Agriculture: Small Grains Pest Management Guidelines

Powdery Mildew

  • Powdery mildew (barley): Erysiphe graminis f. sp. hordei
  • Powdery mildew (oats and wild oats): Erysiphe graminis f. sp. avenae
  • Powdery mildew (wheat): Erysiphe graminis f. sp. tritici
  • Symptoms and Signs

    The disease first appears on lower leaves: white, cottony patches of fungal growth on the upper leaf surface that are opposite chlorotic spots on the underside of the leaf. The patches of white growth turn a dull gray-brown as fruiting structures, called cleistothecia, develop. Plants are often low in vigor.

    Comments on the Disease

    Each type of small grain is attacked by a specific form of the fungus Blumeria graminis (Erysiphe graminis). The fungus overwinters in tiny, dark, spore-forming structures called cleistothecia that release airborne spores (ascospores) in spring. It also can overwinter as mycelium on volunteer wheat, barley, or oat plants and produce spores (conidia) that can cause initial infections; conidia from resulting lesions are windblown for secondary disease cycles at 10-day intervals. Disease development is optimal at 59° to 72°F (15° to 22°C) and is retarded above 77°F (25°C). Disease is favored by dense stands, high nitrogen fertilization, high relative humidity, and cool temperatures.

    The form that attacks barley also attacks other weeds in the genus Hordeum.

    Management

    Resistant cultivars of barley and wheat are available (see BARLEY and WHEAT CULTIVAR tables). Crop rotation, elimination of crop residue, and control of volunteer grains and weed hosts reduce inoculum survival from one season to the next.

    Although normally not economical, foliar fungicides can be used to control disease outbreaks and provide partial disease control. Applications should be made between tillering and heading with the objective being to protect the flag leaf. Depending on weather conditions from tillering to early dough stage, one or more applications may be needed.

    Common name Amount per acre** 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, efficacy, application timing, honey bees, and environmental impact. Always read the label of the product being used.
     
    A. PROPICONAZOLE
      (Tilt) 2–4 fl oz 12 See label
      MODE-OF-ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
      COMMENTS: For use on wheat, barley, triticale, oats, and rye. For wheat, apply until Feekes growth stage 10.5 (full head emergence). For other grains, apply until Feekes 9 growth stage (emergence of flag leaf ligule).
    ** See label for dilution rates.
    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. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.
    1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of action. 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 a fungicide with a different mode-of-action group number.
    Text Updated: 02/09
    Treatment Table Updated: 07/16