No matter how perfect you do everything else, three diseases can derail even the best soybean crop, cautions Ken Ferrie, Farm Journal Field Agronomist. These three diseases include white mold,
sudden death syndrome (SDS) and Asian soybean rust.
With white mold and SDS, the best option is to avoid getting the disease spores in your field in the first place. Once present, you can't get rid of them. All you can do is manage to minimize the effects. With Asian soybean rust—which arrives on spores blown in by the wind—you can reduce yield loss by scouting and timely treatment with fungicide.
Here are some tips to help you keep the "big three” diseases from throwing your yield train off the track.
White mold, or Sclerotinia stem rot, is a threat in most of the North-Central states. A soil-borne fungus, it usually attacks soybeans right after flowering.
"White mold favors cool, moist weather, in which the bottom of the canopy stays wet,” Ferrie says. "The more moisture and the less air circulation through the canopy, the more likely you are to see white mold. That's why you tend to have more disease in narrow rows and high populations.”
"High-yield environments—highly productive fields with tall thick stands of soybeans—favor the development of white mold,” explains Ohio State University (OSU) plant pathologist Anne Dorrance. "It tends to be most severe in areas of fields where moisture collects due to fogs and extended dew periods. Temperatures higher than 90°F arrest disease development.”
You can spot white mold, which looks like frothy white fuzz on the outside of the soybean stem. That is mycelium, the vegetative body of the fungus. Before the mycelium appears, the fungus kills the plant, and the area on the stem looks bleached.
The fungus survives as structures called sclerotia, which are hard, black, irregular in shape and resemble rat feces. The sclerotia are able to germinate directly, or they can produce a small mushroom-like fruiting body, which produces spores.
Although there are no highly resistant white mold varieties, a number of varieties have moderate or partial resistance, Dorrance says.
Topsin M fungicide is labeled for white mold control. "But we recommend spraying only in fields where white mold has limited yields in the past,” Dorrance says. "Successful control requires that the spray penetrate the canopy and reach the flowers.”
- November 2008