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UC Pest Management Guidelines

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Floriculture and Ornamental Nurseries

Cottony Rot

Pathogen: Sclerotinia sclerotiorum

(Reviewed 1/02, updated 1/02)

In this Guideline:

Symptoms
Comments on the disease
Management

SYMPTOMS
Under high humidity the fungus produces a mass of cottony hyphae or mycelia on the soil surface. Later, large (2-10 mm), black sclerotia (hard, dark mass of hyphae) are formed on infected plant parts. Frequently the sclerotia are found inside of dead stems. Plant tissues killed by the fungus often take on a bleached appearance. Gray mold (Botrytis cinerea) causes a similar bleaching and also has black sclerotia, but they are smaller than those of Sclerotinia; it also doesn't produce the mass of white, cottony growth that Sclerotinia does.

COMMENTS ON THE DISEASE
Cottony rot, also called sclerotinia rot, affects many kinds of plants. It is primarily a disease of vegetables, such as beans, carrots, celery, and lettuce. The fungus often infects the plant near the soil line but infections can occur at any height. Moisture and high humidity are necessary for development of the disease and this is one reason the disease is found lower in the plant.

Sclerotinia sclerotiorum does not produce conidia. Sclerotia formed by the fungus undergo a dormant period that is broken by low temperatures (optimal is 56° to 59°F) and high soil moisture. In fall and spring when temperatures are in the optimal range, the sclerotia germinate to form apothecia (saucer-shaped, dime-sized structures on stalks) that produce spores. The spores are discharged forcibly into the air and are carried by air currents. They do not directly infect healthy tissue, but if they land on injured tissue in the presence of moisture, infection occurs. Flower petals of many plants are susceptible. Foliage may become infected if there is an injury or if the tissue is senescent. If diseased tissue comes in contact with healthy tissue, the fungus can invade the healthy tissue. Infections frequently occur at the soil level because plants can be infected directly by sclerotia in the soil that germinate to produce vegetative strands (hyphae).

MANAGEMENT
Protective fungicides as well as steam treatment or fumigation of the growing medium can be helpful.

Common name

(trade name) Amount to Use

A. IPRODIONE

(Chipco 26019) 0.4 lb/100 gal water

COMMENTS: Apply as a drench (1-2 pt/sq ft) at seeding or transplanting.

B. THIOPHANATE METHYL

(FungoFlo, Cleary's 3336F, etc.) 20 fl oz/100 gal water

COMMENTS: Apply as a drench or heavy spray (1-2 pt/sq ft).

C. PCNB

(Terraclor) 0.5-1 lb a.i./1000 sq ft

COMMENTS: Inhibits germination of sclerotia when incorporated into top 2 inches of soil.

PUBLICATION

UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries
UC ANR Publication 3392
Diseases
R. D. Raabe (emeritus), Environmental Science, Policy, and Management (ESPM), UC Berkeley
M. E. Grebus, Plant Pathology, UC Riverside
C. A. Wilen, UC IPM Program, UC Cooperative Extension, San Diego Co.
A. H. McCain (emeritus), Environmental Science, Policy, and Management (ESPM), UC Berkeley

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