilizer, and herbicide residue can

cause root damage. Under normal conditions the impaired root

systems may go unnoticed, but

root damage, coupled with water

stress, can result in severe drought

damage to aerial parts of the

plants. At first, leaves become

flaccid during the heat of the day

and lower leaves become yellow

and fall. If soil moisture is not

replenished, wilting becomes more

severe and the remaining lower

leaves turn brown and dry from

the margin inward. These leaves

eventually die but will not fall.

Affected plants are considerably smaller than those with

healthy roots in surrounding fields

enduring the same drought.

Lightning Injury

Lightning injury to young soybean plants is usually indicated by

sudden death of young plants in

circular areas as large as 50 feet

in diameter. When plants have

reached their maximum heights,

affected areas are usually much

smaller. In an area affected by

lightning injury,' both weeds and

soybean plants are killed. The

clearly defined margins of the

areas and absence of any signs of

a parasite usually enable one to

distinguish lightning injury from

diseases caused by parasites.

"^ U. S. GOVERNMENT PRINTING OFFICE: 1966 791-936