White Mold of Dry Beans

White Mold of Dry Beans

White mold is a common and serious disease of dry beans grown in Nebraska. Average yield loss in the state from this disease is 20 percent but can go as high as 65 percent. The disease is caused by the fungus Sclerotinia sclerotiorum (Lib.) deBary. The causal organism has a very broad host range. The fungus also causes stem rot and the disease white mold on hosts from 64 plant families, 225 genera, and 361 species. Included in the list are dry beans, sunflowers, canola, alfalfa, cabbage, lettuce, tomato and potato. The fungus survives from year to year as sclerotia in plant debris, mixed with seed or in the soil. Sclerotia germinate in late spring or early summer and produce apothecia or mycelium. Apothecia are capable of producing millions of airborne ascospores almost daily over a 7 to 10-day period if favorable environmental conditions prevail. Primary infection occurs on senescing blossoms or injured tissue. Free moisture must be present on the plant surface for infection to occur. When blossoms are infected the fungus obtains the nutrients necessary for appressoria production and then colonizes healthy tissue via direct penetration. Disease development is favored by cool, wet temperatures and a thick canopy, arising as a result of narrow row spacing or use of vining type varieties. Secondary spread occurs when healthy tissue comes into contact with mycelium produced on infected plants. This commonly occurs when plants lodge. Sclerotia produced on plant tissue may remain attached to debris, be released onto the soil or be harvested and contaminate the seed. If seed is not thoroughly cleaned, sclerotia may be planted along with seed the following season. Contaminated seed is the most common means of spreading the disease to new fields. Inoculum in the form of sclerotia, mycelium, and ascospores can also be spread in contaminated irrigation water.

Symptoms

Initial symptoms begin to appear during pod development. Leaves wilt and plants turn gray-green before turning brown and dying. One of the primary signs of a Sclerotinia infection is the presence of white mycelial growth on flowers, stems and pods. Sclerotia later develop in the mycelial mats. Infected stems are initially water-soaked then become tan and eventually bleached. Normally senescing plants (especially pintos and great northerns) and those infected by other pathogens would be tan in color rather than bleached. Stem infections may result in girdling and lodging. Bleached stems have a pithy texture and shred easily, releasing sclerotia onto the soil. Sclerotia may be found both externally and inside infected stems.

Plant Health Management

Implement Cultural Practices That Promote Good Air Circulation

Included in this category are: adequate row spacing (greater than 15 inches), use varieties with an upright growth habit and restrict the use of vining types to coarse textured soils and well-sloped land, avoid overhead irrigation during flowering, and fertilize at recommended rates to avoid excess canopy development.

Plant Clean Seed (Free of Sclerotia)

Crop Rotation

Rotate to monocots such as corn, sorghum or wheat. The effectiveness of this practice may be limited due to the wide host range of the pathogen.

Resistant Varieties

Unfortunately, varieties that are high yielding are also highly susceptible to white mold. Joint bean breeding programs between the University of Nebraska and Colorado State University are aggressively working to develop varieties that are disease resistant and/or have a more open plant canopy.

Do not use contaminated collection water as a source of irrigation

Fungicide Application

Fungicides labeled for control of white mold include benomyl (Benlate), thiophanate-methyl (Topsin-M) and iprodione (Roval) (1997). Successful control with these products tends to be better on plants with an upright growth habit.

References

Hall, R., ed. 1991. Compendium of bean diseases. American Phytopathological Society, St. Paul, MN. 73 pp.

Purdy, L.H. 1958. Some factors affecting penetration and infection by Sclerotinia sclerotiorum. Phytopathology 48: 605-609.

Sherf, A.F. and A.A. Macnab. 1986. Vegetable diseases and their control. John Wiley & Sons, New York, NY. 728 pp.

Steadman, J.R.,Schwartz, H.F. and E.K. Kerr. 1992. White mold of dry beans. Cooperative Extension, Institute of Agriculture and Natural Resources, University of Nebraska-Lincoln. NebGuide G92-1103-A.

Useful Links

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Caveat

This description is presented for information only and no endorsement is intended for products listed, nor criticism meant for products not mentioned. Always consult the product label before purchasing and using any pesticide.

Material contained on the Links from the page are the responsibility of the linked page's author(s).

This page was drafted by: Jane Christensen, Department of Plant Pathology, University of Nebraska-Lincoln

Disease images were provided by: Dr. David Wysong, Department of Plant Pathology, University of Nebraska-Lincoln

Questions, Comments, Complaints and Complements?

This page is authored and maintained by:
Dr. J.E. Partridge, Department of Plant Pathology, University of Nebraska-Lincoln

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