Phytophthora root rot (PRR), caused by Phytophthora sojae, is a considerable threat to soybean production in Iowa, particularly in wet summers. Currently, PRR is primarily managed by planting soybean varieties with genes that confer resistance (Rps genes) to P. sojae. The effectiveness of a resistance gene relies on its ability to recognize the pathogen’s corresponding avirulence gene (Avr). Using Rps genes as a management tool for PRR has had only short-term success because Avr genes in the pathogen continually evolve to escape detection by the plant. There are 15 known Rps genes in soybean, and the pathogen is classified into pathotypes depending on the combination of Rps genes they interact with in soybean. Most commercially available Phytophthora-resistant soybean varieties in Iowa have the Rps1k resistance gene. However, P. sojae continues to change genetically such that most resistant varieties are no longer completely effective. In recent years, Iowa producers have started to report losses to PRR on soybean resistant varieties with Rps1k.
From 2004 through 2007, we collaborated with soybean pathologists from across the Midwest as a member of the North Central Soybean Research Program (NCSRP) Phytophthora research group. The goal of this group was to improve our understanding of the soybean-P. sojae pathosystem, and thereby improve management practices available to producers. At the conclusion of the NCSRP Phytophthora research group, we secured funding from the Iowa Soybean Association to continue my research on this pathosystem. We have been collaborating with Dr. Silvia Cianzio and Dr. Madan Bhattacharyya at ISU, Dr. Anne Dorrance at The Ohio State University, and Dr Saghai Maroof at Virginia Tech.
Currently we are investigating the genetic diversity of Avr genes in the P. sojae population in the north central U.S. When genetic changes occur within an Avr gene, the corresponding Rps gene in soybean no longer recognizes the pathogen, andP. sojae is able to infect the plant and cause disease. Thus this research should to enhance our understanding of how Avr genes evolve to allow P. sojae to escape detection by Rps genes in soybean and cause disease, and consequently lead to improvements in management of this pathosystem.
We also have projects identifying novel Rps genes, and QTL markers for partial resistance.We expect these projects to result in the release of germplasm lines with novel and/or partial resistance to P. sojae for use in developing commercial soybean varieties.