Resource Guide for Organic Insect and Disease ... - Cornell University

lowing spores. If in contact with diseased tissues, any part of the plant may become infected.
Lower leaves that are deep in the foliage are prime target areas. In shipment or on the shelf,
pods may be affected if they are in contact with diseased petals, and the crop could be a mass
of fungus by the time it reaches market.
The first symptoms on leaves are grayish lesions that may become brownish and dry. In damp
conditions, the diseased areas become fuzzy, with gray spores produced in masses (Photo 5.9).
Infected fruit develops water-soaked, yellowish to greenish brown, irregular lesions, which can
become somewhat soft. When conditions are favorable, the fungus grows very quickly, and the
crop becomes a gray, fuzzy mass of rotting tissue and spores.
Botrytis overwinters as mycelium on plant debris or as sclerotia either in the soil or on plant
debris. In the spring, conidia are produced and spread by moving air, plant debris, or soil.
Spores need a wet surface on which to germinate and are produced in large numbers when
conditions are cool and damp. Infection rarely occurs in warm temperatures, but once
infection occurs, the fungus grows well even when it is warm. The germinating spore cannot
penetrate healthy tissue; however, senescing or wounded tissue is easily colonized (e.g., flower
petals, dying foliage, etc.), which can lead to growth into the living tissue. Too much nitrogen
or too little potassium have been reported to make pods more susceptible to gray mold.
Cultural Control:
1. Cultural management is the key. Since it has such a wide host range and is ubiquitous,
avoiding this disease is difficult. Crop rotation and sanitation may help (corn and
grasses are not hosts) but will rarely prevent the problem if the weather favors the
disease.
2. Close plant spacing and a dense canopy favor disease development. Plant in welldrained
fields with good air movement. Avoid overcrowding, overwatering, weedy
sites, and wet mulches.
3. When possible, use drip irrigation instead of overhead irrigation. If using overhead
irrigation, water at times that do not extend the period of leaf wetness, e.g., very early
in the morning on a sunny day.
Materials Approved for Organic Production:
Products containing Bacillus subitlis (Serenade) and potassium bicarbonate (Kaligreen, MilStop)
have been shown to be effective.
WHITE MOLD (Sclerotinia sclerotiorum)
White mold has a very wide host range of vegetables, including beans. Other common hosts
are carrots, tomatoes, peppers, and lettuce. It is not common in peas. If a field has a history
of white mold problems, beans are in a crowded or weedy situation, or the weather is damp,
white mold is more likely to be a problem. The disease starts on the stems, leaves, or pods as
dark green, water-soaked lesions that rapidly increase in size and later develop a mass of very
white, cottony mycelium (Photo 5.10). Black sclerotia, which look like small black pebbles,
develop in the white mass (Photo 5.11). Whole branches of the plant may be consumed, and
eventually the whole planting can be lost if conditions favor the fungus.
Sclerotia can remain viable in the soil for more than five years. Exposure to moist, cool
conditions for several weeks preconditions the sclerotia to produce apothecia, tiny, cupshaped
fruiting bodies from which spores are released. After preconditioning, moderate soil
temperature and near field capacity soil moisture stimulate apothecia and spore production.
The spores are primarily carried by the wind, and in order to complete successful germination,
they require senescing tissue (e.g., flower petals) as a food source. For this reason, white mold
does not usually take hold until the beans flower. Spores can remain viable on a plant surface
for up to two weeks. Once germinated, mycelia in the senescent tissue can survive for up to a
month, waiting for sustained moist conditions during which it can invade healthy plant tissue.
Spread from one plant to another is common by contact with the mycelia.
48 Organic Resource Guide