Potato Late Blight


Found on tomato and potato plants, late blight is caused by the fungus Phytophthora infestans and is common throughout the United States. True to its name, the disease occurs later in the growing season with symptoms often not appearing until after blossom.

Late blight first appears on the lower, older leaves as water-soaked, gray-green spots. As the disease matures, these spots darken and a white fungal growth forms on the undersides. Eventually the entire plant will become infected. Crops can be severely damaged

Unlike other fungal diseases, this plant problem does not overwinter in the soil or on garden trash. Instead the spores are introduced by infected tubers, transplants or seeds. Wind will also carry the disease from nearby gardens. Warm temperatures (70-80˚F) and wet, humid conditions promote its rapid spread.

Note: Late Blight was responsible for the Irish potato famine (1845-1849). 


Treatment

  • Destroy all cull and volunteer potatoes.
  • Plant late blight-free seed tubers.
  • Do not mix seed lots because cutting can transmit late blight.
  • Use a seed piece fungicide treatment labeled for control of late blight (current list of fungicides can be found in the NDSU “Fungicide Guide,” PP622). Recommended seed treatments include Revus, Reason and mancozeb.
  • Avoid planting problem areas that may remain wet for extended periods or may be difficult to spray (the center of the pivot, along powerlines and tree lines).
  • Avoid excessive and/or nighttime irrigation.
  • Eliminate sources of inoculum such as hairy nightshade weed species and volunteer potatoes.
  • Scout fields regularly, especially in low, wet areas, along tree lines, at the center of the pivot and other areas that remain wet for longer periods where late blight first may occur.
  • Use foliar fungicides on a regular and continuing schedule. Once late blight is present, only foliar fungicide applications can manage late blight in the field. (A current list of fungicides can be found in the NDSU “Fungicide Guide,” PP622).
  • Keep up to date on late blight forecasts. In our region, the NDSU Potato Blightline operates during the growing season and provides weekly late blight updates and forecasting.
  • Quickly destroy hot spots of late blight.
  • Kill vines completely two to three weeks before harvest.
  • Applying phosphorous acid to potatoes after harvest and before piling can prevent infection and the spread of late blight in storage.
  • Monitor home garden and market tomatoes near you for late blight. Late blight can move from these local sources to potato fields.



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