11/17/97

Contacts:  Paula Flynn, Extension Plant Pathology, (515) 294-0581
           Elaine Edwards, Extension Communication Systems, (515) 
294-5168

Yard and Garden Column for the Week Beginning Nov. 21

WHITE MOLD

By Paula Flynn
Extension plant pathologist
Iowa State University

White mold may affect more than 100 different plant species. The 
common name white mold often is used because the name of the 
fungus that causes the disease is quite a mouthful -- Sclerotinia 
sclerotiorum.

Many vegetable and ornamental plants are susceptible to white 
mold. The lengthy list of plants that may be infected includes 
beans, broccoli, cabbage, cucumber, lettuce, melons, onions, peas, 
peppers, potatoes, pumpkins, squash, tomatoes, watermelons, 
chrysanthemums, dahlias, daisies, delphiniums, euonymus, 
forsythias, hollyhocks, irises, peonies, petunias, snapdragons, 
sunflowers, zinnias and many others. In Iowa, the primary plant 
host of concern is soybean.

Plants attacked by the fungus usually show a wet rot on the stem. 
When humid conditions occur, a cottony white mold is formed on the 
stems. Clumps of the cottony mold form that eventually become hard 
black structures called sclerotia. Sclerotia vary in size, usually 
ranging from pinhead-size to the size of a pea.

Many times the black sclerotia can be seen on the outside of the 
stems. However, sclerotia also may be found embedded throughout 
the infected tissues. You may be able to feel these hard 
structures by pinching the stem. Interior sclerotia can be seen 
easily by splitting open diseased stems. Because the fungus 
eventually constricts the plant tissue, water and nutrients cannot 
move to upper plant parts, and as a result entire branches or 
plants may be killed.

Sclerotia are special survival structures that can survive in the 
soil for five or more years. When spring arrives and moist 
conditions occur, brown, saucer-shaped structures form on stalks 
that grow from the sclerotia. These structures, called  apothecia, 
are about one-eighth to three-eighth of an inch in diameter and 
project slightly above the soil surface. Apothecia release fungal 
spores that may then infect a susceptible plant that is growing in 
the vicinity. Spores may be spread by wind, water, insects (such 
as bees) or gardening equipment.

Several practices help control white mold. Since the fungus 
thrives in abundant moisture,  practices that favor drying of the 
plant tissues, such as wide spacing at planting, will help reduce 
the chance of infection. If the disease does occur, all diseased 
plant parts should be cleaned up and destroyed to prevent or 
reduce the number of sclerotia added to the soil.

Because the sclerotia can survive for a number of years in the 
soil, rotation with a crop that is not susceptible to the disease 
is helpful, but will not eliminate the pathogen. Deep plowing of 
the soil will help to speed decomposition or breakdown of the 
sclerotia. Various broadleaf weeds, such as pigweed, velvet leaf, 
ragweed and Canada thistle, are susceptible to the disease. Weed 
control, therefore, is important in preventing the fungus from 
increasing its population densities.

Depending on the plant, there may be certain cultivars available 
that are more resistant to disease than other cultivars. For 
example, beans with an upright and open leaf structure tend to be 
less prone to disease than bushy plants.  Fungicides are labeled 
to help control the disease in certain crops, such as soybeans. 
However, these materials may not be readily available or practical 
for use in the home garden.

                              -30-
ml: isugarden