Sclerotinia blight caused by the fungus Sclerotinia minor is the most devastating disease found on Texas Peanuts. No other disease produces the level of damage on infected plants. Fortunately for Texas producers the most devastating disease is not the most widespread.
When soil surface moisture is maintained in infested soil for 24 or more continuous hours while the soil temperature holds at or below 82 degrees the resting spores or sclerotia of the fungus germinate on the soil surface. They appear as small pinhead sized tufts of white mycelial growth on the soil, which quickly spread to the plant if temperature and moisture conditions persist.
Sclerotinia on the peanut plant is characterized in initial states by white cottony tufts of mycelial growth appearing in leaf axils within 2 to 4 inches of the soil surface. This white cottony growth can quickly spread to all plant parts both above and below ground. As the small dark raisin-like sclerotia form within the plant membranes, the plant tissue actually seems to explode. By the time an infected plant goes through a harvester, nothing but dust exits. There is zero yield.
Field to field spread of the disease can occur by livestock, wildlife, and machinery and at a low level in seed. The infective agents of the disease can pass through the digestive system of livestock, deer and birds and still be infective. Even though seed accounts for a very low level of disease movement, it is possible. Infested harvesters can quickly transport the disease organisms from field to field if not properly cleaned and disinfected.
Why some fields?
The question often arises as to why some fields develop severe outbreaks of the disease and others do not. It is believed that this is due to the different levels of antagonistic organisms present in different fields. The fungus Trichoderma is highly parasitic on Sclerotinia. The level of Trichoderma varies widely from field to field, depending on several factors. Certain fungicides containing chlorothalonil greatly reduce soil levels of Trichoderma fungi and thus predispose a field for Sclerotinia development.
Fields rotated with a grass crop such as wheat, sorghum, or corn develop increased levels of Trichoderma and consequently may develop lower levels of Sclerotinia. Corn appears to be the best rotation for Trichoderma development.
When all else fails, growers are forced to look at fungicidal management of Sclerotinia blight. Several fungicides are cleared for use on Sclerotinia but only two presently offer acceptable control. Omega 500 by Syngenta and Endura by BASF have shown remarkable control of Sclerotinia blight. Both products are much better at preventing spread of the fungus than they are at eliminating an infection.
Since Sclerotinia does not develop until soil temperature drops below 82 degrees with free moisture, growers should time applications to these conditions. We seldom see soil temperatures drop to that level before we get a full plant canopy and/or an extended cloudy, rainy spell. Although some producers want to wait until first signs of the disease, they would usually profit with a preventive application about July 15. This is usually about the time of full canopy development.
A second application about 30 days later will usually carry full season. The high cost of these chemicals, however, makes this feasible only in fields with known history of the disease. In fields where Sclerotinia is seen for the first time chemical control is helpful but first applications should be made immediately after first signs of infection.
This is one of those diseases that, thankfully, will not infect every field. But it is a disease whose level of damage cannot be ignored.