Agriculture: Floriculture and Ornamental Nurseries Pest Management Guidelines

Cottony Rot

  • Sclerotinia sclerotiorum
  • Symptoms and Signs

    Under high humidity the fungus produces a mass of white cottony mycelia on the soil or plant surface. Later, large (1/4 to 1 inch long), black sclerotia (hard, dark masses of hyphae) are formed on and inside infected plant parts. Frequently the sclerotia are found inside hollow 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 much smaller than those of Sclerotinia. Additionally, Botrytis produces gray mycelium and spores instead of the mass of white, cottony growth that Sclerotinia does.

    Comments on the Disease

    Cottony rot, also called Sclerotinia rot or white mold, affects many kinds of plants. It is also a disease of vegetables, such as beans, carrots, celery, and lettuce. 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. Infection can either be soilborne or airborne.

    Sclerotinia sclerotiorum does not produce asexual 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, sclerotia germinate and can infect the plant near the soil line either directly by producing vegetative strands (hyphae) or by forming apothecia (saucer-shaped, dime-sized structures on stalks) that produce ascospores (sexual spores). Ascospores 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 can occur on any aboveground part. 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.

    Management

    Protective fungicides, as well as steaming (at 140°F for 30 minutes), solarization (double-tent at 160°F for 30 minutes or 140°F for 1 hour), or fumigation of the growing medium can be helpful. 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 REI‡ PHI‡
    (Example trade name) (hours) (days)
    Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.
    A. BOSCALID/PYRACLOSTROBIN
      (Pageant) 12–18 oz/100 gal 12  
      MODE OF ACTION GROUP NAME (NUMBER1): Carboximide (7) and quinone outside inhibitor (11)
     
    B. CYPRODINIL/FLUDIOXONIL
      (Palladium) 2–4 oz/100 gal water 12
      MODE OF ACTION GROUP NAME (NUMBER1): Amino acids and protein synthesis (9) and signal transduction (12)
     
    C. IPRODIONE
      (Chipco 26019 N/G) 6.5 oz/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.
     
    D. PCNB
      (Terraclor 400) Label rates 12  
      MODE OF ACTION GROUP NAME (NUMBER1): Aromatic hydrocarbon (14)
      COMMENTS: Inhibits germination of sclerotia when incorporated into top two inches of soil. Insoluble in water. 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.
     
    E. THIOPHANATE-METHYL
      (Talaris 4.5 F) 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) after sowing. 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.
    1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions. 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 (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing.
    Text Updated: 11/20
    Treatment Table Updated: 11/20