Phytophthora root and stem rot (PRR), caused by Phytophthora sojae, is an economically important disease of soybean in the United States. Currently, the disease is primarily managed by planting varieties with genes that confer resistance (Rps genes) to the pathogen. The effectiveness of an Rps gene relies on its ability to recognize the pathogens corresponding avirulence (Avr) gene.
From 2007 through 2011, researchers at Iowa State University conducted a microplot study to determine if cultivar rotation could be used to effectively minimize pathotype shifts in a population of P. sojae. The plots were inoculated with rice infested with a single isolate, PRl, which is virulent on Rps 7 only. Cultivar rotations included continuous susceptible, continuous partial resistance, continuous resistant, rotations of susceptible and resistant, and partial resistance and resistant varieties. Immediately prior to sowing and a harvest in each year, soil samples were collected from each microplot and baited for P. sojae. Isolates of the pathogen that were recovered were pathotyped on a standard set of 14 differential lines. Throughout the four years, 121 isolates of the pathogen were recovered from the microplots. Over half of the isolates were virulent on one to four additional Rps genes including Rps l a, l b, l e, l d, lk, 2, and 3b. What changes occurred in these genes to enable the pathogen to cause disease on soybean with these Rps genes?
The proposed project will complement another ISA-funded project, in which they have been studying how Avr genes differ genetically within populations of P. sojae that were collected from farmers fields in Iowa and Ohio. For many of the isolates in these two populations, they have no data on the history of the field in terms of Rps genes used. The proposed project is novel in that this is a unique population of P. sojae that was recovered from a carefully controlled study in which different sources of resistance to P. sojae were used to impose selection pressure on a known population of P. sojae. The data from this proposal will extend the current state of knowledge of (i) genetic changes that occur in Avr genes that enable them to cause disease on P. sojae resistant cultivars and (ii) the impact of Rps genes on Avr gene evaluation. These data will enable improved management of Phytophthora root rot through an improved understanding of how this pathogen continues to diversify.