Population dynamics of Phytophthora sojae in Indiana and the influence of management practices on the pathogen

Phytophthora root rot of soybeans is a primary yield loss concern for midwestern soybean producers and losses have increased during the 1990s. This study documents race diversity and prevalence of Phytophthora sojae, causal agent of Phytophthora root rot, over a five-year period. Soil samples from soybean fields were randomly collected from 81 Indiana counties that yielded 816 P. sojae isolates. This survey revealed presence in Indiana fields for 26 of the 55 known races. Eleven dominant races (1, 3, 4, 13, 25, 28, 29, 33, 43, 44, and 45) occurred throughout the state. Many of the prevalent races possess virulence to Rps1-k, the most commonly employed form of genetic resistance. Virulence to Rps1-a was identified at a significantly higher frequency than virulence to Rps1-b, 1-c, 1-d, 1-k, 3-a or 6. Conversely, virulence to the Rps 3-a allele was identified at a significantly lower frequency throughout the study. The Rps 3-a gene can be combined with the Rps1-k or 1-c gene in soybeans to improve resistance and minimize yield losses. A separate phase of the study identifies the impact of crop management practices (host resistance, tillage, post-herbicide injury, and crop rotation) on the population dynamics of P. sojae . Data were collected in 1999 and 2000 from the long-term tillage plots at Purdue University\u27s Agronomy Research Center that have a history of P. sojae damage. Soil samples were collected where resistant (Rps1-k) vs. susceptible (rps) soybean cultivars had been planted in a split-plot section of the tillage study for over 10-years. Thirteen races (races 1, 3, 11, 13, 23, 25, 28, 29, 32, 33, 41, 44, 45) were identified. The race diversity was similar whether a resistant or susceptible cultivar was grown during the previous decade. However, P. sojae had 5-times the population density in field plots where susceptible cultivars had been grown compared to field plots where the Rps1-k soybeans were planted. A significantly greater isolate frequency was verified in soil samples from the no-till compared to the plowed plots. Differences between the crop rotation treatments (corn-soybean vs. continuous soybeans) and sub-plot treatments (post-herbicide injury vs. untreated) were not significant; however there was a consistently higher P. sojae frequency in the corn-soybean, no-till plots than in the other treatment combinations