Soil, Crop and Pest Management

Sclerotinia Blight of Vegetable Crops

The Plant Disease Diagnostic Laboratory

Prepared by Dr. Robert L. Wick, Department of Plant Pathology, Fernald Hall, Room 111, University of Massachusetts Amherst, MA 01003. Tel. (413) 545-1045 Fax. (413) 545-2532

Revised February 12, 1993 V4

Introduction

Sclerotinia sclerotiorum infects more than 360 species of plants, among them several important vegetable and flower crops as well as weeds. Diseases of vegetable crops include asparagus stalk rot, whitemold of beans, fruit rot, drop and soft rot of lettuce and cabbage, and damping-off. Sclerotinia also causes storage rots of vegetables especially to beans and carrots. The fungus survives winters in plant refuse and soil by forming hard, black structures called sclerotia. Sclerotina blight is managed by crop rotation and in some cases, fungicides.

Life History

In the northeastern United States, sclerotia must pass through the winter before they can germinate. Germination is most prevalent during cool and rainy weather. Ideal temperatures are between 55 and 60 F but a few will germinate from 40 up to 80 F. For germination to occur, the soil must be at field capacity for 10 days.

Sclerotia have two modes of infection; they may infect directly or they may produce fruiting structures (apothecia) which disseminate ascospores. The spores themselves, do not have enough stored energy to directly infect and colonize healthy, unwounded tissue. However, when senescing or dead flowers become colonized and fall onto a stem or fruit, the fungus then has enough energy to invade the plant. One sclerotia may produce one or more fruiting bodies which in turn produce thousands of ascospores that are easily blown about by the wind. In beans and tomatoes, timing of fungicides should be based on blossom development as well as on soil moisture. In crops such as cabbage, infection usually starts at bottom leaves that are in contact with the ground; fungicide applications would not be very effective.

When sclerotia are in direct contact with stems or fruit and conditions are favorable for germination, they may produce hyphae which can colonize stems or leaves.

Symptoms

Cankers that occur on stems appear dark and water-soaked at first but soon become gray and dry. Sclerotia are very diagnostic because they are frequently associated with diseased tissues and are large (1/8 to 1/2 inch in length). Stem cankers must be split in half to reveal the sclerotia. With rots of tomato, cabbage, lettuce, beans, and cucurbits, the sclerotia are apparent on the surface.

Early stages of disease are evident on leaves of crucifers and lettuce that are in contact with the ground; conspicuous white to gray mold with characteristic strands of mycelium develop. Extensively colonized heads become soft-rotted and imbedded with many conspicuous sclerotia. Tomato fruit develops a soft watery rot and snap beans are soft at first but may shrivel and dry out. In either case, mycelium and sclerotia are produced in abundance.

Management

Sclerotinia is a soil-borne fungus meaning that the soil is its natural habitat. Sclerotia are produced in large numbers following diseased crops but when the land is free of susceptible hosts for several years, they decline.

Plant susceptible crops on well drained soils. Rows should be planted in the direction of prevailing winds. Avoid sites that are surrounded by woods or other features that result in slower drying of soil and plants.

Infested soils should be planted to resistant crops and weeds should be eliminated as much as possible. A two year rotation with corn, grasses, beets, onions or spinach will help to reduce the number of sclerotia.

Sanitation, if practical, can be effective. This entails the removal of diseased

plants and fruit before the sclerotia become incorporated into the soil. Discing or plowing under a diseased crop insures that the fungus will be present next year.

Cultivars that do not create dense canopies allow for better air drainage and thus reduce the possibility of germination and infection.

Fungicides can protect against infection resulting from spore dispersal but are of limited value against sclerotia that are in direct contact with stems and fruit. However, disease that occurs during epidemics is largely the result of spores rather than sclerotia.

* This chart is only a guide. See the label for specific rates, precautions and days to harvest.