Sclerotinia stem rot is beginning to show up in a few fields in northern Ohio. This is very surprising due to the hot, dry conditions that occurred across the state during flowering. As I recall, there were heat indexes of over 100 F as we took stand counts and as we talked about the lack of canopy closure in many areas of the state. We pulled some weather data from August, and there was a cold spell, with 70 F during the nights of the first week of August with some light rains. The fields where we are spotting white mold are also those fields that had very high incidence during 2009. They were planted to corn last year. We need another week to see if the fields are as badly affected as in 2009 and if the affected fields stretch as far south as they did in 2009. Based on observations from fields last week, we saw a few lesions on the main stems on a few plants, while the majority of plants had lesions further up the plant or on side branches. Lesions that are on the lower part of the main stem have the greatest impact on yield as the whole plant is killed. Lesions on the upper stem, tend to be slower to develop and do not reduce yields to the same extent. But some losses will undoubtedly occur.
Symptoms of Sclerotinia are quite striking. There will be dead plants standing straight above the canopy. The leaves start off with a gray-green color and wilt. When you open the canopy you will see white cottony growth of the fungus on the affected area. Sclerotinia will produce hard-black structures that resemble rat droppings. These will fall onto the soil during harvest and provide inoculums for the next soybean crop. It is important to note the fields that have white mold now, and mark those fields. Save these to harvest last. This will avoid contaminating more fields as combines will move sclerotia from field to field.
There is an excellent new publication on white mold from the North Central Soybean Research Program’s Plant Health Initiative http://www.planthealth.info/whitemold_basics.htm. You can download it from the Web site.