Gardeners and small-scale potato growers can help prevent late blight

Good cultural practices used by everyone growing potatoes are the first line of defense against Phytophthora infestans.

The recent dramatic increase in the number of local farmer’s markets, very small commercial producers and the increasing interest in home vegetable gardening are exciting and positive developments. However, these factors can result in an increased risk of crop diseases finding new refuges. Potato and tomato producers of all sizes should be careful to control late blight. The disease can spread between tomatoes and potatoes and poses a very serious threat to the potato industry across Michigan.

Large commercial producers have adopted aggressive fungicide programs to prevent and control late blight outbreaks. Use of certified seed potatoes and timely cover sprays of carefully selected fungicides provide excellent crop protection. But uninformed or careless gardeners or farm market growers nearby who do not control late blight can create sources of disease inoculum for the next year. These sources have the potential to infect fields within a radius of several miles if conditions are right.

Potato and tomato growers should follow these tips:

  • Use only certified seed potatoes. Do not plant seed potatoes from last year’s garden.
  • Destroy “volunteer” potato plants (those that grow from last year’s unharvested tubers).
  • Leave diseased tubers and waste tomatoes on the surface of the ground to freeze over winter.
  • Fungicides are available for small growers and gardeners that can prevent the disease from spreading between plants and crops. Daconil or Manzate are appropriate products.
  • Time watering plants to allow a drying period during the day.
  • Fungicides should be applied every seven days or as directed on the label. Apply in water at a rate of 50 gal/A (or about 1.2 gallons/1000 square feet).
  • Copper fungicides are less effective than those mentioned above, but will help on plants that are not already infected (can mix Coppers with Daconil or Echo).
  • If symptoms develop, pull up the diseased plant from the roots, place in a plastic bag, seal the bag and dispose of the bag without reopening it. The spores of this disease can be carried in the air.
  • Check plants daily and dispose of diseased plants immediately.
  • In early spring, do not spread waste potatoes outside, they can survive this period as can the disease.
  • After harvest, check tubers regularly as infected tubers can quickly decompose. Discard infected tubers in a sealed plastic bag.

For more details on late blight, see Extension Bulletin E-22945,Michigan Potato Diseases: Late Blight.

For detailed information on a wide variety of potato diseases, visit the Michigan Potato Diseases website.

To learn about growing potatoes in the home garden, read Grow Potatoes in Your Home Garden! by Michigan State University (MSU) and the MSU Potato Breeding and Genetics Program.

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