1999 Potato Late Blight Action Plan Summary


    Occurrence in Idaho

    • All genotypes found in Idaho are classified as aggressive and resistant to metalaxyl (Ridomil).

    Spreading of Late Blight

    • Inoculum is produced on volunteer plants or from cull piles or infected seed.
    • Infected plants produce spores that can be carried in moist air for up to several miles.


    Before Planting

    • Reduce or eliminate inoculum sources: cull piles, infected volunteer potato plants from previous season, and infected seed tubers.
    • Plant only certified seed.
    • Select fields for planting potatoes having good water infiltration and drainage characteristics.
    • Do not plant potatoes in areas of fields where plants cannot be sprayed with fungicide.
    • Consider not planting the area under the first few nozzles of a center pivot system where plants remain wet for extended periods.

    During Planting

    • Do not mix seed lots to avoid possible contamination of healthy seed by infected seed.

    Early Season

    • Cultivate fields to increase water infiltration and control weeds.
    • Form high, wide hills to reduce exposing tubers to late blight spores that may be washed from infected plants.
    • Scout fields regularly concentrating in areas that remain wet for extended time periods.
    • Look for early indications of late blight on volunteer potatoes and monitor cull disposal sites.


    • Apply heavier, less frequent water applications rather than light, frequent ones.
    • For night irrigation sets, consider beginning after midnight when dew would normally wet the leaves anyway.
    • Continue scouting, especially low lying areas, pivot corners, field borders, and weedy patches.
    • Although late blight readily spreads with moist air, to reduce any risks, people entering fields may want to wear high boots that can be disinfected between fields, or wear disposable boots and pants that can be changed between fields, and reused after washing and drying.
    • Destroy small patches of late blight-infected potato plants in the field.

    Late Season

    • Avoid excessive irrigation.
    • Avoid late season fertilizer applications.
    • Continue scouting to identify and mark late blight-infected spots in fields.
    • Consider using a sprout suppressant to reduce volunteers the following year (check label for timing of application).
    • Kill vines at least 2 to 3 weeks prior to the anticipated harvest date making sure vines are completely dead.

    Harvest and Storage

    • Remove as many decayed tubers as possible coming into storage during harvest.
    • Avoid harvesting during wet conditions, before skins are mature, and minimize skinning, cuts, and shatter bruises.
    • Remove vines, loose soil, and anything else that may interfere with air distribution in the pile.
    • If foliar late blight was present in a field, dry tubers as quickly as possible in storage.
    • Immediately begin observing potatoes in storage for developing hot spots.
    • Supply additional air to hot spots in storage, and remove potatoes as soon as possible.
    • Use shallow tillage practices that leave tubers on the surface or within the top two inches of soil to encourage freezing during the winter.


    Seed Piece Treatments

    • Use a seed piece treatment labeled for controlling late blight on all seed lots suspected of having late blight.

    Selecting a Fungicide

    • A list of fungicides labeled for potatoes for controlling late blight is included in the action plan.
    • The fungicide selected is not as important as having complete coverage and proper timing.
    • Using copper or tin fungicides alone is not recommended for controlling late blight. These products provide excellent control when used in combination with other fungicides.

    Consider These Factors When Selecting and Using an Application Method

    • Chemical label instructions dictate if a fungicide may be applied.
    • Field size, shape, tillage practices, and obstacles that may hinder application.
    • If using air or ground applications, be certain the equipment will be available when needed.
    • Be sure the field is completely covered and fungicides are applied at the proper time.
    • With air or ground applications, an irrigation may be needed to redistribute the chemical uniformly through the crop canopy.

    Application Methods

    • Ground Application

      • Apply at least 20 gallons of water per acre, 50 gallons may improve coverage.
      • Adjust sprayer pressure towards the upper operating range recommended for the nozzle type.
      • Use hollow cone and extended range flat fan nozzles.
      • Recalibrate the sprayer often, and replace nozzles that are under or over applying by more than 10 percent.
      • Raise the boom height as the crop grows to maintain the proper overlap in spray pattern.

    • Air Application

      • Use 5 gallons of water per acre, more water does not improve coverage and distribution.
      • Calibrate nozzle output often and replace those that are under or over applying by more than 10 percent.
      • Skips can be avoided by marking spray passes with permanent flags and alternating spray passes on the flags and between flags on subsequent applications.
      • Use a ground applicator to spray areas missed or inaccessible by air application.

    • Sprinkler Application

      • Use appropriate chemigation equipment making sure the injection pump operates the entire set.
      • Use the highest labeled rate of fungicide to ensure an effective concentration on the leaves.
      • For all irrigation system, make sure there are no potatoes outside the water coverage area.
      • For solid-set or set-and-move systems, inject the fungicide at the end of the irrigation set, or make a separate application between irrigations. Make sure the fungicide has flushed out of the end nozzles before shutting off the system.
      • For center pivot systems, adjust the revolution time to the fastest setting to reduce fungicide wash off.

    When to Apply Fungicide

    • Initial applications

      • All fields should be sprayed with a protectant fungicide before row closure (plants touching between rows) followed by a second application in 7 to 10 days.

    • Applications after initial applications and up to late season

      • Rigorously scout for late blight and monitor late blight hot line or use University of Idaho wed site.
      • If late blight is found in the area or weather conditions are conducive for disease development, continue spraying protectant fungicides.
      • Spraying schedules will be issued regularly throughout the growing season as necessary.

    • Late season

      • Continue fungicide applications at intervals based on weather conditions and recommendations.
      • Protectant fungicides may need to be applied even after vine desiccation until all green vines are completely dead if late blight was present in the region or field.

    • Recommendations to reduce late blight tuber rot

      • Provide season-long control of late blight fungus on the vines (leaves and stems).
      • Follow recommendations above to reduce chances of tuber infection during harvest.



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