By Anne Dorrance, Ohio State University Extension plant pathologist
There are many of things to choose from as we prepare for the 2011 planting season. Lots of different packages, choices, and we all remember the challenges of the past few field seasons. In parts of the eastern soybean belt, we have more challenges than most other areas of the Midwest soybean production region. This is a review of the key pathogens in the state that are very well managed with resistance –- if the soybean variety has it.
Phytophthora sojae is our number one soil borne pathogen for major portions of the state. We see it every time the wrong variety is planted and we have heavy rains. We get stem rot. When stem rot occurs we lose substantial amounts of yield. For a variety with low levels of partial resistance, we can still lose 50% in yield, and if there is no partial resistance, the whole field can be lost.
In Ohio, where we have poorly drained soils and high proportions of clay — we have a great diversity of pathotypes or races. Rps1a is no longer effective against the populations we have in this state. Rps1c and Rps1k are effective to approximately 30% to 40% of the populations. Rps3a is bit higher with approximately 50%. But this continues to change. We recovered a few isolates of P. sojae this past summer, where none of the Rps genes were effective, including Rps8. Within a field there is a great deal of variability so these individual genes may still be effective for some of the isolates but not all. Therefore, for this region, partial resistance becomes the most important part of the Phytophthora resistance package in Ohio.
Partial resistance is known by several different names, tolerance, field resistance — but what you will see is little to no yield loss, the stem rot phase never develops when this is high. Every company has their own scoring system for partial resistance. In the Ohio Performance Trials (when I have nontreated seed in time to test) — we use a score of 1 to 9, in which 9 is dead. Scores of the 3 to 5 represent the best levels for the field. Other companies use a scoring system where 9 or 10 is best. So this is one situation to read the fine print.
Soybean cyst nematode
For SCN there are several sources of resistance that each have several genes that are integrated into new varieties. The interesting thing with this is that not all of the genes get moved into each variety. Right now there are varieties that have resistance genes from PI88788, Peking and PI 437654 (Hartwig). PI88788 is the most common. Based on 3 years of data, the yield drag is no longer an issue here with resistance. However, like Phytophthora, we do have some SCN populations where some of the PI88788 resistance genes are no longer effective. If you have SCN and your populations keep increasing, even when you use resistance, it is time to change varieties to either a different combination of PI88788 genes or Peking or the PI 437654 Hartwig. The primary key here is don’t plant the same variety year after year. If you do have SCN, keep monitoring those populations and rotate to ensure that they stay at levels below 200 eggs per cup of soil, where there is no yield loss.
Sclerotinia stem rot or white mold
For this disease, we had a fairly widespread outbreak in 2009. So for some fields that were in corn last year, we are moving right back into those fields this next year. The sclerotia are there, happy as can be. Resistance to Sclerotinia is challenging, but we have had two fairly good years to get rid of the super susceptible varieties. Again read the company scores for this on whether or not this will be an issue.
Frogeye leaf spot
This foliar disease is more common in the southern U.S. We have not had an outbreak in a few years but it is still one to ask about. This fungus will survive through the winter on residue, so you need to watch for it in fields where you are planting back-to-back soybeans and you had frogeye there at the end of the season. It really needs a resistant line, better yet plant corn.
Brown Stem Rot
We have this here in Ohio, but it does not seem to cause the same level of disease as what they find in Northern Illinois and Wisconsin. Symptom development is related to low pH. For areas where the pH is 6 or below, choosing varieties with this resistance package is important.
Sudden death syndrome
A lot of work has gone into this one over the past few years. For soils that have SCN, compacted soil conditions and otherwise poor drainage, add this pathogen to the list. We have seen several situations where the SDS symptoms occurred right to the row where the varieties switched out. Based on surveys from myself, county educators, and crop consultants comments, I think this is now throughout the state. This is another one that is difficult to screen, but southern Illinois does a lot of testing and on their Web site they have some good information on resistance levels in some varieties at http://soybean.siuc.edu/2010%20SIU%20SDS%20Commercial%20Report.pdf.
Excellent yields are key to making profits, but without disease resistance, there will be big losses if the environment turns favorable for the pathogens. Don’t forget this part of the total package as you work with your seed dealers.