Midseason soybean diseases

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Across the state we have just about every condition possible in soybean this year. With the cooler temperatures this week here are some things to scout for.

1. Soybean cyst nematode – Unfortunately, this pathogen is not affected by the dry weather and will still be active in the soils. Where there are high populations, soybeans will be stunted and may have symptoms similar to those of nutrient deficiencies. If the soybeans within an area of the field are shorter than they are in another part of the field, dig up the soybeans, shake the soil off of the roots, and look for the ‘pearls’ on the roots. Many, many times I have found SCN in these fields.

2. Phytophthora stem rot – this is caused by the water mold, Phytophthora sojae and is present in fields that tend to be poorly drained. The best time to look for this disease is 1 week after a heavy rain. The key symptom of this disease is a brown chocolate canker that goes from the soil line up the stem, plants will wilt and eventually turn yellow and die. A few places in the state have gotten some dumpers and this provides the perfect conditions for Phytophthora to develop. The Rps genes are not providing 100% protection and we are relying on the “field resistance”, partial resistance, part of the resistance package. Searching for the stem rot phase is a good key to know if that resistance is working for your field.

3. Frogeye leaf spot – this is caused by the fungus, Cercospora sojina. Now is the time to check fields for the presence for this fungal disease. I also have a favor to ask if you find it. Could you please send me a few leaves with spots (from Ohio only please). A colleague at Univ. of Illinois is evaluating the population for resistance to the strobilurin class of fungicides (Headline, Quadris). There are still a few highly susceptible varieties out there, and we have also had some early morning fogs in a few places. Fields that are in continuous soybean and planted to susceptible varieties are the most likely candidates for this disease.

4. White mold – Sclerotinia stem rot. This disease hits some areas of the state, northeast Ohio and north central on an annual basis. Again, variety resistance plays a key role in this, as well as thick stands, and canopy closure during flowering. Despite the heat of last week, we have had some cool nights and fog – but the soil has been dry. I’ve found sclerotia in some fields that are soft and healthy, but no apothecia. Last year, apothecia formed late in some fields during a period when night time temperatures were down in the ‘60s. Once these apothecia form, they shoot their spores onto the flowers of plants. This gives them a great carbon source as they germinate and grow and form the white fluffy growth on the stems of the plants.

5. Soybean rust. No, it is not here in Ohio. This disease we monitor in the southern states. To date it has been identified in Texas, Alabama and Florida in counties where it is hugging the Gulf as well as a few counties in Southern Georgia. The heat will slow the development down, thus making it hard to build up inoculum that may affect this region.  (http://sbr.ipmpipe.org/cgi-bin/sbr/public.cgi). Once again, many thanks to our colleagues in the south who scout the kudzu patches to monitor for this disease.


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