Late Blight Alert for Tomatoes and Potatoes

Last week, late blight was found on greenhouse tomato transplants in the westernmost county in Maryland, which is approximately 250 miles from the eastern border of Kentucky. While unlikely, the possibility exists that with a swirling, southwesterly weather pattern, spores from infected plants there could affect tomato and potato crops in eastern KY, particularly if those spores are deposited under wet conditions.

If late blight is suspected, the local county Extension agent should be contacted as soon as possible. Like other water mold-induced diseases, such as blue mold and cucurbit downy mildew, late blight spreads rapidly within and between fields, and quickly becomes a grower community problem.

A Few Things to Know

  • The pathogen is Phytophthora infestans, which is a water mold. Many of the systemic fungicides typically used for early blight and Septoria (such as Fontelis, Cabrio, or Topsin) will not be effective against the late blight pathogen.
  • This pathogen recently found in Maryland was genotyped as US-23, the most commonly found genotype in recent years. US-23 genotypes can infect both tomato and potato and have varying sensitivity to Ridomil (according to Steven Rideout at Virginia Polytechnic Institute).
  • Late blight can destroy a field in a matter of weeks during wet, cool weather. It is often too late to save the crop if fungicide sprays are delayed until symptom development.
  • Sprays with a protectant fungicide (such as chlorothalonil, mancozeb, or copper) are the minimum recommendation for both tomato and potato crops in eastern Kentucky prior to the next rain event. A number of other fungicides have efficacy for late blight (see ID-36, pages 79 and 104); for best results, these should be applied preventatively.

Symptoms of Late Blight

late blight fig 1Figure 1, clockwise from upper left:

  • Under humid or rainy conditions, late blight lesions on stems, leaves, and fruit will develop white-gray sporulation. (Photo: M. McGrath, Cornell University)
  • Sporulation is more common on the underside of plant leaves. (Photo: M. McGrath, Cornell University)
  • Fruit typically develop greasy, gray-brown lesions that expand rapidly and may cause fruit cracking. (Photo: J. Ristaino, NC State University)
  • Under dry conditions, little sporulation will be present and leaves may appear scorched (M. McGrath, Cornell University)


Additional Resources

  • Late Blight of Tomato (PPFS-VG-13)
  • Recognizing Late Blight on Tomato Seedlings (PPFS-VG-14)
  • (USDA information portal on late blight)
  • Vegetable Production Guide for Commercial Growers (ID-36)

By Emily Pfeufer, Extension Plant Pathologist

Posted in Vegetables