A Molecular Marker Identifying Subspecific Populations of the Soybean Brown Stem Rot Pathogen, Phialophora gregata

Diagnosis of Soybean Diseases Caused by Fungal and Oomycete Pathogens: Existing Methods and New Developments

Soybean (Glycine max) acreage is increasing dramatically, together with the use of soybean as a source of vegetable protein and oil. However, soybean production is affected by several diseases, especially diseases caused by fungal seed-borne pathogens. As infected seeds often appear symptomless, diagnosis by applying accurate detection techniques is essential to prevent propagation of pathogens. Seed incubation on culture media is the traditional method to detect such pathogens. This method is simple, but fungi have to develop axenically and expert mycologists are required for species identification. Even experts may not be able to provide reliable type level identification because of close similarities between species. Other pathogens are soil-borne. Here, traditional methods for detection and identification pose even greater problems. Recently, molecular methods, based on analyzing DNA, have been developed for sensitive and specific identification. Here, we provide an overview of available molecular assays to identify species of the genera Diaporthe, Sclerotinia, Colletotrichum, Fusarium, Cercospora, Septoria, Macrophomina, Phialophora, Rhizoctonia, Phakopsora, Phytophthora, and Pythium, causing soybean diseases. We also describe the basic steps in establishing PCR-based detection methods, and we discuss potentials and challenges in using such assays.

Fine Mapping of Resistance Genes from Five Brown Stem Rot Resistance Sources in Soybean

Brown stem rot (BSR) of soybean [ (L.) Merr.] caused by (Allington & Chamb.) T.C. Harr. & McNew can be controlled effectively with genetic host resistance. Three BSR resistance genes , , and , have been identified and mapped to a large region on chromosome 16. Marker-assisted selection (MAS) will be more efficient and gene cloning will be facilitated with a narrowed genomic interval containing an gene. The objective of this study was to fine map the positions of genes from five sources. Mapping populations were developed by crossing the resistant sources 'Bell', PI 84946-2, PI 437833, PI 437970, L84-5873, and PI 86150 with either the susceptible cultivar Colfax or Century 84. Plants identified as having a recombination event near genes were selected and individually harvested to create recombinant lines. Progeny from recombinant lines were tested in a root-dip assay and evaluated for foliar and stem BSR symptom development. Overall, 4878 plants were screened for recombination, and progeny from 52 recombinant plants were evaluated with simple-sequence repeat (SSR) genetic markers and assessed for symptom development. Brown stem rot resistance was mapped to intervals ranging from 0.34 to 0.04 Mb in the different sources. In all sources, resistance was fine mapped to intervals inclusive of BARCSOYSSR_16_1114 and BARCSOYSSR_16_1115, which provides further evidence that one locus provides BSR resistance in soybean.

Anatomical response and infection of soybean during latent and pathogenic infection by type A and B of Phialophora gregata

Growth and anatomical responses of plants during latent and pathogenic infection by fungal pathogens are not well understood. The interactions between soybean (Glycine max) and two types of the pathogen Phialophora gregata were investigated to determine how plants respond during latent and pathogenic infection. Stems of soybean cultivars with different or no genes for resistance to infection by P. gregata were inoculated with wildtype or GFP and RFP-labeled strains of types A or B of P. gregata. Plants were sectioned during latent and pathogenic infection, examined with transmitted light or fluorescent microscopy, and quantitative differences in vessels and qualitative differences in infection were assessed using captured images. During latent infection, the number of vessels was similar in resistant and susceptible plants infected with type A or B compared to the control, and fungal infection was rarely observed in vessels. During pathogenic infection, the resistant cultivars had 20 to 25% more vessels than the uninfected plants, and fungal hyphae were readily observed in the vessels. Furthermore, during the pathogenic phase in a resistant cultivar, P. gregata type A-GFP was limited to outside of the primary xylem, while P. gregata type B-RFP was observed in the primary xylem. The opposite occurred with the susceptible cultivar, where PgA-GFP was observed in the primary xylem and PgB-RFP was limited to the interfascicular region. In summary, soybean cultivars with resistance to BSR produced more vessels and can restrict or exclude P. gregata from the vascular system compared to susceptible cultivars. Structural resistance mechanisms potentially compensate for loss of vessel function and disrupted water movement.

Influence of Soybean Monoculture on Phialophora gregata f. sp. sojae IGS-Genotype B Isolate Aggressiveness

Many soybean accessions described as resistant to brown stem rot (BSR) are preferentially colonized by isolates of Phialophora gregata f. sp. sojae genotype B. These isolates are generally considered less aggressive than isolates of P. gregata f. sp. sojae genotype A because they cause minor or no foliar symptoms characteristic of BSR. However, variation in aggressiveness has been observed among isolates of P. gregata f. sp. sojae genotype B. To determine if BSR-resistant soybean accessions would preferentially select for more aggressive isolates of P. gregata f. sp. sojae genotype B, monocultures of both BSR-resistant or BSR-susceptible accessions were established at the Arlington Agriculture Research Station, Arlington, WI. BSR-susceptible cv. Corsoy 79 and BSR-resistant plant introduction (PI) 567.157A were inoculated under greenhouse conditions with a total of 39 isolates of P. gregata f. sp. sojae genotype B obtained from the different monocultures. BSR severity was determined as the percentage of symptomatic foliar and internal stem tissue. Overall, BSR severity was low and did not exceed 20% for either foliar or stem symptoms. Isolates of P. gregata f. sp. sojae genotype B caused more severe foliar (P < 0.0001) and stem (P = 0.0008) symptoms of BSR on PI 567.157A than on Corsoy 79. Analysis of BSR stem symptom severity indicated an interaction (P = 0.0124) between soybean accession and the origin of isolates of P. gregata f. sp. sojae genotype B. Isolates of P. gregata f. sp. sojae genotype B obtained from the monoculture of a BSR-susceptible or -resistant accession were more aggressive than isolates from a mixed resistant and susceptible soybean monoculture. The relationship between the origin of isolate of P. gregata f. sp. sojae genotype B and isolate aggressiveness was more apparent for PI 567.157A than for Corsoy 79. Results of this study indicate that the monoculture of resistant or susceptible soybean favors an increase in the aggressiveness of isolates of P. gregata f. sp. sojae genotype B. Furthermore, results suggest that resistance to genotype A may be genetically different from resistance to genotype B.

Influence of Monocropping Brown Stem Rot-Resistant and -Susceptible Soybean Accessions on Soil and Stem Populations of Phialophora gregata f. sp. sojae

Brown stem rot (BSR)-resistant and -susceptible soybean accessions were continuously cropped in an area never previously seeded to soybean to study the influence of monocultures on soil and stem populations of Phialophora gregata f. sp. sojae. P. gregata f. sp. sojae population size and genotype composition were determined by dilution plating, isolation of P. gregata f. sp. sojae and standard polymerase chain reaction (PCR), and by quantitative real-time PCR (q-PCR. In general, the sizes of P. gregata f. sp. sojae populations in soil were similar regardless of monoculture. The percentage of P. gregata f. sp. sojae genotype B was greater than A in soil following the monoculture of both BSR-susceptible and -resistant soybean accessions. Following the monoculture of a BSR-resistant accession, the percentage of P. gregata f. sp. sojae genotype B was greater than A. Overall, P. gregata f. sp. sojae populations in stems of a BSR-susceptible accession were greater than those in stems of a BSR-resistant accession. P. gregata f. sp. sojae genotype B was detected more often than A in stems of both resistant and susceptible accessions planted following a BSR-resistant monoculture. P. gregata f. sp. sojae genotype B was also detected more often than A in stems of a BSR-resistant accession planted following a BSR-susceptible monoculture. P. gregata f. sp. sojae genotypes A and B were isolated at similar frequencies from stems of a BSR-susceptible accession planted following a BSR-susceptible monoculture. However, q-PCR results indicate that the percentage of P. gregata f. sp. sojae genotype A was greater than B in stems of a BSR-susceptible accession planted following a BSR-susceptible monoculture. Among BSR-susceptible accessions, those with the soybean cyst nematode (SCN)-resistant cv. Peking in their parentage had the largest populations of P. gregata f. sp. sojae and a greater percentage of P. gregata f. sp. sojae genotype B. Similar results were observed for BSR-resistant accessions derived from SCN-resistant PI 88788.

Real-time PCR assays for the quantification of Phialophora gregata f. sp. sojae IGS genotypes A and B

Populations of Phialophora gregata f. sp. sojae, the causal agent of brown stem rot (BSR) of soybean, consist of two genotypes, designated A and B. These genotypes are differentiated by an insertion or deletion in the intergenic spacer region (IGS) of ribosomal DNA. The two genotypes differ in the type and severity of symptoms they cause and have displayed preferential host colonization. Methods to quantify populations of P. gregata f. sp. sojae and to distinguish between the two genotypes are essential to understanding this host-pathogen interaction and to improving control of BSR. A real-time, quantitative polymerase chain reaction (qPCR) assay was developed for the specific detection and quantification of P. gregata f. sp. sojae genotype A. This assay is specific to P. gregata f. sp. sojae genotype A, sensitive to 50 fg of DNA, and unaffected by the presence of soybean or soil DNA. When the P. gregata f. sp. sojae genotype A-specific primer/probe set is used in a multiplex qPCR assay with a previously developed primer/probe set which indiscriminately amplifies both genotypes, the quantity of P. gregata f. sp. sojae genotype B can be indirectly determined. This multiplex assay provides a rapid and robust method for studying both the population size and genetic structure of P. gregata f. sp. sojae in its soybean host and in the soil.

Detection and Quantification of Phialophora gregata in Soybean and Soil Samples with a Quantitative, Real-Time PCR Assay

Brown stem rot of soybean, caused by the soilborne fungus Phialophora gregata, is a common and widespread disease of soybean (Glycine max) in the midwestern United States. This pathogen is challenging to study due to a long latent period and slow growth. A TaqMan probe-based quantitative, real-time polymerase chain reaction (qPCR) assay was developed for sensitive and specific detection and quantification of genotypes A and B of P. gregata in plant and soil samples. It is sensitive with detection limits of 50 fg of pure genomic DNA, 100 copies of the target DNA sequence, and approximately 400 conidia. The qPCR assay is approximately 1,000 times more sensitive in detecting DNA and conidia of P. gregata, and is more rapid and less sensitive to PCR inhibitors from soybean stems than a standard PCR (sPCR) assay. Using this single-step qPCR assay, low levels of infection were detected in soybean stems at least 1 to 2 weeks prior to symptom development and before P. gregata was detected with sPCR. This assay also was used to detect the pathogen in field-grown plants and in naturally infested field soils. This new qPCR assay is a powerful tool for rapid, specific, and sensitive detection, diagnosis, and quantification of P. gregata in plants and soil, and for advancing studies of the ecology of P. gregata and its interactions with host plants.

Genotypes A and B of Cadophora gregata Differ in Ability to Colonize Susceptible Soybean

Growth chamber experiments were conducted to determine if extent of colonization of soybean stems by genotypes A and B of Cadophora gregata (Phialophora gregata), the causal agent of brown stem rot (BSR) of soybean, is similar in soybean plants resistant or susceptible to genotype A. Upon introduction of the two genotypes separately into the base of stems of 2-week-old seedlings, genotype A advanced with the growing tips of susceptible but not resistant genotypes. In contrast, genotype B did not advance with the growing tips of either resistant or susceptible soybean. In similar experiments, 5 weeks after introduction of genotype A, both mean percent stem length colonized by C. gregata and mean percentage of symptomatic trifoliate leaflets were significantly less for resistant than for susceptible genotypes. For genotype B, there was no or a slight difference between resistant and susceptible soybean genotypes in mean percent stem length colonized and no difference in mean percentage of symptomatic trifoliate leaflets 5 weeks after introduction of the pathogen. These results indicate that genotype A and genotype B differ not only in the severity of foliar symptoms they cause on genotype A-susceptible soybean plants, but also in how severely they colonize the stems of these soybean plants. In our experiments, genotype A and genotype B did not differ consistently in their ability to cause internal stem discoloration. The two genotypes of C. gregata can be distinguished based on how severely they colonize stems of genotype A-susceptible soybean. Thus, a BSR resistance screening method, which relies on assessment of stem colonization by C. gregata, works only for screening soybean lines resistance to genotype A. In light of these results, it is important to distinguish soybean resistance to genotype A versus genotype B of C. gregata. Whether genotype B causes yield loss and whether soybean plants can be distinguished as resistant or susceptible to genotype B needs to be investigated.

Soybean Stem Colonization by Genotypes A and B of Cadophora gregata Increases with Increasing Population Densities of Heterodera glycines

Growth chamber experiments were conducted to investigate whether parasitism by increasing population densities of Heterodera glycines, the soybean cyst nematode, increases the incidence and severity of stem colonization by the aggressive genotype A and the mild genotype B of Cadophora gregata (Phialophora gregata), causal agents of brown stem rot of soybeans. Soybean genotypes with three combinations of resistance and susceptibility to H. glycines and genotype A of C. gregata were inoculated with each genotype of C. gregata alone or each genotype with two population densities of H. glycines eggs, 1,500 or 10,000 per 100 cm³ of soil. Stems of two H. glycines-susceptible soybeans were more colonized by both aggressive and mild genotypes of C. gregata in the presence of high than in the presence of low H. glycines population density.

A New Greenhouse Method to Assay Soybean Resistance to Brown Stem Rot

Greenhouse, growth chamber, and field experiments were conducted to develop a method to assess resistance of soybeans to Cadophora gregata (Phialophora gregata), causal agent of brown stem rot (BSR). In the new method, C. gregata is introduced at the base of the stems of 2-week-old soybeans, and the presence of the fungus is assessed in the tips of the stems 5 weeks later. To test the effectiveness of the method, two populations of soybeans and 10 checks were inoculated at the stem base and then assayed for fungal colonization of the stem tips, percentage of symptomatic leaflets, and percent internal stem length discolored. The lines also were planted in naturally infested fields to assess for percent internal stem length discolored, and were tested for the presence/absence of a BSR-resistant molecular marker. Greenhouse, field, and molecular marker data were compared. Linear regression analysis suggested that percentage of plants with colonized stem tips explained 41 to 64% of the variability (P < 0.0001) in percent stem length discolored in the field and 58 to 85% of the variability (P < 0.0001) in molecular marker data for BSR resistance. Percent stem length discolored assessed in the greenhouse had the lowest correlation with percent stem length discolored in the field and with the molecular marker. Of three incubation temperatures tested, 22°C was the most conducive for distinguishing resistant/susceptible soybeans using the colonization method.

Population Dynamics of Phialophora gregata in Stem Residue of a Resistant and a Susceptible Soybean Cultivar

Previous studies on the saprophytic survival of Phialophora gregata were conducted with soybean residue derived from a susceptible cultivar and did not address genotypes of P. gregata. This current study monitored the saprophytic population density of P. gregata in stem residue derived from a susceptible and a resistant soybean cultivar placed in the field. A second phase of the study followed the frequencies of genotypes A and B of P. gregata in stem residue derived from a susceptible cultivar. The population density of P. gregata declined 10-fold in stem residue from the initiation of sampling to the end of this 16-month study, regardless of cultivar or whether residue was positioned on the soil surface or buried. The population density of P. gregata was greater in buried residue of the resistant cultivar compared with the susceptible cultivar after 12 to 14 months, but equalized after 16 months. The population density of P. gregata was similar in residue derived from the susceptible and resistant cultivars if positioned on the soil surface. Genotype B was detected more frequently than genotype A of P. gregata at each sampling date regardless of residue placement.

The traditional Chinese medicine Cordyceps sinensis and its effects on apoptotic homeostasis

Cordyceps sinensis is a medicinal fungus of Traditional Chinese Medicine. While there are a wide range of reported uses of Cordyceps sinensis in the literature, the reports that extracts of this fungus may alter apoptotic homeostasis are most intriguing. However, there are significant challenges regarding research surrounding Cordyceps sinensis, such as the difficulty identifying the various species of Cordyceps and the many conflicting reports of pharmacological function in the literature. In this review we outline what is known about the ability of Cordyceps sinensis to alter apoptotic homeostasis, attempt to reconcile the differences in reported function, identify the challenges surrounding future Cordyceps sinensis research, and delineate options for overcoming these critical hurdles.

Resistance to Brown Stem Rot in Soybean Germ Plasm with Resistance to the Soybean Cyst Nematode

The soybean cyst nematode (SCN) and Phialophora gregata f. sp. sojae, the causal agent of brown stem rot (BSR), are two pathogens of soybean commonly found in the same field throughout the north-central United States. Field experiments designed to study the role of SCN-resistant germ plasm in soybean production have led to data suggesting that some sources of SCN resistance also may provide resistance to BSR. Soybean germ plasm with resistance to SCN was evaluated in greenhouse and field environments for resistance to BSR development based on the percentage of host tissue symptomatic of BSR. Comparison of SCN-resistant cultivars and plant introductions (PI) to standard BSR-resistant and -susceptible checks were conducted in two greenhouse experiments using a root-dip inoculation with a single isolate of P. gregata. For both greenhouse experiments, PI 209332 was the only source of SCN resistance with resistance to BSR similar to standard BSR-resistant checks. Nine other sources of SCN resistance, including PI 88788 and Peking, expressed BSR symptom severity similar to BSR-susceptible checks. Cultivars derived from most SCN-resistant sources, including PI 209332, also were susceptible to BSR development, while four of the five cultivars derived from PI 88788 were highly resistant to BSR development. SCN-resistant cultivars derived from PI 88788, Peking, and PI 209332 were planted along with standard BSR-resistant and -susceptible checks at two field locations naturally infested with P. gregata and SCN or P. gregata alone. As in greenhouse experiments, four of the five cultivars derived from PI 88788 expressed resistance to BSR development equal to or better than standard BSR-resistant checks at both locations. In contrast, cultivars derived from PI 209332 and Peking expressed varying levels of disease development depending on field environment. Yields observed for PI 88788-derived cultivars were higher than BSR-resistant checks regardless of the presence of SCN. Data from both greenhouse and field experiments suggest that cvs. Williams and Williams 82 may contain a gene or genes for BSR resistance that require one or more modifier genes, possibly located in the genome of PI 88788, for complete expression.

Detection of the biocontrol agent Colletotrichum coccodes (183088) from the target weed velvetleaf and from soil by strain-specific PCR markers

Diagnostic molecular markers, generated from random amplified polymorphic DNA (RAPD) and used in polymerase chain reaction (PCR), were developed to selectively recognize and detect the presence of a single strain of the biocontrol fungus Colletotrichum coccodes (183088) on the target weed species Abutilon theophrasti and from soil samples. Several isolates of C. coccodes, 15 species of Colletotrichum, a variety of heterogeneous organisms and various plant species were first screened by RAPD-PCR, and a strain specific marker was identified for C. coccodes (183088). No significant sequence similarity was found between this marker and any other sequences in the databases. The marker was converted into a sequence-characterised amplified region (SCAR), and specific primer sets (N5F/N5R, N5Fi/N5Ri) were designed for use in PCR detection assays. The primer sets N5F/N5R and N5Fi/N5Ri each amplified a single product of 617 and 380 bp, respectively, with DNA isolated from strain 183088. The specificity of the primers was confirmed by the absence of amplified products with DNA from other C. coccodes isolates, other species representing 15 phylogenetic groups of the genus Colletotrichum and 11 other organisms. The SCAR primers (N5F/N5R) were successfully used to detect strain 183088 from infected velvetleaf plants but not from seeded greenhouse soil substrate or from soil samples originating from deliberate-released field experiments. The sensitivity of the assay was substantially increased 1000-fold when nested primers (N5Fi/N5Ri) were used in a second PCR run. N5Fi/N5Ri selectively detected strain 183088 from seeded greenhouse soils as well as from deliberate-released field soil samples without any cross-amplification with other soil microorganisms. This rapid PCR assay allows an accurate detection of C. coccodes strain 183088 among a background of soil microorganisms and will be useful for monitoring the biocontrol when released into natural field soils.

Cultivar Preference and Genotype Distribution of the Brown Stem Rot Pathogen Phialophora gregata in the Midwestern United States

Brown stem rot (BSR), caused by Phialophora gregata f. sp. sojae, is an important yield-limiting disease of soybean (Glycine max) in the midwestern United States. Midwestern populations of P. gregata are separated into genotypes A and B based on intergenic spacer sequences of nuclear ribosomal DNA. Genotype A causes both leaf and stem symptoms, and genotype B typically causes internal stem symptoms only. Data are limited on the geographic distribution of genotypes A and B. It is not well understood whether cultivars may be infected preferentially by a genotype. Field plots were established at five sites in Illinois, three sites in Wisconsin, and two sites in Minnesota in two different years. Soybean cvs. Bell, BSR101, Dwight, Sturdy, Williams 82, LN92-12033, and LN92-12054 were sown with two to four replications at each field site. From each plot, 5 to 10 stems were harvested arbitrarily at the R8 growth stage and assayed by polymerase chain reaction to detect the A and B genotypes. Both pathogen genotypes were detected at all locations except Urbana, where only genotype A was detected, and St. Paul, where only B was detected. Genotype A was the predominant genotype detected in susceptible cvs. Williams 82 and LN92-12054, with 70 and 78% of infected stems, respectively, positive for A. The other susceptible cultivar, Sturdy, yielded predominantly genotype A at four of the seven Illinois and Wisconsin locations where both pathogen genotypes were present, but yielded predominantly B at the Minnesota location where both genotypes were detected. Genotype B was the predominant type detected in partially resistant cvs. Dwight, LN92-12033, and Bell, with 56, 85, and 99% of the infected stems, respectively, testing positive for B.

Characterization and Distribution of Two Races of Phialophora gregata in the North-Central United States

ABSTRACT Genetic variation and variation in aggressiveness in Phialophora gregata f. sp. sojae, the cause of brown stem rot of soybean, was characterized in a sample of 209 isolates from the north-central region. The isolates were collected from soybean plants without regard to symptoms from randomly selected soybean fields. Seven genotypes (A1, A2, A4, A5, A6, M1, and M2) were distinguished based on DNA fingerprinting with microsatellite probes (CAT)(5) and (CAC)(5), with only minor genetic variation within the A or M genotypes. Only the A1, A2, and M1 genotypes were represented by more than one isolate. The A genotypes dominated in the eastern Iowa, Illinois, and Ohio samples, whereas the M genotypes were dominant in samples from western Iowa, Minnesota, and Missouri. In growth chamber experiments, isolates segregated into two pathogenicity groups based on their aggressiveness toward soybean cvs. Kenwood and BSR101, which are relatively susceptible and resistant, respectively, to brown stem rot. In both root dip inoculation and inoculation by injecting spores into the stem near the ground line (stab inoculations), isolates of the A genotypes caused greater foliar symptoms and more vascular discoloration than isolates of the M genotypes on both cultivars of soybean. All isolates caused foliar symptoms in both cultivars and in three additional cultivars of soybean with resistance to brown stem rot. Greater differences between the A and M genotypes were seen in foliar symptoms than in the linear extent of xylem discoloration, and greater differences were seen in Kenwood than in BSR101. Inoculation of these genotypes into five cultivars of soybean with different resistance genes to brown stem rot showed a genotype x cultivar interaction. A similar distinction was found in an earlier study of the adzuki bean pathogen, P. gregata f. sp. adzukicola, and consistent with the nomenclature of that pathogen, the soybean pathogens are named the aggressive race (race A) and the mild race (race M) of P. gregata f. sp. sojae.

Pathogenic Characterization of Genotypes A and B of Phialophora gregata f. sp. sojae

Genetic studies of Phialophora gregata f. sp. sojae, the causal agent of brown stem rot (BSR) of soybean, have led to the development of species-specific primers capable of separating isolates into two distinct genotypes, A and B. To determine whether genotypic characterization could be related to differences in BSR symptom expression, five soybean cultivars, Pioneer 9234, Corsoy 79 (both BSR susceptible), Williams, BSR 101, and Jack and plant introduction (PI) 437970 (all BSR resistant), were inoculated with a total of 27 isolates of each genotype in four greenhouse experiments conducted from February to November 2000. BSR severity was calculated as the percentage of symptomatic foliar, internal stem, and internal root tissue. Genotype A isolates caused significantly more severe (P < 0.0001) BSR foliar symptoms than genotype B isolates on Pioneer 9234, Corsoy 79, Williams, and BSR 101, while Jack and PI 437970 expressed minimal foliar symptoms regardless of isolate genotype. Overall, internal stem symptoms caused by genotype A isolates were more severe than those caused by genotype B isolates on Pioneer 9234, Corsoy 79, Williams, and BSR 101. Conversely, Jack and PI 437970 did not differ significantly in severity of stem symptoms when inoculated with isolates of genotype A or B. Internal root symptoms for genotype A isolates were generally more severe than for genotype B isolates on all soybean genotypes tested. Our data strongly suggest that A and B genotypes of P. gregata f. sp. sojae differ in the severity of symptoms they cause, and that these genotypes correspond to the Type I (defoliating) and Type II (nondefoliating), respectively, pathotypes previously proposed for this vascular pathogen of soybean.