It may sound too early to talk about white mold when soybeans just emerge. However, it's time to talk about the disease because one of the management measures I will discuss takes time to prepare. Results of our 1995 survey show that about 5 percent of soybean fields had at least 5 percent white mold-infected plants. Most of these fields will be in soybean this year.
||Soybean white mold
What to do. Generally, soybean white mold is managed by variety selection and use of row spacing, which should be done before or during planting. After planting, there are still some things you can do to reduce the disease risk.
Fungicides. Benomyl (also called Benlate) and Thiophanate (also called Topsin) provide good control for soybean white mold if applied properly. The fungicides have to be applied during flowering stage, 25-50 percent of full blooming for Benlate and about 50 percent for Topsin. Topsin is registered for the control of soybean white mold. Benomyl is not specifically registered for soybean white mold and is labeled for control of other soybean diseases and dry bean white mold caused by the same fungus. If you want to use fungicides to control white mold, check with chemical dealers for information about the chemicals.
Cultivation. Although plant pathologists are still doing experiments, observations indicate that cultivation may reduce disease damage. White mold fungus overwinters in soil. Theoretically, cultivation reduces surface soil moisture, which reduces the production of apothecia, tiny mushroom-like structures that produce spores to infect plants. Fewer apothecia mean lower disease pressure. The white mold fungus also is very sensitive to the surrounding soil environment. Disturbing the soils by cultivation could prevent the fungus from producing spores.
Further, white mold fungus has a very broad host range and infects weeds in the absence of soybean. The fungus may increase its population by reproducing on weeds if weed control in the corn season is poor. Major weed hosts are: lambsquarters, pigweed, velvet leaf, ragweed, nightshade, Canada thistle, and mustard.
Who should prepare. People who have fields with a high risk of white mold should consider using chemical controls. For individual fields, the level of white mold risk depends on how much white mold you had in 1995 and how the disease was distributed in the fields. Assuming this summer will be cool with normal precipitation, you can follow general rules to assess the risk:
- if the disease incidence was less than 1 percent in 1995, the risk will be minimum this year;
- if light disease occurred in small patches in 1995, the risk of having significant damage is limited to those portions of the field;
- if the incidence was 5-10 percent in 1995 with diseased plants scattered throughout the field, the risk is high when highly susceptible varieties are used, and
- if the disease was severe in 1995, the risk also will be high this summer.
Further, if you grow soybean for seed in white mold-infested fields, applications of fungicides will help produce healthy seeds.
This article originally appeared on page 92 of the IC-478(12) -- June 9, 1997 issue.