Agriculture: Tomato Pest Management Guidelines

Early Blight

  • Alternaria solani
  • Symptoms and Signs

    Plants infected with early blight develop small black or brown spots, usually about 0.25 to 0.5 inch (6–12 mm) in diameter, on leaves, stems, and fruit. Leaf spots are leathery and often have a concentric ring pattern. They usually appear on older leaves first. Spots on fruit are sunken, dry, and may also have a concentric pattern; frequently they occur near the calyx end of the fruit.

    Comments on the Disease

    Early blight is not common in California; it occurs in coastal areas and mainly affects tomatoes exposed to rain. Damage can occur if conditions remain cool and humid for several days after a rain. The early blight fungus survives in the soil on residue of infected tomatoes, potatoes, and nightshade weeds. The fungus is spread by spores (oval shaped with a long beak) carried by wind or splashed in water. Germination of spores and infection require free moisture. Disease development stops in dry, hot weather.

    Management

    Most California varieties are susceptible to early blight, but treatment is rarely needed because its occurrence is uncommon.

    • Destroy nightshades and volunteer tomato and potato plants.
    • Proper crop rotation is important to ensure infected plant debris decomposes.
    • Apply a fungicide when environmental conditions are favorable and the first sign of disease becomes apparent.
    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. CHLOROTHALONIL
      (Bravo Weather Stik) 1.5–2 pt 12 0
      MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M5)
     
    B. MANCOZEB
      (Dithane M-45) 1.5–2 lb 24 5
      (Dithane F-45 Rainshield) 1.2–1.6 qt 24 5
      (Penncozeb 75DF) 1.5–2 lb 24 5
      MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M3)
     
    C. FIXED COPPER#
      (various products) Label rates
      MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M1)
      COMMENTS: Not all copper compounds are approved for use in organic production; be sure to check individual products.
     
    D. BACILLUS SUBTILIS#
      (Serenade Max) 1–3 lb 4 0
      MODE OF ACTION: A biological fungicide.
      COMMENTS: For suppression begin applications when plants are 4 to 6 inches tall. Repeat on a 5-to-7-day interval or as needed.
    ** See label for dilution rate.
    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 until harvest can take place. In some cases the REI exceeds the PHI. The longer of these two intervals is the minimum time that must elapse before harvest may take place.
    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 fungicide with a different mode-of-action Group number.
    # Acceptable for use on organically grown produce.
    Text Updated: 12/13
    Treatment Table Updated: 12/13