Phytophthora root rot, caused by Phytophthora sojae, is a very destructive disease of soybean. Races of P. sojae have been identified from Indiana, Iowa, Ohio, and other states that are not controlled by the commonly used race-specific resistance genes in soybean. The prevalent races of P. sojae in Illinois are not known. The objectives of this study were to isolate and identify races of P. sojae in the state of Illinois. Thirty-three isolates were obtained from 192 soil samples collected throughout Illinois in 1997 and were identified to race by inoculating Rps isolines of the soybean cultivar Williams. Most P. sojae isolates were races 1, 3, 4, or variants with the addition of Rps1d virulence. One new race, 54, with virulence on the Rps1d and Rps7 alleles, accounted for 50% of the isolates. Another new race, 55, also was identified in one sample. Two isolates were obtained from plants with race-specific resistance and were virulent on those resistance alleles, and were identified as races 41 and 43. Many of the isolates were obtained from the east-central part of the state. Combining currently used resistance alleles with alleles at other loci could potentially control P. sojae in Illinois.

Phytophthora root rot, caused by Phytophthora sojae Kaufmann and Gerdeman, primarily attacks the roots of soybean [Glycine max (L.) Merr.] plants. Partial resistance and field tolerance in 14 commercial glyphosate [N-(phosphonomethyl)glycine] tolerant soybean cultivars with the Rps1k resistance gene were studied. Partial resistance to compatible P. sojae races 28 and 30 was evaluated by the agar layer technique. Relative to the percentage of the control, all of the commercial cultivars with the Rps1k had reductions in top mass and plant height that were not significantly different from the partial resistant check ‘Conrad’ that had 83% top mass and 77% plant height reduction; two of the 14 commercial cultivars had significantly lower root mass (28 and 31% lower) than Conrad (84%). In addition, there was no significant difference in disease ratings (root or whole plant) of the 14 commercial cultivars with the Rps1k compared with Conrad. Field tolerance, studied in six field experiments at Urbana, IL, during 2002–2004, was identified when there were no significant differences between the yield of inoculated treatments with or without mefenoxam [methyl N-(methoxyacetyl)-N-(2,6-xylyl)-D-alaninate] fungicide seed treatment or between inoculated and noninoculated treatments. There were no significant cultivar x inoculation x fungicide treatment interactions found in any of the field experiments, and a significant cultivar x inoculation treatment interaction was found in only one field experiment. Therefore, most of the cultivars appeared to be tolerant to P. sojae. It should be noted that field tolerance was not distinguished from partial resistance in the field component of this study.

Several genetic maps of the soybean have been developed during the past decade. Different markers have been used to construct these maps including RFLP, AFLP, RAPD, SSR, and classical markers. However, virtually none of the maps and linkage groups developed has been associated with specific soybean chromosomes. For example, there are 13 single dominant resistance genes at 7 different loci that control Phytophthora sojae. These resistance genes (Rps genes) have not been located on any chromosomes, but several have been associated on classical and molecular maps. For example, the Rps1 locus is associated with molecular linkage group N. The objective of this study was to locate the Rps1 locus on a specific soybean chromosome using primary trisomic analysis. Crosses were made between 10 soybean trisomic lines and cv. Resnik (containing Rps1-k). The F₂ populations from trisomic parents were inoculated with race 3 of P. sojae to determine the ratio of resistant to susceptible plants. Nine of the F₂ populations tested segregated in a normal 3:1 ratio. The F₂ population of triplo 3 segregated in a 2:1 ratio, the expected segregation ratio for a single dominant gene if the gene is located on the extra chromosome, suggesting that the Rps1 locus is on chromosome 3. Thus, chromosome 3 corresponds to molecular linkage group N of the integrated genetic linkage map.

New sources of soybean (Glycine max) resistance to Phytophthora sojae are needed to provide effective resistance because of the rapidly changing race patterns of P. sojae in fields. The objectives of our study were to develop a method to screen Glycine soja for resistance to P. sojae and then use this methodology to screen G. soja lines for resistance to P. sojae races 1, 3, and 20. An agar plug-inoculation method, in which a 3-mm-diameter mycelial plug of the fungus was placed mycelium side down on cotyledons of 10-day-old soybean seedlings, was directly compared with the traditional hypocotyl inoculation method. There was no significant difference between the hypocotyl- and plug-inoculation methods when tested on four soybean differential lines using three P. sojae races. The plug-inoculation method then was used to screen 430 G. soja accessions for resistance to P. sojae race 3. Nine G. soja accessions were retested with races 1, 3, and 20. Of the 430 G. soja accessions tested, 22 accessions had survival rates higher than 75% and nine had rates higher than 90% against race 3. Additionally, five of the nine accessions that were tested again had greater than 60% survival against races 1, 3, and 20. These results suggest that the plug-inoculation method can be used as an alternative to the hypocotyl-inoculation method. Potential sources of new P. sojae resistance and/or tolerance may be present in G. soja, but additional genetic research is needed to determine if these sources are different from sources currently found in G. max.

Phytophthora sojae causes damping-off, root rot, and stem rot of soybean, particularly in poorly drained soils. Soybean cultivar resistance is one of the primary methods to control this disease, with Rps1c, Rps1k, and Rps1a being the most commonly used genes. The Varietal Information Program for Soybeans (VIPS) at the University of Illinois evaluates soybean cultivars for resistance to a number of diseases including Phytophthora root rot (PRR). The objectives of this research were to evaluate PRR resistance among commercial cultivars or advanced lines, and to compare these results with the information on PRR resistance provided by the company that entered the cultivar in VIPS. Each year from 2004 to 2008, between 600 and 900 cultivars were evaluated for resistance to either race 17 or 26 of P. sojae using the hypocotyl inoculation method. P. sojae single resistance genes were reported in 1,808 or 51% of the entries based on company information. Of these, the most commonly reported resistance genes were Rps1c (50%), Rps1k (40%), and Rps1a (10%). To a much smaller degree, companies reported using Rps3a (0.3%), Rps1b (0.2%), and Rps7 (0.2%). For the duration of the 5-year testing period, almost half of the cultivars (46%) were entered in VIPS with no reported resistance genes, and only nine out of a total of 3,533 entries (less than 0.3%) reported a stacked combination of resistance genes. Agreement between company-reported genes and any resistance found in the VIPS PRR evaluation was highest for those cultivars claiming to have Rps1c (90%) and Rps1k (83%), followed by Rps1a (70%). On average, 54% of the cultivars submitted to VIPS each year were new, reflecting the rapid development and turnover of soybean cultivars provided by the soybean seed companies.