Agriculture: Potato Pest Management Guidelines

Pythium Leak (Water Rot)

  • Pythium spp.
  • Symptoms and Signs

    Pythium leak, also known as water rot, causes spongy, wet internal rot of tubers. The diseased flesh is clearly demarcated from healthy tissue by a dark boundary line. In advanced infections, hollow cavities form and all that remains of some infected tubers are tuber shells with thin papery skins.

    Comments on the Disease

    The pathogen lives in the soil and enters through wounds made during harvest. The disease is primarily a problem in Kern County and causes the most damage when saturated soil occurs along with warm temperatures.

    Management

    Avoid overwatering near harvest. Allow tubers to mature completely before harvest. Minimize bruising the tubers. In areas where leak is a problem, apply fungicide 2 and 4 weeks before harvest.

    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. MEFENOXAM/MANCOZEB
      (Ridomil Gold MZ) 2.5 lb 48 14
      MODE OF ACTION GROUP NAME (NUMBER1): Phenylamide (4)/ Dithiocarbamates and relatives (M3)
      COMMENTS: Make the first application at flowering and another 14 days later.
     
    B. MEFENOXAM/CHLOROTHALONIL
      (Ridomil Gold Bravo SC) 2 pts 48 14
      MODE OF ACTION GROUP NAME (NUMBER1): Phenylamide (4)/ Chloronitriles (phthalonitriles) (M5)
      COMMENTS: Make the first application at flowering and another 14 days later.
    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: 03/19
    Treatment Table Updated: 03/19