Agriculture: Floriculture and Ornamental Nurseries Pest Management Guidelines

Gray Mold

  • Botrytis cinerea
  • Symptoms and Signs

    Gray mold is one of the most destructive plant pathogens and attacks a wide variety of plants. Flower petals and ripening fruits and vegetables are particularly susceptible to infection, but leaves and stem tissues may also get infected. Young seedlings of several crops can be killed if infected with gray mold. In high relative humidity, the fungus may sporulate on infected tissues and produce masses of characteristic gray or brownish spores that become airborne, which is primarily how the fungus is disseminated. Spores must have moisture to germinate and infect.

    Comments on the Disease

    Botrytis does not invade healthy green tissue such as leaves and stems unless (a) an injured or dead area is present, or (b) it grows directly from a food base such as a fallen petal or leaf. The fungus will first colonize the food base and then attack healthy green tissues. However, flower petal tissue differs significantly from leaf and stem tissue, and Botrytis can directly invade petals of African violet, aster, begonia, carnation, chrysanthemum, cyclamen, cymbidium, gerbera, geranium, gladiolus, hydrangea, marigold, orchid, petunia, poinsettia, primrose, ranunculus, rose, snapdragon, zinnia, and others.

    Although the fungus is capable of growth within a wide range of temperatures from about 28° to 90°F, growth is very slow at the extremes. Optimum temperature range for growth is 70° to 77°F. The fungus, which is more active below 70° than it is above 77°F, is particularly troublesome under conditions of moderate temperature and high humidity.

    Management

    Refrigeration at temperatures near 32°F will retard but not completely stop the development of gray mold; when infected tissue is warmed, decay can proceed rapidly.

    Moisture often is more of a limiting factor than temperature. Free moisture is necessary for germination of Botrytis spores. Moisture is also necessary for growth within plant tissues, and low humidity may result in arrested growth of the fungus. However, growth can resume when moisture again becomes available.

    Gray mold is most severe during times of the year when the humidity is high. In California, this is usually in the late fall and winter months, when rainfall is common. The worst time for disease development is from September to December, because there is an abundant amount of herbaceous vegetative material (crop refuse and dying summer plants) available for fungal colonization and, as a consequence, many spores are present in the air and on plant parts.

    Cultural Control

    Botrytis cinerea produces innumerable asexual spores (conidia) that are moved by air currents. Because spores may readily develop in decaying vegetation and old flowers, elimination or reduction of sources of the spores is an important part of any control program.

    • Remove old flowers before they become infected and function as spore sources. The fungus can develop and sporulate at low temperatures, so do not overlook old flowers and foliage in refrigerators.
    • Avoid condensation of water on susceptible plant parts, as free moisture is necessary for germination and infection
    • Avoid overhead watering during blooming. If this is the only method of irrigation available, irrigate early in the day so that the foliage can dry as rapidly as possible.
    • Maximize the period between irrigations to further enhance drying of foliage and flowers.
    • Increase plant spacing to increase ventilation and minimize leaf wetness. This can help reduce both disease incidence and severity.

    Chemical Control

    Numerous fungicides are effective against B. cinerea but not all of them can be used on all crops. Some products can damage plants. To avoid damage and the development of fungal strains that are resistant to fungicides, growers should alternate different fungicides. The fungicides are preventives and must be applied before infection. In some crops, such as chrysanthemum, the lower foliage of crowded plants becomes infected and acts as a source of spores that then infect the flowers. In these crops, it is important to apply fungicides at an early stage when the lower foliage can be adequately covered by the chemical.

    Common name Amount to use REI‡
    (Example trade name) (hours)
    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. AZOXYSTROBIN
    (Heritage) 4–8 oz/100 gal water 4
    MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
    COMMENTS: Apply as a broadcast or banded spray targeted at the foliage or crown of the plant. A locally systemic fungicide.
     
    B. BOSCALID/PYRACLOSTROBIN
    (Pageant) 12–18 oz/100 gal 12
    MODE OF ACTION GROUP NAME (NUMBER1): Carboximide (7) and quinone outside inhibitor (11)
     
    C. CHLOROTHALONIL
    (Daconil WeatherStik) 1.375 pt/100 gal water 12
    MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M 05)
    COMMENTS: Do not apply to either green or variegated Pittosporum or to Schefflera. Effective for the control of Botrytis spp., Alternaria spp., Rhizoctonia spp., as well as other leaf-spotting fungi on many ornamentals.
     
    D. CYPRODINIL/FLUDIOXONIL
    (Palladium) 4–6 oz/100 gal water 12
    MODE OF ACTION GROUP NAME (NUMBER1): Amino acids and protein synthesis (9) and signal transduction (12)
     
    E. FENHEXAMID
    (Decree 50 WDG) 0.75-1.5 lb/100 gal water 12
    MODE OF ACTION GROUP NAME (NUMBER1): Hydroxyanilide (17)
    COMMENTS: Apply as a spray; very effective and can be applied after infection.
     
    F. FLUDIOXONIL
    (Medallion WDG) Label rates 12
    MODE OF ACTION GROUP NAME (NUMBER1): Phenylpyrroles (12)
     
    G. FLUOXASTROBIN
    (Disarm 480 SC) 4–8 fl oz/100 gal 12
    MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
    COMMENTS: Preventative application.
     
    H. 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. Some resistance has been reported with this material. Effective against Rhizoctonia damping-off, Sclerotinia, and gray mold. Some iprodione is absorbed by plant parts.
     
    I. MANCOZEB
    (Dithane 75DF) 1–1.5 lb/100 gal water 24
    MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M 03)
    COMMENTS: Protects against leaf spots, Botrytis, rusts, and blight. Not systemic so thorough coverage is important for control.
     
    J. 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. Some resistance has been reported with this material. Thiophanate-methyl is 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.
     
    K. DICLORAN
    (Botran 75-W) Label rates 12
    MODE OF ACTION GROUP NAME (NUMBER1): AH-fungicides (14)
    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