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Strategies for Effective and Durable Management of Phytophthora
Mon - March 21, 2016
Phytophthora variety trial
Phytophthora variety trial
by Alison Robertson, Soybean Plant Pathologist, Iowa State University

Phytophthora sojae is an oomycete pathogen which causes Phytophthora root and stem rot.  The pathogen can infect soybean at all growth stages. Phytophthora often ranks as second or third among the most important diseases in the United States affecting yield.

Phytophthora has been managed with single dominant resistance genes (Rps genes) for the past 50 years.  Twenty-one  Rps genes in soybean have been identified which confer resistance to P. sojae, some of which have been incorporated into commercial cultivars. However, as in many other host–pathogen systems, the pathogen adapts to the specific Rps gene in soybean. Thus, the effectiveness of Rps genes has eroded at a fairly rapid pace as new pathotypes have emerged. The durability of Rps effectiveness in the field is estimated to be 8 to 20 years.

Soybean plant pathologists in the north-central region have been working collaboratively to understand the genetic diversity of P. sojae and its relationship to pathotype diversity.  In a recent study funded by the Iowa Soybean Association, the Ohio Soybean Council, and the United Soybean Board, we found a high level of pathotype diversity and a low to moderate level of genotypic diversity within fields and among states (IA, OH, MO, SD).  None of the Rps-gene differentials (test cultivars containing a specific set of Rps genes) were resistant to all of the isolates.  The results provide further evidence of high diversity within this pathogen, and suggest that each state may have their own or several regional populations of P. sojae.

Because of the diversity and increasing complexity of P. sojae populations, the simple rotation of Rps genes is probably no longer a viable disease management strategy. The identification of new and novel Rps genes will continue to be of importance because virulence to many of the Rps genes is maintained within these populations. More important, however, are strategies such as quantitative resistance, also called partial resistance or tolerance, that put less selection pressure on the pathogen population. These strategies should become a high priority in Phytophthora disease management and cultivar development.
Scouting for Phytophthora Root and Stem Rot, 2015
Population Structure Among and Within Iowa, Missouri, Ohio, and South Dakota Populations of Phytophthora sojae Plant Disease, 2016.