Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/7/2000
Publication Date: N/A
Interpretive Summary: Phytophthora root rot of soybean, caused by Phytophthora sojae, is a devastating disease that caused an estimated yield loss of 560,300 metric tons in 1994 in the United States alone. Currently there are 53 identified races of P. sojae pathogenic on soybean, which are identified using a differential set of cultivars with the different race-specific resistance genes. The objectives of this study were to isolate and identify races of P. sojae in Illinois soybean fields to determine if the current resistance sources being used are effective as a control measure for P. sojae. The races of P. sojae that occur in Illinois are reported for the first time. Two new races of P. sojae that are not controlled by the commonly used race-specific resistance genes in soybean were identified. Combining currently used resistance sources could potentially control P. sojae in Illinois. Knowledge of new races present is essential for plant breeders to know when to incorporate new sources of genetic resistance into elite cultures. This information is useful to the growers, breeders and the seed industry.
Technical Abstract: 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.