Strategies for Effective and Durable Management of Phytophthora
by Alison Robertson, Soybean Plant Pathologist, Iowa State UniversityPhytophthora sojae
Phytophthora variety trial
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 Rp
s 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
, 2015Population Structure Among and Within Iowa, Missouri, Ohio, and South Dakota Populations of Phytophthora sojae. Plant Disease, 2016.