Agriculture: Alfalfa Pest Management Guidelines

Common Leaf Spot

  • Pseudopeziza medicaginis
  • Symptoms and Signs

    Symptoms of common leaf spot include small (0.12 inch), circular, brown-to-black spots on leaves. Margins of spots are characteristically toothed or uneven. As the disease progresses, infected leaves turn yellow and drop. In cool, moist weather circular, raised, brown fruiting bodies, called apothecia, are visible within the spots with the use of a hand lens.

    Comments on the Disease

    Common leaf spot is a cool-season foliar disease that requires moisture. The causal fungus overwinters in undecomposed leaves and leaf debris on the soil surface. In spring, spores are forcibly discharged into the air and some land on alfalfa leaves and initiate new infections.

    Management

    Start looking for leaf spot in spring. Harvest infected alfalfa early because the severity of this disease increases over time. Although the disease does not kill plants, defoliation reduces vigor, hay quality, and yield. In irrigated fields in California, common leaf spot can cause more leaf loss during drying and harvesting than before harvesting. Fungicides are generally not necessary. Some cultivars may be less susceptible than others.

    Crop rotation can reduce inoculum in the field. For more information, see CROP ROTATION.

    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 and application timing, honey bees, and environmental impact. Always read the label of the product being used.
     
    A. PYRACLOSTROBIN
      (Headline) 6–9 fl oz 12 14
      MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
     
    B. AZOXYSTROBIN
      (Quadris) 6–15.5 fl oz 4 14
      MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
    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 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.
    Text Updated: 03/17
    Treatment Table Updated: 03/17