By Suzanne Venter
Like love triangles, disease triangles in agriculture never end well. This season’s unusually volatile weather, with the arrival of higher humidity, cooler evenings and spores of Sclerotinia, has completed the ideal triangle for the outbreak of this disease, which leads to stem rot and ear rot.
“Sclerotinia is a weather-related disease. First, the pathogen must be present on the host and if you then also get higher humidity and cooler nights, the disease triangle is complete and leads to a fungal outbreak,” says Nelis Potgieter, area manager of Pioneer in the North West. He says they are currently observing that sunflower fields in particular are affected in patches, mainly in the western parts of the summer rainfall area.
“The outbreak is not yet as bad as it was three years ago. If the cloudier weather had come in February as we expected and hoped, it would not have been such a problem, but it was now a month too late. That is precisely why we are particularly concerned about the sunflower plantings that were done later,” says Potgieter.
Jan Erasmus, agriculturalist from Pannar in the North West, says they are also observing that Sclerotinia is occurring in patches. “I was in a sunflower field this week that was very badly affected and that guy is going to suffer a lot of damage, but that is the exception. At this stage it is occurring in patches and it is difficult to say what the later sunflower plantings will do. We will just have to keep an eye on them.”
Not just sunflowers
A farmer near Stella, North West, was struggling with Sclerotinia on his peanuts. “It came out in a few places and one spot was particularly badly affected. It was about 22 ha that I didn’t rip and just ploughed. I suspect that may be part of why it happened.
“Those peanuts have all rotted and are lost, but the others are still okay and look good,” says the farmer, who wishes to remain anonymous.
“Sunflowers are particularly susceptible to it and once it spreads to the heads and ear rot sets in, it is a big problem and so harmful that it cannot even be used for feed. In South Africa, there are more than a thousand crops, including peanuts, soybeans and certain types of weeds, that can serve as host plants for Sclerotinia,” says Potgieter.
“Peanuts are particularly prone to getting Sclerotinia in the stem if the plant has been mechanically injured at some point by something like a bollworm before it was sprayed.”
Erasmus says there is still no herbicide available to stop Sclerotinia. “The best plan remains to apply crop rotation and to plant in parcels so that you have different flowering stages,” says Erasmus.
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