Late season diseases are early this year

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Based on visits to some fields and samples sent in to the lab here in Wooster, Ohio, there are some late season diseases that are appearing. They are: Phytophthora root and stem rot, Diaporthe stem canker, Sudden Death Syndrome, Soybean Cyst Nematode (SCN), Charcoal rot, and Frogeye leaf spot. 

At this stage, the infections have taken place, at this time fungicides will not prevent any further disease development. All of these diseases are managed with host resistance.  Scouting fields to see where these problems are will greatly help identify what disease resistance package the variety should have in the field the next time. Links to pictures of these diseases and fact sheets can all be found at
http://oardc.osu.edu/ohiofieldcropdisease/t01_pageview2/Home.htm

Phytophthora stem rot. The Phytophthora sojae populations in Ohio are very diverse and we have found that the Rps genes are not effective in all fields. If stem rot is developing that indicates that the field resistance (tolerance, partial resistance) part of the package was not up to snuff. Read the fine print on the scales – Universities and Companies all use different scales – and choose a variety with the highest level of this resistance for the next soybean crop.

Diaporthe stem canker. This disease can be distinguished from Phytophthora stem rot by how much of the stem is girdled. For Phytophthora the stem lesion will begin in the roots and move up the stem. For Diaporthe canker the lesion typically is at the 3rd or 4th node of the plant and there will be small black dots (fungal structures) in the lesion. This is a residue borne disease so rotation will be important in these fields as well as choosing a resistant variety.

Sudden death syndrome. Symptoms were severe in a couple of fields, with the yellowing and brown necrosis between the veins. The plants had not begun to defoliate. This was also a SDS susceptible and SCN susceptible variety. For those parts of the state where this is a perennial problem, adding SDS to the resistance package will be important.

Soybean cyst nematode. It is getting easy to pick out the areas of the fields with high populations. They are about half the height of the neighboring beans. These are also the areas of the fields that will turn yellow first. It is worth taking out a shovel and digging these roots up to see the females, there will be some shiny white ones on the roots.

Charcoal rot. This is very apparent in the areas of the state that have been hit with drought.  This also causes premature yellowing and dying of soybean plants. To distinguish this from these other diseases, split the tap root open and there will be black flecks embedded in the tissue. These are the microsclerotia which will serve as survival structures. Most of the research on this disease has been done in Kansas as well as Tennessee. One management strategy for this, in addition to resistance, is planting soybean at reduced populations. We have several studies this year in Ohio which are comparing high and low plant populations to see if this will be a viable management strategy for our conditions in Ohio.

Frogeye leaf spot. Found one leaf spot in one field, this leaf spot has purple margin with gray center and on the underside of the lesion the conidia (spores) look like whiskers. This fungus can only infect new growth and once the leaf has expanded it is resistant. The key for this disease is to check at the end of the season, this can overwinter in Ohio. We have had some experience here in the state if it builds up in a field one year and the same or another susceptible variety is planted back into the same field we can run into some serious disease loss issues.


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