Agriculture: Potato Pest Management Guidelines

Late Blight

  • Phytophthora infestans
  • Symptoms and Signs

    Late blight lesions can occur on all aboveground plant parts. On leaves, lesions typically first appear as small pale to dark green water-soaked spots that are irregular in shape and surrounded by a zone of yellowish tissue. Under conducive conditions, lesions expand rapidly and become brown to purplish black as tissue is killed. Under sufficient humidity, white sporulation of the fungus can be observed at the periphery of lesions, principally on the underside of leaves. On stems and petioles, lesions are brown to black and may also support sporulation of the fungus. Infected tubers develop a firm brown decay that starts on the outside and may later extend to include the outer 0.125 to 0.5 inch (3–12 mm) of tissues.

    Comments on the Disease

    Late blight occurs commonly in coastal environments and in the southern San Joaquin Valley in California. In other California potato-growing areas its occurrence is sporadic, depending on the presence of the pathogen and cool, damp weather conditions. Inoculum of Phytophthora infestans can originate from seed tubers, cull piles, volunteers, closely related weed hosts such as nightshade, and adjacent plantings of potatoes or tomatoes that are affected. Late blight can develop and spread rapidly if inoculum is present and conditions are conducive. High humidity (above 90%) and average temperatures in the range of 50° to 78°F favor the disease.

    Management

    Late blight is controlled by eliminating cull piles and volunteer potatoes, using proper harvesting and storage practices, and applying fungicides when necessary. Air drainage to facilitate the drying of foliage each day is important. Under marginal conditions, overhead sprinkler irrigation can favor late blight; in Tule Lake under solid set sprinklers, conditions conducive to late blight development are enhanced by day time irrigation but not night time irrigation.

    Plant certified seed tubers. When late blight has developed on foliage and tubers are at risk of infection, make sure that vines are completely dead for 2 to 3 weeks before harvest. Phytophthora infestans does not survive very long in dead foliage.

    In districts that are commonly subjected to outbreaks of late blight, preventive applications of fungicides are advised when environmental conditions are favorable for the disease. Continue fungicide applications at 7- to 10-day intervals as conditions require. Shorter intervals may be needed under cool, rainy conditions. In interior districts where late blight occurs sporadically, watch for disease symptoms during routine field monitoring, and record your results (example formPDF). Apply fungicides when late blight lesions appear in the field or in nearby plantings.

    Strains of Phytophthora infestans have developed resistance to mefenoxam (Ridomil Gold).

    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. AZOXYSTROBIN
      (Quadris) 6.2–15.4 fl oz 4 14
      MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
     
    B. CYMOXANIL
      (Curzate 60 DF) 3.2 oz 12 14
      MODE OF ACTION GROUP NAME (NUMBER1): Cyanoacetamide-oxime (27)
      COMMENTS: Use only in combination with a protectant fungicide (e.g., mancozeb, chlorothalonil). For tank mixes, observe all directions for use on all labels, and employ the most restrictive limits and precautions. Never exceed the maximum a.i. on any label when tank mixing products that contain the same a.i. Begin applications when conditions are conducive to disease development, repeat at 5- to 7-day intervals. Do not exceed 7 applications/growing season (see label for additional restrictions).
     
    C. CHLOROTHALONIL
      (various products) Label rates 12 7
      MODE OF ACTION GROUP NAME (NUMBER1): Chloronitriles (phthalonitriles) (M5)
     
    D. DIMETHOMORPH
      (Forum) 4–6 oz 12 4
      MODE OF ACTION GROUP NAME (NUMBER1): Carboxylic acid amides (40)
      COMMENTS: Begin applications when plants are 4 to 6 inches high or at onset of disease. See label for additional information.
     
    E. FAMOXADONE/CYMOXANIL
      (Tanos) 6–8 oz 12 14
      MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)/ Cyanoacetamide-oxime (27)
    COMMENTS: For best results suppressing Black Dot, tank-mix with a mancozeb or maneb fungicide.
     
    F. FENAMIDONE
      (Reason 500SC) 5.5–8.2 fl oz 12 14
      MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
     
    G. MANCOZEB
      (various products) Label rates 24 14
      MODE OF ACTION GROUP NAME (NUMBER1): Dithiocarbamates and relatives (M3)
     
     
    H. MEFENOXAM/CHLOROTHALONIL
      (Ridomil Gold Bravo SC) 2.5 pts 48 14
      MODE OF ACTION GROUP NAME (NUMBER1): Phenylamide (4)/Chloronitriles (phthalonitriles) (M5)
      . . . or . . .
      MEFENOXAM/COPPER
      (Ridomil Gold Copper) 2 lb 48 14
      MODE OF ACTION GROUP NAME (NUMBER1): Phenylamide (4)/Multi-site contact (M1)
      . . . or . . .
      MEFENOXAM/MANCOZEB
      (Ridomil Gold MZ) 2.5 lb 48 14
      MODE OF ACTION GROUP NAME (NUMBER1): Phenylamide (4)/Multi-site contact (M3)
      COMMENTS: Apply Ridomil Gold Copper with 0.8 lb a.i. per acre of mancozeb or chlorothalonil. For tank mixes, observe all directions for use on all labels, and employ the most restrictive limits and precautions. Never exceed the maximum a.i. on any label when tank mixing products that contain the same a.i.
     
    I. PROPAMOCARB HYDROCHLORIDE
      (Previcur Flex) 0.7–1.2 pt 12 14
      MODE OF ACTION GROUP NAME (NUMBER1): Carbamate (28)
      COMMENTS: Apply as a tank mix with chlorothalonil or mancozeb in a preventive program. For tank mixes, observe all directions for use on all labels, and employ the most restrictive limits and precautions. Never exceed the maximum a.i. on any label when tank mixing products that contain the same a.i. Do not apply more than 6 pt/growing season.
     
    J. PYRACLOSTROBIN
      (Headline) 6–12 oz 12 3
      MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
     
    POSTHARVEST - STORED POTATO TUBERS
    A. PHOSPHITES
    (Phostrol) 0.1 gallon/ton of tubers/0.5 gal 4 NA
    MODE-OF-ACTION GROUP NAME (NUMBER1): Phosphonates (33)
    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.
    NA Not applicable.

    Important Links

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