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Control of White Mold
High levels of resistance to white mold have not been incorporated into commercially acceptable pinto and navy beans, although some cultivars have more resistance (tolerance) than others (see Circular A-654, North Dakota Dry Bean Performance Testing).
Fungicides can help control the disease. Boscalid (Endura, 70%) Iprodione (Rovral 4F)and thiophanate methyl (Topsin M) are registered for this use. These materials are carried through the plant to protect the new foliage. However, they are carried primarily upward in the plant. Since the disease usually begins on the dead blossoms and lower parts of the plant, control depends on thorough coverage. Use drop nozzles (between the rows) and 100 psi to obtain satisfactory coverage and canopy penetration. It is difficult to obtain thorough coverage after bean rows have closed. Low volume fungicide application (5 gallons) by aircraft may give poor coverage where needed unless special care is taken to get canopy penetration. High volume application (7-10 gallons) by aircraft has provided good coverage and good control.
The most economic use of fungicide is a banded or directed spray applied at early bloom. Higher rates provided better control. Fungicide decision guidelines, in use since 1987, have proved helpful. A fungicide is required if: 1) wet weather maintains soil surface wetness for 10-14 days before flowering, and maintains plant wetness for 2+ days during flowering; 2) the yield potential is over 2000 pounds per acre; and 3) white mold has affected crops in the area in previous years. Severe white mold infection can cause yield losses of 300-1000 lbs/Ac.
Keeping rows open longer can help provide aeration which dries plants more rapidly. Planting more upright (navy, black turtle, kidney) beans rather than vining (pinto) and planting at wide row spacings (at least 30 inches) helps keeps rows open.
Deep plowing (6-8 inches) aids in microbial decomposition of debris and some sclerotia. Shallow incorporation (1-2 inches) may increase the danger from this disease.
Sanitation is important to keep the fungus from spreading between fields. Proper cleaning of combines and proper disposal of seed screenings, bin cleanout, etc. can reduce dispersal of the pathogen.
The fungus attacks many crops (sunflowers, canola, crambe, mustard, dry peas, buckwheat, lentils, garbanzos, safflower, and sometimes potatoes or flax) and many weeds (Canada thistle, pigweed, lambsquarter, marsh elder, etc.). Flax is only rarely attacked and does not produce sclerotia; semi-leafless dry peas are not attacked nearly as frequently or severely as vine-type dry peas.
Long rotations between susceptible crops and good broad leaf weed control can assist in the control of white mold. Planting to non-susceptible crops (grains, corn, sorghum) and fallow also reduces the amount of fungus in a field.
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