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X. Disease and Insect Pests

X. Disease and Insect Pests

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144



MARTIN G. WEISS



new habitat sufficiently long for many new pathogens to develop and

become adapted to this new host.

Some pathogens were undoubtedly imported with the seed brought

into this country. I n this connection it is of interest. to note th a t a considerable number of the soybean diseases reported in this country carry

over into the next generation of plants in or on the seeds of the soybean

plant. Certain other pathogens which were prevalent on crops already

grown in this country, were found to also attack soybeans. An excellent summary of soybean diseases reported in the United States prior to

1943 has been made by Johnson and Koehler (1943). This reference

has been used freely in the following discussion. Only the presently

more important diseases will be discussed herein.

a. Stem Diseases. Several diseases of increasing importance on stems

and pods are included in this section. Diaporthe phaseolorum var. batatatis (Hart. and Field) Wehmeyer, reported by Welch and Gilman

(1948) as causing a stem canker, gives evidence of becoming one of the

most serious of the stem pathogens. It is prevalent throughout the corn

belt states. Field infection is characterized by the occurrence of scattered premat,urely-killed plants throughout the field during the period

of pod formation. Most of the leaves and pods remain on the infected

plants. Close examination reveals that the stem has been girdled by a

lesion, usually diagonal to the axis of the stem, and th a t parts of the

plant above this lesion are dead. This lesion, considered the point of

infection, usually occurs a t the junction of a branch or petiole with the

stem, most. frequently near the base of the plant although girdling a s

high as 2 feet above the soil sometimes occurs. No fruiting bodies are

found to be associated with this type of lesion.

A closely related fungus, Diaporthe phaseolorum var. sojae (Lehman)

Wehmeyer, causes the pod and stem blight of soybean. This fungus is

only weakly parasitic, attacking senescent plants or those weakened from

other causes. Pycnidia of the fungus may be found aligned in rows on

the stems or uniformly scattered over the pods of diseased plants, usually late in the growing season.

The occurrence of brown stem rot was increased by the repeated

cropping of land in the corn belt with soybeans during World War 11.

The causal organism, thought by Presley and Allington (1947) to be a

Cephalosporium, was identified as Cephalosporium gregatum by Allington and Chamberlain (1948). The organism invades the pith and xylem

of the stem below the ground level, moves upward with only slight external symptoms, and suddenly causes interveinal chlorosis, subsequently

necrosis, and death of the top leaves of the plant. In the corn belt the

external symptoms usually appear in late August. Retention of these



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dead leaves causes the field as a whole to resemble the symptoms attributable to an early autumn frost. Subsequently, the rotted, weakened

stems result in excessive lodging. Bccausc serious infections of brown

stein rot seem limited to soils on which soybeans have been grown repeatedly, adequate control appears possible through crop rotation. The

organism is thought to bc soil borne. Brown stem rot is prevalent

throughout the corn belt and, according to Hildebrand (1948), has recently been found in Ontario. High temperatures were found by Allington (1946b) to be highly unfavorable for the development of this fungus.

Lack of severe damage in 1947 was attributed by Chamberlain (1948) to

high mean temperatures during the month of August. No varietal resistance has been found to this parasite.

Premature killing of plants is also caused by the anthracnose fungus,

Glmerella glycinas (Hori) Lehm. and Wolf. The symptoms are similar

to those of Diaporthe pod and stem blight. Differentiation of the two

diseases is possible by examination of the small black fruiting bodies

which, in the case of anthracnose, occur irregularly over the stems rather

than in rows. On magnification the antliracnose fruiting bodies appear

very spiny. Originally considered a disease prevalent only in southeastern

United States, it has recently been reported in the corn belt. Although

symptoms described in this country pertain to plants in the pod development stage, it was reported by Ling (1940) to attack soybeans in the

seedling stage in China. It overwinters in infected stems, is seed borne,

and in China is thought to overwintcr in soils.

Bud blight is one of the three virus diseases of soybeans reported

in the United States. According to Allington (1946a) the causal virus

is indistinguishable from the tobacco ring-spot virus. Plants infected

early usually develop only rudimentary pods and are somewhat stunted

in growth. Late infection of plants results in production of distorted,

shrunken pods and defective beans. At harvest time these plants are

conspicuous in that. they do not mature until killed by frost. One of the

characteristic symptoms of this disease is the “shepherd’s crook” assumed

by the stems due to the necrosis of the apical bud. Pith discoloration,

particularly at the nodes of the plant, usually accompanies the disease.

Bud blight occurs quite commonly throughout the United States and

southwestern Ontario. During years of heavy infection this parasite has

caused severe losses. No insect vector or wild host has been found. Seed

transmission of the virus is still open to question for, according to Hildebrand and Koch (1947b), seed from infected plants gave rise to only

slightly more diseased plants than seed from apparently healthy plants.

The pathogen, however, can be transmitted by mechanical means. As

yet host resistance has not been found in soybeans.



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b. Root and Crown Diseases. Important among soybean diseases are

5 fungus pathogens which invade and attack the roots or the stem near

the ground level. Subsequent girdling of the stem results in death of the

plant above the infected area. The diseases and causal organisms are

charcoal rot, Macrophomina phaseoli (Maubl.) Ashby, sclerotial blight.,

Sclerotium rolfsii Sacc., Fusarium blight, Fusarium oxysporum f. tracheiphilum (E.F.S.) Snyder and Hansen, stem rot, Sclerotinia sclerotiorum (Lib.) Massee, and Pythium rot, Pythium debaryanum Hesse.

Charcoal rot occurs generally in the central Mississippi Valley, sclerotial

and Fusarium blight in the sandy soils of the south, and stem and

Pythium rots are believed to have general distribution. All are thought

to be soil borne. Charcoal rot can be identified by numerous small, black

sclerotia uniformly distributed in the subepidermal layer of the stem.

The epidermis must be removed to examine them. The symptoms for

sclerotial blight are similar but the sclerotia are larger, rounder, and

brown in color. Infection with stem rot results in large black sclerotia

developing on and within the stem. This pathogen is most severe during

early summer when cool wet weather prevails. I n most, instances damage

is confined to scattered small areas in which young plants have been

killed. In Ontario, however, the almost complete destruction of more

than three acres of soybeans was reported by Hildebrand (1948). Brown

or black discoloration of the xylem tissues of the root or stem identifies

infection with Fusarium, and Pythium infection can usually be identified

by typical “damping-off” symptoms and root necrosis. None of the

above organisms is restricted to attacks on soybeans. The causal organism of Fusarium blight has been proven to be indistinguishable from that

causing cowpea wilt. The other organisms attack a wide variety of

plants. Resistance to Fusarium blight has been found in field varieties

of soybeans but a high type of resistance to the other pathogens has not

been found.

Root knot ranks as one of the most serious diseases of the south.

Caused by the soil-borne root knot nematode, Heterodera marioni

(Cornu) Goodey, its chief symptom is the formation of galls in the roots

of the plants. The tops of the plants are thereby reduced in vigor, size

and green coloration. Differences in degrees of infection have been observed among varieties of soybeans.

c. Foliage Diseases. Probably the most common and conspicuous of

the soybean diseases are three bacterial leaf spots, bacterial blight, bacterial pustule, and wildfire. All are generally common throughout the soybean production area, overwinter in diseased plant tissue, and are also

seed borne. Bacterial blight, caused by the organism Pseudomows glycinea (Coerper) Stapp, is characterized by small, angular, yellow spots



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on che leaves, which soon bccome dark brown to bIack in color with thc

initiation of necrosis, and frequently drop out, t,hen giving the leaf a

ragged effect. The symptoms of bacterial pustule, caused by Xanthomoms phaseoli var. sojensis (Hedges) (Starr) Burkholder, are similar to

those of bacterial blight. The brown areas are more irregular in size

and are surrounded by yellow margins. This disease occurs particularly

in the southern United States. Symptoms of both of the above diseases

may also occur on the pods. Genetic resistance to bacterial pustule is

available, particularly in the variety CNS. Chamberlain (1948) reported

that resistance to bacterial blight is also available. From among 1100

varieties tested, 50 showed relatively little infection and three appeared

highly resistant.

Wildfire, caused by Pseudomonas tabaci (Wolf and Foster) Stevens,

a8 described by AIlington (1945), is characterized by light brown, variable-sized, necrotic spots which are nearly always surrounded by a

distinct, wide, yellow halo. The symptoms develop somewhat later than

those of the two preceding bacterial diseases.

The leaf spot diseases, brown spot and frog-eye, caused by the fungi

imperfecti Septork glycines Hemmi and Cercospora daizu Miura, respectively, are prevalent in the southeastern states. Both organisms overwinter on diseased leaves and stems and are also seed borne. Brown

spot is recognized by angular brown or reddish-brown lesions which occur

on the soybean leaves, particularly of young plants. Severe infection

results in basal defoliation. Moderate resistance is available to brown

spot. Round, dark-margined spots are the characteristic symptoms of

frog-eye on soybean leaves. Symptoms on stems and pods are less conspicuous and appear late in the season. Late varieties usually show

heavier degrees of infection than early varieties.

A seed-borne transmissable virus has been found to be the causative

agent of certain types of crinkling in soybean leaves, usually known as

soybean mosaic. Delayed maturity and interveinal, dark green puckers

are typical of the disease. In certain instances the puckers are aligned

in rows contiguous to the veins whereas in other instances they occur

uniformly over the leaflet. A moderate type of resistance is available

in certain field varieties while certain vegetable varieties and the wild

soybean, Glycine ussuTiensi8, Regal and Maack, are particularly susceptible. The causative virus of soybean mosaic has been designated by

Conover (1948) as Soja virus I . Symptoms were found to be influenced

by air temperatures, being severe a t 18.5"C. and largely masked a t

29.5"C. Soja virus 1 was found to produce systemic infection only on

soybeans. A conspicuous yellow mottle of soybean leaves was found by

Conover to be caused by another virus, Phaseolus virus 2. This disease,



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MARTIN G. WEISS



called yellow mosaic, induces systemic mottling on numerous species.

Yellow mosaic was not found to be seed transmitted and all soybean

varieties bested reacted similarly.

Downy mildew, caused by Peronospora manchurica (Naoum.) Syd.,

is sometimes found on soybean foliage during damp periods. The symptoms consist of grayish brown to dark-brown lesions surrounded by

chlorotic margins, and grayish masses of conidiophores on the under

surface of the lesions. This phycomycete has been shown by Jones and

Torrie (1946) to be systemic. The mycelia proceed up the stem in the

pith and phloem parenchyma tissues. Infected beans frequently have

milky-white crustaceous masses of the oospores on their surface. Oospores also develop within the vegetative tissue, constituting a source of

infection the following year. Several Manchu selections, Mandarin,

Habaro, Dunfield, and Mukden exhibited resistance to the disease whereas

Richland and Illini were found susceptible. Yield reductions due to

infection during a season in which the disease was epidemic were not

serious.

d. Control. Differential varietal reaction to some of the diseases has

been cited in the preceding sections. To certain diseases a high type of

resistance has been found whereas to other diseases the differential reaction consists mainly of varying degrees of susceptibility. Varietal

resistance is an ideal means of disease control. To a few diseases control

through the use of resistant varieties is presently possible. I n other cases

resistance to certain diseases is being transferred to agronomically desirable varieties t.hrough hybridization and selection. Many other diseases

have not been studied adequately to permit accurate host genotype classification as to resistance. Rapid progress is being made and development

of varieties resistant to many of the diseases seems highly possible.

Many soybean disease organisims overwinter in diseased plant

residues or in soil. This mode of overwintering permits effective control

of the disease through crop rotation. Certain diseases, such as brown

stem rot, seldom make their appearance except in fields cropped with

soybeans in two or more consecutive years.

Seed treatment offers a means of control of those pathogens which

carry over on the surface of the seed. Response to seed treatment is

discussed under Section VI-4-b. Successful treatment for “deep” infection

of the seed by certain organisms such as the one causing downy mildew,

pod and stem blight, etc., is difficult if not impossible, and planting of

disease-free seed is likely to be the only effective means of control.



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2. Insects



Severe damage to snybeans by insects has not been widespread in the

inaj or producing areas. However, in local areas severe yield reductions

have been caused by insects during certain years. Heavy infestations

of grasshoppers have completely destroyed sizeable fields in the northwestern part of the corn belt. Blister beetles also have caused substantial

tlefoliation damage in this area and in southern states, and Japanese

beetles have inflicted similar damage in the eastern states. The green

clover worm, which is the caterpillar of a moth, Plathypena scabra Fab.

is a defoliating insect widespread throughout the South Atlantic Coastal

Plain and Eastern Corn Belt. Alt,hough this insect constitutes a constant

menace, it falls prey to a considerable number of insect parasites, and

has therefore not caused widespread damage. Complete failure of soybean crops in eastern North Carolina has been caused by caterpillars

(Kulash, 1948). The principal caterpillars are the velvetbean caterpillar, Anticarsia gammatiles Hbn., which is a ravenous feeder on soybean

foilage, and the corn earworm, Heliothis armigera (Hbn.), which feeds

on the green pods. Other caterpillars commonly found attacking soybeans in North Carolina are the yellow striped armyworm, Prodenia

ornitholgalEi Guenee, the fall armyworm Laphygma frugiperda S. and A.

and species of the Autographa group of caterpillars. Excellent control

was provided by application of benzene hexachloride or DDT. Symptoms of the grapevine colaspis, Colaspis brunnea Fubr. are usually

noticeable in the corn belt during late June. Branches with shrunken

stems and withered leaves are symptoms of the stems hollowed out by

this insect. Leafhoppers of several types cause damage to varieties

lacking upright pubescence.

Differential damage is usually noticeable when several varieties of

soybeans are exposed to a population of insects. As explained by Snelling (1941) in a review of literature on plant resistance to insects, diferential damage to varieties may be the result of clearcut resistance, or

may merely constitute differential levels of susceptibility or tolerance.

This is well illustrated in soybeans. Upright, dense pubescence, such

as occurs on most field varieties, was found by Johnson and Hollowell

(1935) to constitute a high type of resistance to leafhoppers, Empoasca

fabm, Harr. Glabrous varieties, on the other hand, were severely damaged by the sucking insects, whereas variet.ies with appressed pubescence

were intermediate in resistance and sustained only slight damage. Although Coon (1946) reported differential damage by Japanese beetles,

Popilliu japonica, Newm., to a group of soybean varieties, the differences

merely constituted varying degrees of susceptibility.



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MARTIN G. WEISS



XI. THEREGIONAL

APPROACH

TO SOYBEAN

RESEARCH

Prior to the past decade research in soybeans was conducted largely

by individual agrirult,urnl experiment stations of states in which soybeans gave promise of becoming a major crop. Many of these studies

were in cooperation with t,he Division of Forage Crops and Diseases,

Bureau of Plant Industry, U. S. Department of Agriculture. Cultural

and varietal recommendations frequently differed in adjacent states having similar soil and climatic conditions. Regionalization of agronomic

research in soybeans was made possible in 1936 with the organization of

the U. S. Dept. of Agr. Regional Soybean Industrial Products Laboratory

(1947). Organized as a coopcrativc laboratory between the U. S. Department of Agriculture and tlic agricultural experiment stations of t.he

12 states in the North Central region, it cnabled coordination of research

in these states and augmented the projects of the expcriment stations

with leadership and facilities not prcviously available.

Considerable impetus was given to agronomic research on soybeans

by the organization of the Laboratory. Of great immediate usefulness

were the regional variety tests, organized and implemented by the agronomy section of this Laboratory. Regional evaluation rapidly revealed

the merits of varieties, such as Earlyana, Patoka, Gibson, Chief, Viking,

and Boone, which were subsequently distributed to farmers by the states

where these varieties are best adapted. The efficiency of the extensive

iegional testing program was again aptly demonstrated when hybridization programs, conducted cooperatively by several state agricultural experiment stations and the Laboratory, attained the selection-testing stage.

As described by Cartter (1947), reduction in the period required for testing was thereby made possible. Whereas development of varieties of

normally self-pollinated crops is generally considered to require 14 to 15

years, some of the varieties emanating from the regional program, such

as Hawkeye (Weiss et al., 1947b), were released to growers within 10

years from t.he time the cross was made. The material reduction in the

time required for development was permitted by the combination of early

generation testing, as discussed under Section VIII-2-cJ and the extensive

regional testing program.

The cooperative hybridization programs have resulted in the development and distribution of several superior varieties such as Lincoln,

Hawkeye, Adams, Monroe, and Wabash.

Expansion of the U. S. Regional Soybean Laboratory in 1942 to

include cooperative research with 12 southern states resulted in equivalent advantages for this region. The regional testing program promptly

revealed the superiority of Ogden, a variety already developed but not,



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extensively grown. The development of superior oil-type varieties for

the south is well underway. Continued benefits from the coordinated

testing programs in the entire soybean production area are to be expected.

Emphasis on the composition of soybeans was permitted largely by

the analytical facilities of the Laboratory. As a consequence of thorough

analytical examination, commercial varieties with unsuitable composition

were rapidly eliminated from the experiment station lists of recommended

varieties and high oil content, suitable protein content and iodine number

of oil are required attributes of new varieties. The exchange of breeding

material between states has been strongly stimulated by regional cooperation. Segregrating populations and selections are furnished by the states

with substantial cooperative breeding programs t o those states in which

the soybean acreage has not justified extensive breeding programs. Final

selection of varieties adapted to the local conditions is thereby possible.

Materially improved varieties in the areas marginal to the regions of

heavy production may be expected in the near future.

The soybean disease investigations of the Division of Forage Crops

and Diseases have been closely coordinated with the U. S. Regional Soybean Laboratory. As described by Morse and Johnson (1946) the disease

studies are closely integrated with the breeding programs to facilitate

the product,ion of improved, disease-resistant varieties. Although the

pathological investigations are relatively new, much progress has been

made in the identification of pathogens, the determination of relative

damage attributable to the various diseases, the search for resistant host

germ plasm, transmission of the parasites, and control measures. These

cooperative pathological investigations will undoubtedly contribute

greatly toward sustaining high production of soybeans in the United

States.

Cultural methods are again claiming the attention of various research

agencies. Preliminary cultural studies were conducted by many agricultural experiment stations when soybeans were introduced as a new crop.

Thereafter, researches were largely directed in other channels. Wit,h the

mechanization of the soybean crop and development of varieties more

resistant t o lodging, the need for additional information on cultural methods has been realized and several intensive projects have been initiated

by experiment stations. Researches in weed control have also gained

momentum and are being coordinated through the Regional Weed Control

Conferences.

Although numerous studies have been conducted on the effect of soybeans on succeeding crops, relatively few data have been reported as to

the merits of various cropping systems when soybeans are included in

the rotation. Long-time rotational studies involving soybeans have been



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