Diseases & Pests
Helping Nebraskans enhance their lives through research-based education.
Late Blight of Potato and Tomato
by Don Janssen, UNL Extension Educator
Late blight is one of the most devastating diseases of potato and tomato worldwide. It was responsible for the devastating Irish potato famine of the 1840's, and has continued to be important to the present. Since 1990, late blight has caused widespread damage across the United States and Canada. If left unmanaged, this disease can result in complete destruction of potato or tomato crops.
Late blight appears on potato or tomato leaves as pale green, water-soaked spots, often beginning at leaf tips or edges. The circular or irregular leaf lesions are often surrounded by a pale yellowish-green border that merges with healthy tissue. Lesions enlarge rapidly and turn dark brown to purplish-black. During periods of high humidity and leaf wetness, a cottony, white mold growth is usually visible on lower leaf surfaces, at the edges of lesions. In dry weather, infected leaf tissues quickly dry up and the white mold growth disappears. Infected areas on stems appear brown to black, and entire vines may be killed in a short time when moisture persists. Infected tomato fruit have a firm chocolate brown rot through the fruit.
On potato tubers, late blight appears as a shallow, coppery-brown, dry rot that spreads through the tuber. Late blight will attack any green tissue on the plant, and in wet weather can wipe out a planting in a week or two. Early blight typically starts on older lower leaves and works its way up the plant.
University of Nebraska-Lincoln Extension in Lancaster County is your on-line yard and garden educational resource. The information on this Web site is valid for residents of southeastern Nebraska. It may or may not apply in your area. If you live outside southeastern Nebraska, visit your local Extension office
Contact InformationUniversity of Nebraska-Lincoln
in Lancaster County
Web site: lancaster.unl.edu
444 Cherrycreek Road, Suite A,
Lincoln, NE 68528
1. Spray a fungicide with the active ingredient chlorothalonil (Daconil, Fungonil and others) on a weekly basis to protect tomatoes.
2. Good garden sanitation – cleaning up the tomato residue, and burning or landfill it at the end of the season.
3. Watering practices that do not wet leaves. If you must overhead irrigate, do so early in the morning so the period of leaf wetness is not lengthened.
4. Rotate crops so you are not growing tomatoes or potatoes in the same part of your garden more often than once every three years.
5. Do not allow potato volunteers to grow in gardens or compost piles - the only known way for late blight to overwinter is in potato tubers.
In a wet year, spores can blow into the area from great distances, and preventive fungicidal sprays are the main line of defense against late blight, and can also control the other leaf and fruit diseases as well. For organic gardeners, copper fungicide sprays are about the only effective option. Keep in mind, that fungicides are preventive – they don’t cure established infections, they only protect healthy tissue.