Sunflower, followed by dry edible beans, are the most susceptible crops to white mold, or sclerotinia.
“White mold thrives in cool, wet condition for many years, and unfortunately, we saw that across the whole state last year, leading to yield loss,” said Sam Markell, NDSU plant pathologist, at the Western Crop and Pest Management school.
For those unfamiliar with white mold, Markell pointed out that every broadleaf crop, plant or weed “will get white mold. White mold is a beast.”
Flax is the only broadleaf that seldom gets white mold - for unknown reasons.
The system starts with sclerotia, or black structures, which overwinter, and can last from three to five years.
“We got tons of sclerotia out in the fields right now,” he said “These black structures on the surface or just under the surface will mostly germinate when water gets on them.
When they germinate, spores are sent out.
“The spores can’t affect healthy tissue,” Markell said.
The broadleaf weed, crop or plant is affected during flowering in cool wet, 60-80 degree weather.
The disease seldom grows in hot weather.
“Once you get that affection, it progresses until it kills the plant or a lesion will develop and kill the plant from the top up, and black sclerotia will result,” he said.
In white mold, there can be plant-to-plant spread.
A spore can land on a leaf. If that touches a sunflower stem, for example, the lesion can start progressing on its own.
“Sunflower is my favorite crop, but it is so sensitive to white mold,” he said. “We don’t recommend using fungicides on sunflower for white mold because they are too sensitive.”
It is possible for the sclerotinia to get into the sunflower root system, resulting in a basal lesion at the base.
“If you have had white mold in the crop, don’t rotate into sunflower,” Markell said. “Even in warm weather, the white mold can grow up through the sunflower plant from the roots.”
White mold can skeletonize the head of the sunflower.
How to manage white mold:
- Crop rotation is important, especially in no-till.
- With tillage, sclerotia is the problem. Do you bury it or leave it on surface?
“If you bury it, the spores won’t germinate and will sit there for years until it works its way back up to the surface,” he said. “If you leave it on the surface, and put water on it, it will germinate eventually.”
- There really isn’t resistance for white mold in a crop variety. Partial resistance exists in most crops - except sunflower or dry beans.
Soybeans have some partial resistance to white mold.
Some soybean resistance occurs in some maturities, but since soybean varieties disappear after a short time, that good variety with partial resistance may soon be gone.
- Wide row spacing could be helpful with white mold but it reduces yield. It is not the best idea. Don’t change what you normally do.
- Fungicides can be applied if the canopy is closed at R1. If it is not closed, wait until R2. By R3, it is too late.
If the canopy is closed, you can’t cover the tissue with fine droplets. With white mold, it is difficult to cover tissue. Fungicides go to the tip of leaf, but nowhere else. They are not systemic like herbicides are.
“When thinking about fungicide, think about the risk and the timing first, then the product,” Markell added.
- Chickpeas are sensitive for white mold but producers already manage for ascochyta, so plenty of fungicide is already being used for protection.
- If your soil is dry, it is doubtful you have mold. But with frequent rains, and cool weather, you are “pretty sensitive” to mold.
For exact information on fungicides and white mold, see the Carrington Research Extension site and Michael Wunsch, NDSU plant pathologist’s data at https://www.ag.ndsu.edu/CarringtonREC/gsearch?q=fungicide%20and%20white%20mold.