By Dusty Sonnenberg, CCA, Ohio Field Leader

Since 2016, numbers tell the story. Three states, four years, 128 farms, and 1,280 soil samples. Linda Weber, a graduate student researcher in plant pathology at OARDC has been collecting samples and screening over 400 isolates of Phytophthora sojae against the single resistance genes that are currently available in today’s cultivars. “Researchers evaluate 14 single resistance genes, however there are only about 6 genes that are currently available in cultivars,” Weber said. This survey is funded by money from the Ohio Soybean Check-off through the North Central Soybean Research Program. Weber is evaluating the effectiveness of single resistance genes against the Phytophthora sojae pathogen.

Phytophthora sojae is a soil borne pathogen that causes Phytophthora Root Rot and Stem Rot. “This is a major cause of soybean yield losses annually,” Weber said. “Soil types found in Ohio, such as the heavier clay soils, when they are saturated in the spring, are ideal for Phytophthora Root and Stem Rot to develop.”

The project is titled “Phytophthora sojae Pathotype Variability across Ohio, Indiana and Kentucky. Soil samples have been collected from soybean fields across the three states that have been known to have a disease history and that were measured in the past. The pathogens from those samples have been collected and are being used in the survey. The Rps1a, Rps1c, Rps1k, Rps3a, and Rps6 single resistance genes are being screened against the pathogens. “Resistance genes are ideal because they are easy to breed into the soybean. The down side is that the pathogen can easily overcome the single resistance. Single resistance genes have a limited life span, roughly 8-20 years,” Weber said. “The Rps1a and Rps1c genes have been used the longest and are mostly ineffective as single resistance genes. They remain in the survey to monitor their level of effectiveness currently. The Rps1k gene was used a lot in the 1980’s and now has also become mostly ineffective as a single resistance gene against the pathogen. The Rps3a and Rps6 genes still remain very effective as single resistance genes against the pathogen.”

“At this point in the study, some general conclusions can be drawn,” Weber said. “Companies should begin to incorporate other Rps genes in the soybean variety line up.   Since most of the Rps genes are ineffective in this region, farmers should focus on selecting cultivars with high levels of partial resistance. Not every isolate in every field has adapted to the Rps genes, thus, these single resistance genes may still be effective, especially when combined with a high level of partial resistance.” Final results from the survey should be available in early 2020.


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