Project Details:

Title:
The effect of rotations of host resistance genes (Rps) on pathogen avirulence genes (Avr) in the soybean Phytophthora sojae pathosystem

Parent Project: This is the first year of this project.
Checkoff Organization:Iowa Soybean Association
Categories:Soybean diseases, Breeding & genetics
Organization Project Code:
Project Year:2014
Lead Principal Investigator:Alison Robertson (Iowa State University)
Co-Principal Investigators:
Keywords: Pathogenicity Studies, Phytophtora Root Rot

Contributing Organizations

Funding Institutions

Information and Results

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Project Summary

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.

Project Objectives

1. Determine the genetic changes that occur in Avr genes that enable them to cause disease on P. sojae resistant cultivars.
2. Determine the impact of Rps genes on Avr gene evaluation.

Project Deliverables

Progress of Work

Final Project Results

• The presence or absence of Avr1a, 1c, 1d and 3b in isolates recovered from a microplot rotation study that were virulent on the respective Rps genes varied as determined by PCR using specific primers. Similarly, transcripts of the genes also varied among isolates;
• Pathotype differences were detected in PR1 (race 1 of P. sojae; virulent of Rps7 only) in a culture that was grown in the lab on dV8 agar for 33 generations;
• Similarly pathotype differences were detected among three generations of single zoospore isolates recovered from PR1. These data indicate that changes in virulence can occur in the absence of selection pressure by Rps genes; and
• Three rotations of soybean have been planted in the greenhouse. A fourth rotation is in progress.

Benefit to Soybean Farmers

Performance Metrics

Project Years