Agriculture: Potato Pest Management Guidelines

Early Blight

  • Alternaria solani
  • Symptoms and Signs

    Early blight is primarily a disease of stressed or senescing plants. Symptoms appear first on the oldest foliage. Affected leaves develop circular to angular dark brown lesions 0.12 to 0.16 inch (3–4 mm) in diameter. Concentric rings often form in lesions to produce characteristic target-board effect. Severely infected leaves turn yellow and drop. Infected tubers show a brown, corky dry rot.

    Comments on the Disease

    Between crops, the early blight fungus can overwinter on potato refuse in the field, in soil, on tubers, and on other solanaceous plants. Infection occurs when spores of the fungus come in contact with susceptible leaves and sufficient free moisture is present. Spore germination and infection are favored by warm weather and wet conditions from dew, rain, or sprinkler irrigation. Alternately, wet and dry periods with relatively dry, windy conditions favor spore dispersal and disease spread. Tubers can be infected as they are lifted through the soil at harvest. If sufficient moisture is present, spores germinate and infect the tubers.


    Early blight can be minimized by maintaining optimum growing conditions, including proper fertilization, irrigation, and management of other pests. Grow later maturing, longer season varieties. Fungicide application is justified only when the disease is initiated early enough to cause economic loss. Watch for disease symptoms during routine monitoring, and keep records of your results (example formPDF). When justified, apply fungicides as soon as symptoms appear; continued protection requires application at 7- to 10-day intervals.

    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.
      (Quadris) 6.2–15.4 oz See label 14
      MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
      COMMENTS: Do not apply more than 2.88 qt per season.
      (Endura) 2.5–4.5 oz 12 10
      MODE OF ACTION GROUP NAME (NUMBER1): Succinate dehydrogenase inhibitors (7)
      (various products) Label rates 12 7
      MODE OF ACTION GROUP NAME (NUMBER1): Chloronitriles (phthalonitriles) (M5)
      (Tanos) 6 oz 12 14
      MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)/Cyanoacetamide (27)
      (Reason 500SC) 5.5–8.2 fl oz 12 14
      MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
      (Rovral 4F) 2 pt 24 14
      MODE OF ACTION GROUP NAME (NUMBER1): Dicarboximide (2)
      (various products) Label rates 24 14
      MODE OF ACTION GROUP NAME (NUMBER1): Dithiocarbamates and relatives (M3)
      (Headline) 6–9 fl oz 12 3
      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.

    Important Links

    Text Updated: 08/07
    Treatment Table Updated: 03/19