Phytophthora root and stem rot symptoms were first observed in Indiana in 1948 and in Ohio in 1951. The causal agent was identified in Ohio and North Carolina in 1954. Since that time, Phytophthora rot has been reported in Argentina, Australia, Brazil, Canada, the People's Republic of China, Hungary, Italy, Japan, the former Soviet Union, and throughout the soybean-growing regions of the United States.
The disease may cause plant stand losses and complete yield reductions in very susceptible soybean cultivars. The estimated reduction in yield in 1994 was 560,300 metric tons. The severity of losses depends on cultivar susceptibility, rainfall, soil type, tillage, and compaction. Phytophthora rot is most severe in poorly drained clay soils that are readily flooded. Plant loss can occur in lighter soils and on well-drained soils if they are saturated for an extended period of time when plants are young.
Soybean is the only important host of the causal fungus. It has been isolated
from Lupinus spp. native to the United States. In greenhouse inoculation
tests but not in the field, it has been found to be pathogenic to alfalfa, sweet
clover, snap bean, and crane's-bill. For several years, Phytophthora
isolates from alfalfa, soybean, and other hosts were considered to be the same
species, leading to confusing reports on the host range of the species that
infects soybean. There is one report of P. parasitica, a pathogen
with a large host range, causing soybean stem rot.
Phytophthora rot may be found in soybean at any stage of development. Early symptoms are seed rot and pre- and postemergence damping-off (Fig. 1), which may necessitate extensive replanting. These symptoms occur in soils flooded soon after planting and are commonly misidentified as water damage. Postemergence damping-off and seedling root and stem rot (Fig. 2) also occur under the same conditions and reduce stands.
Symptoms on older seedlings depend on the relative susceptibility or tolerance of the cultivar. In the most susceptible cultivars or in those with low tolerance, seedling stems may appear water soaked, leaves may turn yellow, and the plants wilt and die. In the least susceptible or highly tolerant cultivars, root rot may cause stunting but plants are not killed.
Older plants of cultivars with low tolerance may die throughout the season
from infections that occur soon after planting. Symptoms include yellowing
between veins and leaf margins and chlorosis of upper leaves followed by
wilting. Leaves remain attached after the plants die. Foliar symptoms are
preceded by destruction of the lateral roots, severe taproot rot, and girdling
stem lesions that may progress up the stem as high as 10 nodes (Fig. 3) before
the plants wilt (Fig. 4). The cortex and vascular tissues are discolored. In
moderately susceptible plants, stem lesions consist of long, narrow, sunken,
brown lesions that progress up one side of the stem (Fig. 5). Plants with
unilateral stem lesions may not wilt.
In older plants of highly tolerant cultivars, the only symptoms are generally rot of the secondary roots and discoloration of the taproot. These plants are not killed by the fungus but may be stunted and slightly chlorotic, and symptoms may resemble those of mild nitrogen deficiency or flooding. Occasionally, a one-sided stem lesion may occur in such cultivars. These mild symptoms, referred to as hidden damage, may reduce yield by as much as 40%. Hidden damage can be seen and measured by comparing nontreated plants with those treated with a fungicide such as metalaxyl or by comparing near-isogenic lines with and without race-specific resistance.
Foliar blight has been reported after heavy rains and can be produced in a
growth chamber on young leaves. Symptoms consist of progressive, light brown
lesions with yellow margins that can form on young leaflets under continuous
misting. Older leaves are resistant to foliar blight, a phenomenon referred to
as age-related resistance.
P. sojae produces simple, indeterminate sporangiophores. Typically, terminal sporangia (conidia) are obpyriform (32–53 × 42–65 µm) and nonpapillate (Fig. 6). Sporangia germinate indirectly by extruding fully formed zoospores into a thin, delicate, membranous, evanescent vesicle, which quickly expands and ruptures. Zoospores are sometimes trapped within sporangia and germinate there. The resulting germ tubes penetrate the sporangium wall. Empty sporangia commonly proliferate internally (Fig. 6), forming new sporangia terminally or within the old sporangia. Sporangia may also germinate directly, functioning as conidia. Hyphal swellings are commonly formed.
The optimum temperature for direct germination is 25°C, for indirect germination 14°C, and for zoospore production 20°C. Zoospores are ovoid, bluntly pointed at one or both ends, and flattened on the sides. They have two flagella, a short one directed anteriorly and the other, four to five times as long, directed posteriorly. At the end of the motile period, which may last up to several days, zoospore movement becomes sluggish and jerky and encystment occurs. Encystment may be triggered prematurely by agitation or by bivalent or trivalent cations. The cysts frequently germinate immediately and directly, producing germ tubes, which generally swell to form appressoria when they contact a solid surface. After a short time, normal growth of mycelia is resumed from the appressoria. Cysts sometimes germinate by producing secondary zoospores, leaving the cyst membrane behind. Rarely, a miniature sporangium is formed at the tip of the germ tube. Cysts germinate more vigorously in nutrient solutions than in distilled water.
P. sojae is homothallic. Antheridia and oogonia develop in abundance on corn meal, lima bean, V8, or potato dextrose agar. The antheridia are diclinous and generally paragynous, although amphigynous antheridia may be found. The oogonia (29–58 µm) are thin walled and spherical or subspherical. An oospore develops after an antheridium fertilizes an oogonium. Dormant oospores (Fig. 7) have thick, smooth inner and outer walls, fine-grained cytoplasm, a spherical, refractive body in the center, a well-developed reserve granule, and a pair of pellucid bodies at the outer edge of the cytoplasm. Germination may occur after the pellucid bodies merge and fuse. At germination, the pellucid bodies are no longer detectable, the smooth inner wall erodes, the central refractive body is absorbed, and the spore has the appearance of a sporangium. When the oospore germinates, the inner wall is absorbed and the germ tube produces either a sporangium or mycelium.
Oospores may germinate about 30 days after formation. Germination occurs in distilled water and is increased by low levels of nutrients and root exudates. Light increases the percentage of germination but inhibits subsequent formation of sporangia. This inhibition of sporangial formation is reversed by root exudates. Oospore germination is initiated within 2 days of separation from mycelium but is nonsynchronous and may continue for 30 days or more. The optimum temperature for formation and germination of oospores is 24°C.
Sporangia develop in dilute extracts of lima bean or other plant material. They can also be induced to form on solid agar if washed repeatedly with water. Sporangia will form abundantly in young cultures grown on lima bean extract that are washed repeatedly with Chen-Zentmyer salt solution. The medium and age of the culture affect oospore size and sporangial shape and size.
Mycelium is coenocytic when young and becomes septate with age. It branches mostly at right angles, and there is a slight constriction at the base of each branch. Hyphae are 3–9 µm wide and frequently slightly curled. The optimum temperature for growth of most isolates is 25–28°C. Isolates of P. sojae vary widely in cultural characteristics, morphology, and virulence to specific resistance genes. There are 37 described races of the pathogen and also many isolates not yet described that do not fit any race designations. Races can be distinguished by differential reactions on eight near-isogenic lines, each with a different Rps resistance gene. These races are identified by a number or by a list of the resistance genes that they can defeat (virulence phenotype) (Table 1). The latter method is preferred. Additionally, five Rps genes have been used to further describe the races, and many lists have been generated from various sources. Many races no longer exist or differ from their original descriptions.
Three common selective media used for isolating P. sojae from plants
are P10VP, PARPH, and PBNIC. The first two have a cornmeal agar base and
include pimaricin (10 mg/l) and quintazone (100 mg/l) for selective inhibition
of nonpythiaceous fungi. The first utilizes vancomycin (200 mg/l) and the
second ampicillin (250 mg/l) and rifampicin (10 mg/l) for bacterial control.
The PBNIC medium utilizes benomyl (10 mg/l), quintazone (40 mg/l), and
iprodione (20 mg/l) for control of nonpythiaceous fungi and neomycin sulfate
(100 mg/l) and chloramphenicol (10 mg/l) for bacterial control. Hymexazol (20
mg/l) can be used in all three media for partial control of Pythium
spp. P. sojae cannot be isolated directly from soil. A semiquantitative
baiting method has been described in which soybean leaf disks are floated on
water flooded over soil for 2 hr and then plated on selective media.
Disease Cycle and Epidemiology
P. sojae persists in soils as oospores either in crop residues or free in soil after residues decompose. Many oospores are formed in roots and stems of susceptible and tolerant soybean cultivars (Fig. 8). Oospores can survive for many years without a host. Factors controlling dormancy and germination in soil have not been determined.
P. sojae cannot be recovered directly from soil incubated for an extended period of time at 3°C or lower (conditions comparable to overwintering). The soil must be incubated for at least 1 week at 25°C before P. sojae can be isolated by using soybean seedlings or leaf-disk baits. Little oospore germination occurs until soil temperatures exceed 15°C, and germination is delayed at that temperature. From such observations, it is assumed that in the spring, oospores germinate whenever the temperature is suitable for forming sporangia. The sporangia accumulate until the soil is flooded, at which time zoospores are released. Sporangia also form on the surfaces of infected roots (Fig. 9), providing secondary inoculum. Soils must be flooded or saturated with water for zoospores to be produced. Zoospores can swim short distances (1.0 cm or less) in saturated soil but are disseminated primarily by moving floodwater. Phytophthora rot is essentially a monocyclic disease; plants become more tolerant with age and secondary inoculum does not increase loss.
Zoospores are attracted to seeds and roots by genistein and other isoflavonoid exudates. They encyst on the root surfaces and immediately germinate and penetrate the roots. Hyphae grow intercellularly in root tissues. Intracellular growth has been reported in hypocotyls. Globular and fingerlike haustoria penetrate host cells in susceptible cultivars. Cell necrosis is more prevalent in resistant than in susceptible cultivars. Colonization of the cortex is similar in susceptible and resistant cultivars for about 14 hr. Colonization then ceases in resistant cultivars, and the fungus fails to reach the vascular tissue.
Oogonia and oospores are formed in infected root and stem tissues of susceptible, tolerant, and resistant cultivars. However, more oospores are formed in susceptible and tolerant cultivars than in resistant cultivars. Oospore formation in resistant cultivars is not associated with root rot. Leaf infection, which is rare, results in more severe foliar symptoms than does root infection. Leaf infection occurs when soil particles containing the pathogen are splashed onto leaves during rainstorms. If the weather remains misty and cloudy, leaves can become severely infected and the fungus grows toward the petioles and stems.
Phytophthora root rot is most common in heavy, tightly compacted, fine
(clay) soil or soils with an impervious, shallow hardpan that are subject to
saturation and flooding. Flooding rains within a week after planting create
conditions most favorable for disease development. Highly tolerant cultivars
may escape severe damage if they emerge and reach the first trifoliolate stage
without conditions becoming suitable for zoospore release. Less tolerant
cultivars can be damaged from root rot that develops from midseason infection
periods. Damage caused by P. sojae can be reduced by tile drainage to
remove excess soil water and reduce the duration of saturation and flooding.
Damage increases in areas of reduced tillage, especially no-till, because
these areas absorb more rainfall and are more easily saturated. Population
densities of P. sojae in no-till areas with fine-textured soils are
higher than in no-till areas with moderately textured soils. Monocropping of
soybean increases damage, but rotation with nonhosts may not reduce disease
severity. Application of high levels of fertilizer (KCl) just before planting
may increase damage.
Race-specific resistant cultivars should be used. Thirteen genes at seven loci control race-specific, whole-plant resistance. Resistance results in an incompatible interaction in which the fungus fails to colonize tissue beyond a hypersensitive fleck-type lesion. Cultivars with the Rps1c, Rps1d, Rps1k, or Rps3a gene and several with two genes are available.
A second type of race-specific resistance has been named "root resistance." In some cultivars, the hypocotyls are susceptible but the roots may be partially resistant. This phenomenon has not been clearly defined, and no specific genes have been described.
A third type of reaction is a cultivar-specific quantitative difference in disease that is a type of non-race-specific, partial resistance and has been referred to as "field resistance," "rate-reducing resistance," or "tolerance." Tolerant cultivars include Asgrow A3127, Conrad, Elgin, Jack, Ripley, and Zane.
It is important to know which virulence phenotypes are present in the field before selecting a cultivar with specific resistance. In the Lake Erie Basin, for example, Rps1k and Rps3a or the gene stack of Rps1k + Rps6 are most effective. Elsewhere in the Midwest, Rps1a and Rps1c are still effective. With continued breeding for resistance, other gene stacks or as yet undescribed genes can be incorporated into soybean for future management through resistance.
Metalaxyl, an acylalanine fungicide specific for oomycetes, is often applied to tolerant cultivars as a seed treatment (Apron) or as an in-furrow spray or granule (Ridomil). Metalaxyl is more effective on highly tolerant cultivars. In general, metalaxyl applied to the soil is taken up by the roots and provides longer-lasting control than metalaxyl seed treatment.
Integrated control, which combines high tolerance, improved drainage, and
tillage, is as effective as resistance or fungicides in most soil
environments. Metalaxyl seed treatment may partially compensate for excess
soil moisture resulting from no-till cropping. Crop rotation should be used to
avoid increasing inoculum levels in the field, and the application of potash
fertilizer or animal manure immediately before planting should be avoided.
Very early planting when soil temperatures are below 15°C may allow soybean
to avoid significant damage but increases infection by Pythium spp.
(see Pythium Damping-Off and Root Rot).
Abney, T. S., Melgar, J. C., Richards, T. L., Scott, D. H., Grogan, J., and Young, J. 1997. New races of Phytophthora sojae with Rps1-d virulence. Plant Dis. 81:653-655.
Anderson, T. R. 1986. Plant losses and yield responses to monoculture of soybean cultivars susceptible, tolerant, and resistant to Phytophthora megasperma f. sp. glycinea. Plant Dis. 70:468-471.
Anderson, T. R., and Buzzell, R. I. 1982. Efficacy of metalaxyl in controlling Phytophthora root and stalk rot of soybean cultivars differing in field tolerance. Plant Dis. 66:1144-1145.
Faris, M. A., Sabo, F. E., Barr, D. J. S., and Lin, C. S. 1989. The systematics of Phytophthora sojae and P. megasperma. Can. J. Bot. 67:1442-1447.
Förster, H., Tyler, B. M., and Coffey, M. D. 1994. Phytophthora sojae races have arisen by clonal evolution and by rare outcrosses. Mol. Plant-Microbe Interact. 7:780-791.
Hansen, E. M., and Maxwell, D. P. 1991. Species of Phytophthora megasperma complex. Mycologia 83:376-381.
Ploper, L. D., Athow, K. L., and Laviolette, F. A. 1985. A new allele at the Rps3 locus for resistance to Phytophthora megasperma f. sp. glycinea in soybean. Phytopathology 75:690-694.
Schmitthenner, A. F., and Bhat, R. G. 1994. Useful methods for studying Phytophthora in the laboratory. Ohio Agric. Res. Dev. Cent. Spec. Circ. 143.
Schmitthenner, A. F., Hobe, M., and Bhat, R. G. 1994. Phytophthora sojae races in Ohio over a 10-year interval. Plant Dis. 78:269-276.
Schmitthenner, A. F., and VanDoren, D. M., Jr. 1985. Integrated control of root rot of soybean caused by Phytophthora megaspermaf. sp. glycinea. Pages 263-266 in: Ecology and Management of Soilborne Plant Pathogens. C. A. Parker, A. D. Rovira, K. J. Moore, P. T. W. Wong, and J. F. Kollmorgen, eds. American Phytopathological Society, St. Paul, MN.
Tooley, P. W., and Grau, C. R. 1984. Field characterization of rate-reducing resistance to Phytophthora megasperma f. sp. glycinea in soybean. Phytopathology 74:1201-1208.
Tyler, B. M., Förster, H., and Coffey, M. D. 1995. Inheritance of avirulence factors and restriction fragment length polymorphism markers in outcrosses of the oomycete Phytophthora sojae. Mol. Plant-Microbe Interact. 8:515-523.
Walker, A. K., and Schmitthenner, A. F. 1984. Heritability of tolerance to Phytophthora rot in soybeans. Crop Sci. 24:490-491.