Agriculture: Floriculture and Ornamental Nurseries Pest Management Guidelines

Powdery Mildew

  • Powdery mildew: Erysiphe spp., Golovinomyces spp., Leveillula (=Oidiopsis) taurica, Oidium sp., Podosphaera spp.
  • Symptoms and Signs

    Powdery mildew is the name given to diseases resulting from infection by fungi that produce a white to gray, powdery growth on the surfaces of leaves and sometimes other plant parts. Leaves may yellow, then brown and die. Infected tissues may be distorted and misshapen.

    Comments on the Disease

    There are many kinds of powdery mildew fungi, and most are highly specialized. For example, the powdery mildew that infects squash plants will infect some other plants in the cucurbit family but will not infect roses, and the powdery mildew from roses will not attack zinnias (and vice versa), although the fungus that infects zinnias also infects many other members of the composite family. Powdery mildew fungi are obligate parasites; that is, they can grow only on living plant tissue. When the mildew-infected plant part dies, so does the mildew unless chasmothecia (spherical, resting stages of the fungus) are formed.

    Most powdery mildew fungi grow over the surface of the leaf, sending short food-absorbing projections (haustoria) into the epidermal cells. The fungi produce masses of spores (conidia), which become airborne and spread to other plants. Powdery mildew spores require no external moisture for germination, while most other fungi require free water in the form of dew, guttation, rain, or water from overhead irrigation for germination and infection. Conidia of powdery mildew (except those that infect grasses) die in water. Spores may be dispersed, however, by splashing water.

    The fungus survives in the absence of susceptible host tissues by forming a sexual stage (chasmothecia) resistant to drying and other adverse environmental conditions. With many perennial plants, such as rose, the fungus survives as mycelium in dormant buds or actively on plant tissues. Powdery mildews are particularly severe in semiarid regions, such as most of California, and are less troublesome in high rainfall areas.

    Powdery mildews are favored by warm days and cool nights and moderate temperatures (68° to 86°F). At leaf temperatures above 90°F, some mildew spores and colonies are killed. Shade or low light intensities as well as high relative humidity (greater than 95%) favor powdery mildew fungi. Greenhouse conditions are often ideal for development of the disease.

    Management

    The best control is through the use of resistant cultivars. However, little attention has been paid to the development of resistant cultivars in flower and nursery crops. Increased air movement around the plants in the greenhouse tends to reduce infection potential of mildews. Plants that have been treated with anti-transpirants are less likely to develop powdery mildew infections

    Monitoring and Treatment Decisions

    In general, there are two types of fungicidal control: eradication of existing infections and protection of healthy tissues. In practice, some products provide both protection and eradication, especially when good wetting of the plant is achieved. To achieve good wetting, some of these products may require the addition of surfactants.

    The fungus has developed resistance to some of these fungicides. Rotate the different fungicides to help slow down the development of fungicide resistance within fungal strains.

    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.
     
    PROTECTANTS (Must be applied to healthy tissues before infection takes place)
    A. TEBUCONAZOLE
    (Torque) 4–10 fl oz/100 gal water 12
    MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
    COMMENTS: A systemic fungicide applied as a foliar spray; both a protectant and eradicant.
     
    B. 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)
     
    C. BOSCALID/PYRACLOSTROBIN
      (Pageant) 6–12 oz/100 gal 12
      MODE OF ACTION GROUP NAME (NUMBER1): Carboximide (7) and quinone outside inhibitor (11)
     
    D. FLUOXASTROBIN
      (Disarm 480 SC) 1–4 fl oz/100 gal 12
      MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
     
    E. WETTABLE SULFUR# 3 lb/100 gal water 24
    MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M 02)
    COMMENTS: Use a wetting agent. Effectiveness of sulfur increases with increasing temperature, but the likelihood of plant injury increases also. Plant damage may result if sulfur is applied at temperatures exceeding 90°F. Some plants, such as melons, are sensitive to sulfur. Sulfur can be applied as a dust or as a spray. Repeated applications are generally necessary to protect new growth and also to renew deposits removed by rain or irrigation.
     
    F. MYCLOBUTANIL
    (Rally 40WSP) 3 oz/50 gal water 24
    MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
    COMMENTS: A systemic fungicide applied as a foliar spray; both a protectant and eradicant of rusts or powdery mildew on carnations, crape myrtle, gerbera, roses, and snapdragons.
     
    G. AZOXYSTROBIN
    (Heritage) 1–4 oz/100 gal water 4
    MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
    COMMENTS: Acts as a protectant but has some eradicant properties. A locally systemic fungicide that is an eradicant and protectant against some powdery mildews.
     
    H. TRIADIMEFON
    (Bayleton Flo) Label rates 12
    MODE OF ACTION GROUP NAME (NUMBER1): DMI (Group 3)1 triazole fungicide
    COMMENTS: A long-lasting systemic fungicide that provides for general control of some powdery mildews, some rusts, and leaf blight and spots in greenhouses and commercial nurseries.
     
    I. THIOPHANATE-METHYL
    (Talaris 4.5 F) 10–20 fl oz/100 gal water 12
    MODE OF ACTION GROUP NAME (NUMBER1): Methyl benzimidazole (1)
    COMMENTS: Not as effective against powdery mildew as other materials. 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.
     
    J. PROPICONAZOLE
    (Banner Maxx II) Label rates 12
    MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
    COMMENTS: A preventive fungicide.
     
    K. REYNOUTRIA SACHALINENSIS#
      (Regalia CG) Label rates 4
      MODE OF ACTION GROUP NAME (NUMBER1): Anthraquinone elicitor (P 05)
     
    L. POTASSIUM BICARBONATE
    (Kaligreen)# 1–3 lb/100 gal water/acre 4
    MODE OF ACTION GROUP NAME (NUMBER1): Inorganic salt (NC)
    COMMENTS: Primarily a protectant but it eradicates some existing infections with thorough coverage. Apply at first signs of infection. Thorough coverage is essential for good protection. Labeled for use on roses, field ornamentals, and greenhouse ornamentals.
     
    ERADICANTS
    A. LIME SULFUR 28%
    (Rex lime sulfur solution)# 0.5 gal/100 gal water 48
    MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M 02)1
    COMMENTS: Primarily an eradicant but has some protectant properties. Plant damage may result if applied when temperatures exceed 80°F. Not as effective against powdery mildew as other materials. Not for use in greenhouses.
     
    B. NEEM OIL
    (Triact 70)# Label rates 4
    MODE OF ACTION GROUP NAME (NUMBER1): (NC)
    COMMENTS: A broad-spectrum botanical pesticide derived from the neem tree that is effective against various fungal diseases including black spot on roses, powdery mildew, downy mildew, anthracnose, and leaf spot. Registered for landscape and nursery ornamentals; oils work best as eradicants but also have some protectant activity. When using as a protectant, apply on a 14-day schedule; as an eradicant, apply on a 7-day schedule. Do not repeat oil applications frequently as multiple applications may burn leaves and flowers. Never apply any oil within 2 weeks of a sulfur spray or plants may be injured.
     
    C. PIPERALIN
    (Pipron) 4–8 fl oz/100 gal water 12
    MODE OF ACTION GROUP NAME (NUMBER1): Amine (morpholine) (5)1
    COMMENTS: Requires thorough coverage. For use in greenhouses only. A foliar spray that eradicates powdery mildew on rose, lilac, dahlia, phlox, zinnia, chrysanthemum, and catalpa.
     
    D. STYLET OIL
    (Organic JMS Stylet Oil)# 1 oz/gal water 4
    MODE OF ACTION GROUP NAME (NUMBER1): (NC)
    COMMENTS: A good eradicant for mild to moderate powdery mildew infections; oils work best as eradicants but also have some protectant activity. Registered for use on chrysanthemum, dieffenbachia, philodendron, poinsettia, and roses. May be phytotoxic, especially on greenhouse roses. Do not repeat oil applications frequently as multiple applications may burn leaves and flowers. Do not apply to plants suffering from heat or moisture stress. Never apply any oil within 2 weeks of a sulfur spray or plants may be injured.
     
    E. TEBUCONAZOLE
    (Torque) 4–10 fl oz/100 gal water 12
    MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
    COMMENTS: A systemic fungicide applied as a foliar spray; both a protectant and eradicant.
    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.
    # Acceptable for use on organically grown ornamentals.
    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