Phytophthora sojae is present in many fields across the North Central region and Ontario, Canada, and had been managed very successfully with the deployment of single resistance Rps genes (Rps1a, Rps1c, Rps1k, Rps3a, and Rps6) as well as partial resistance (field resistance or tolerance). However, there are an increasing number of reports where varieties are sold that have Rps genes that are no longer effective towards the regional population or lack sufficient levels of partial resistance. The consequence is stand loss through damping-off or the development of stem rot throughout the season which results in lower yields and added weed pressure.
The most recent survey of P. sojae pathotypes (races) was completed during 2012-2013 with funding provided by Monsanto in which 213 unique pathotypes were identified among 873 isolates collected from 202 fields in 11 states. Two key findings from this sampling included: i) a greater number of regions have a higher proportion of isolates with virulence to key Rps genes, such that the resistance gene will no longer be effective as well as, ii) isolates across the region continue to increase in complexity. This indicates that the ability to recycle Rps genes is highly unlikely. There were several challenges in this study, the first was baiting from soil during a drought year (2012) such that there was low recovery; due to sample processing few isolates were recovered (WI); and few fields were actually sampled (MI, MO, NE, and SD all had less than 10 fields).
In another recent study funded by soybean checkoff dollars (2010-2013), the genotypic diversity among P. sojae populations was examined. That genetic diversity exists in this pathogen has been highly controversial, but with new markers, and in-depth sampling of populations recovered in the 2000s, regional populations of P. sojae in the Midwest were identified. As a soil borne organism, it was believed that this P. sojae should be clonal (every individual is the same within a field and between neighboring fields) – but the findings from this study clearly indicated that the P. sojae populations in the Midwest are not clonal.
These findings indicate that a more thorough assessment of the P. sojae populations in the North Central region is needed. In addition, new Rps genes have been identified but their effectiveness is still highly questionable. Among the 15 Rps genes that were identified prior to 2001, only 5 were effective and incorporated into varieties.
A second Phytophthora, P. sansomeana, also appears to be contributing to seed and seedling damping-off of soybean at greater incidence than previously thought, both from surveys previously reported in Illinois (Malvick), Ohio (2003 and 2015 data), and region-wide (Chilvers USDA-AFRI project data). This is intriguing as P. sansomeana has a larger host range (corn, douglas fir, soybean). As part of this proposes study, state populations of P. sansomeana will also be examined for host range, resistance in current soybean cultivars, and efficacy of seed treatment fungicides.