White mold of soybean, also called Sclerotinia stem rot, begins to show on the lower stem in August. However, infection occurs earlier in the season during flowering.
White mold is caused by the fungus Sclerotinia sclerotiorum and is favored by cool, damp conditions. From the edge of the field, the soybeans have a gray-green appearance. Here are some things to keep in mind when white mold has attacked your soybean fields.
1. There is nothing to spray at this time to stop the infection. The plants that have stem lesions will die and that yield will be lost. The plants that lodge on top of one another, we have seen the mycelium keep colonizing the upper stems – “plant-to-plant”. But overall this will be minimal. Again, nothing can be sprayed on the plants at this time.
2. It takes at least 20% incidence to begin to measure yield loss. We have had several trials out over the past several years, we can measure differences in disease severity/incidence – but in all cases we could not measure yield loss as we were at 15 to 20% incidence.
3. Not all fields have inoculum – there must be sclerotia in the field from a previous crop to have Sclerotinia develop – so farmers should note this field as they will now have this “forever”.
4. Make note of the variety – and remove this from the list. There are more resistant varieties out there, but this year was VERY conducive for infection and I expect that some moderately resistant varieties are going to have a tough time. As I like to say, it is a good year to get data and some of these varieties will be dumped.
5. Plan to harvest these fields last. The reason is the sclerotia, they will get everywhere in those combines and then become introduced into more fields. Let’s keep the spread to a minimum.
6. After harvest, practice no-till. The sclerotia that are left on the surface will degrade more quickly than those that are buried.