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X. Disease and Insect Pests
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
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.
MARTIN G. WEISS
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
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
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
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,
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
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.
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.
MARTIN G. WEISS
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,
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
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
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