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How to Manage Pests

UC Pest Management Guidelines

Floriculture and Ornamental Nurseries

Cottony Rot

Pathogen: Sclerotinia sclerotiorum

(Reviewed 3/09, updated 3/09)

In this Guideline:


Under high humidity the fungus produces a mass of cottony hyphae or mycelia on the soil or plant surface. Later, large (5–10 mm), black sclerotia (hard, dark masses of hyphae) are formed on infected plant parts. Frequently the sclerotia are found inside 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.


Cottony rot, also called Sclerotinia rot or white mold, 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 on any aboveground part. Moisture and high humidity are necessary for development of the disease and this is one reason the disease is found lower in the plant canopy.

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).


Protective fungicides, as well as steam, solarization, or fumigation can be helpful. Steam (at 140°F for 30 minutes), solarize (double-tent at 160°F for 30 minutes or 140°F for 1 hour), or chemically treat growing medium. For flower production in open fields, solarization in warmer climates has been successful for control of Sclerotinia diseases in many crops. Solarization and steaming are acceptable for organic production. In open fields airborne spores can blow in from outside the field, so soil treatment may be limited in its effectiveness.

Common name Amount to Use R.E.I.+
(trade name)   (hours)

Calculate impact of pesticide on air quality
Bee precaution pesticide ratings
When choosing a fungicide, consider the general properties of the fungicide as well as information relating to environmental impact.
  (Chipco 26019) 0.4 lb/100 gal water 12
  MODE OF ACTION GROUP NAME (NUMBER1): Dicarboximide (2)
  COMMENTS: Apply as a drench (1–2 pt/sq ft) at seeding or transplanting. Effective against Rhizoctonia damping‑off and Sclerotinia. Some iprodione is absorbed by plant parts.
  (FungoFlo, etc.) 20 fl oz/100 gal water 12
  MODE OF ACTION GROUP NAME (NUMBER1): Methyl benzimidazole (1)
  COMMENTS: Apply as a drench or heavy spray (1–2 pt/sq ft). Absorbed by plant parts exposed to the chemical; roots may absorb the fungicide (or its breakdown product carbendazim), which moves in the xylem to transpiring leaves.
  (Terraclor) 0.5–1 lb a.i./1000 sq ft 12
  MODE OF ACTION GROUP NAME (NUMBER1): Aromatic hydrocarbon (14)
  COMMENTS: Inhibits germination of sclerotia when incorporated into top 2 inches of soil. Insoluble in water and must be thoroughly mixed with soil to reach its desired depth of control. Works through vapor action and has good residual action. Germination of some seeds may be inhibited and small plants may be stunted by this fungicide.
1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions (for more information, see http://www.frac.info/). Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode of action Group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different mode of action Group number; for fungicides with other Group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode of action Group number.
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing.



[UC Peer Reviewed]

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

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