The first and most economical approach (a.k.a. save money) to manage many of the pathogens that impact soybean in the state of Ohio is to pick varieties that have the right resistance package. If the resistance is effective – then there is no need for any further measures during the season. The soybean plant can take care of itself, especially during years when conditions are favorable for disease development.
We routinely use both resistant and susceptible varieties/germplasm in our test locations. Germplasm are soybean lines that may have come from another country or are in the breeding pipeline of a public breeder – they are not quite ready for “prime time” but the genetics are on the way to keep improving soybean varieties. A major contribution to soybean development from the public land grants has been the assessment and genetic discovery of key genes that contribute not only to yield but also to disease, pest, and abiotic stress.
Some examples from our studies in Ohio.
1. Sudden death syndrome. I don’t talk about this disease too often, as it is mainly limited to a few geographic regions in this state, is very sporadic and unpredictable when it will occur, is associated with soybean cyst nematode and the yield losses have not been as impressive as other pathogens. However, we have evaluated new germplasm every year as well as some of the seed treatments. Here is some data from 2014 when disease symptoms were severe.
Test A. Examined the performance of two cultivars with SCN resistance but one is SDS susceptible (Cultivar A) and the second is Resistant (Cultivar B).
Test B. Evaluated the performance of soybean germplasm for resistance to SDS. These are the checks, and this test demonstrates the variability in disease levels that we seem to face here in Ohio.
Disease severity of sudden death syndrome is measured first by the percentage of the plants within the plot that have symptoms of the disease followed by a score for the severity of the symptoms. The score ranges from 1 to 9, where 1 is no disease; 2 to 5 the amount of the yellow and necrotic spots on the plants; 6 to 8 the amount of defoliation; 9 the amount of prematurely dead plants. nt indicates that the line was not tested.
2. Sclerotinia white mold. The resistance levels to this pathogen have slowly been improving over the years to the point that it is really hard to show the benefit of mid-season fungicide applications. From 2014 we evaluated 4 cultivars in a field with a long and sordid history of white mold.
3. Frogeye leaf spot. This leaf pathogen has a wide range of responses among northern cultivars. For the most part many of our varieties have resistance,however there are a few that are highly susceptible and do require a fungicide treatment mid-season. This year we had two varieties planted at Western Research station and within the study they were placed throughout the study to measure the levels of disease within the field. As you can see, the response was consistent across the field.
4. Phytophthora root and stem rot. This disease is most common on poorly drained soils and we have recovered or found plants with symptoms in all of Ohio’s soybean production regions, it is not just a northwestern Ohio problem. The key to managing this disease is with host resistance which is a combination of Rps genes and partial resistance (also known as tolerance & field resistance). This past year under heavy flooding and disease pressure, several of the new public varieties had significantly higher yields than some of the older standards. These varieties were also planted in 30 inch rows near the end of May, received between 15 and 20 inches of rain within 2 weeks after planting.
For each of these diseases, companies use a different scoring system for resistance and on top of that, each company uses different values. For example, for Phytophthora, companies will list which Rps gene(s) are present in the variety. For Ohio, the gene stacks of Rps1c + 3a; Rps1k + 3a provide the best management, followed by single genes Rps1k, Rps3a, and Rps6.
For partial resistance towards Phytophthora, Sudden Death syndrome, frogeye leafspot or white mold, the scoring is very different. Most often it is on a 1 to 9 scale and depending on the company, 1 can be great or it can be dead. The only way to tell is to read the fine print in the tables. Here is a tool to help you compare and record the information for the 2016 crop:
If you are having trouble assembling this information, work with your seed dealer or your county AGNR educator.
Anne Dorrance is a professor specializing in soybean disease management at The Ohio State University Extension. The article originally appeared on the OSU Extension website and has been reposted with permission.