By Anne Dorrance
OSU plant pathology
(Reprinted from the Aug. 3-10, 2015, edition of Ohio State University’s C.O.R.N. Newsletter.)
All of the rain during May, June and July continues to impact the soybean crop in some areas of Ohio.
Surveys of our plots and some scouting in the lower canopy have turned up some surprises and some we expected.
1. Sclerotinia stem rot. Over the next two weeks we will begin to see above canopy symptoms. Stem lesions are now evident in some of our historic white mold fields below the canopy.
With sclerotinia, white fluffy mycelium is evident on the stem, while leaves turn a gray-green and stay on the plant.
2. Sudden death syndrome (SDS). We received several samples and calls last week on susceptible varieties.
The plants showed the classic yellow spots between the veins on the leaves which will then turn brown.
Symptoms on leaves will continue to progress and eventually the leaflets will drop from the plants leaving the petioles attached. The roots and particularly the tap root are rotted and discolored.
After a rain, the blue spores (conidia) can be observed on the tap root. The fungus infects the plants shortly after planting and continues to colonize the plant slowly, the piths of infected plants are white and the discoloration in the leaves is due to a toxin that the fungus produces.
We have the North Central Soybean Research program soybean variety trial in northwestern Ohio, and only the highly susceptible checks have symptoms of sudden death syndrome.
This is a very good sign that resistance is holding up under the high disease pressure this year. In some of these fields, there is no record of the presence of soybean cyst nematode. I think that in the fields with low to no SCN, the SDS symptoms may be due to the flooding injury that occurred early on.
One of the fields with SDS symptoms received 4 inches of rain in a 24-hour period, and it was saturated for approximately 24 hours. Realistically, this is a best guess for how this disease did develop in some of these fields, the thought of trying to run the experiment to prove this, is more than a bit daunting at the moment.
3. Frogeye leaf spot. This leaf spot disease is present on more varieties than expected, but for some areas we are getting close to the R5 growth stage, where spraying will have little effect.
The R5 growth stage is when flowering has stopped, the plants will not produce too many more new leaves, and the seed begins to fill the pods. The fungus that causes frogeye leaf spot can only infect new leaves, so fungicide applications during late R4 and beyond are no longer necessary.
At this point in time, we have only processed lesions from a few locations and based on the test for the most common mutation indicate that these few samples were sensitive to the strobilurin fungicides.
We will continue to monitor lesions as we receive them for the remainder of the season.
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