Sherman V. Thomson/Extension Plant Pathologist
Scott C. Ockey/Plant Disease Diagnostician
|Irregularly shaped dark lesions (indicated by the arrows) produced by the early blight fungus on a potato tuber.|
|Foliar early blight symptoms on a potato leaf. Upper lesion illustrates the angular appearance of some lesions due to being bound by vein.|
|Characteristic early blight lesion on a potato stem. Stem lesions are present under extreme disease conditions.|
|Early blight lesions on a tomato leaflet. The lesions are angular as they are being bound by the leaflet veins.|
|Foliar early blight lesion on a tomato leaflet. Notice the concentric ring pattern characteristic of advanced lesions.|
|Portion of a tomato field severely affected by early blight.|
Early blight is a fungal disease of potatoes and tomatoes. The name, early blight, is somewhat misleading since the disease usually occurs on mature vines. Early blight may also affect tubers and fruit. The disease varies in severity every year and is most severe in years when late summer precipitation is frequent.
Leaf symptoms begin as pinpoint brown or black spots, usually on the older (lower) leaves. These lesions expand in size and may reach one-half inch across. The spots are frequently surrounded by a yellow halo. Within the enlarged lesions, concentric rings are usually seen. The lesions are irregularly shaped and may be somewhat angular because they are often limited by large leaf veins.
Lesions found on stems are sunken and silvery, ranging from 1/4 to 1/2 inch in size, and may coalesce to produce large infected areas. Stem lesions are an indication of a serious early blight outbreak in a field. Tomato fruit symptoms, not common in Utah, include greenish brown to black sunken lesions with concentric rings like those on the leaves. Fruit lesions are usually found on the stem-end of the fruit.
On the potato tuber, lesions are irregularly shaped and at first dark colored, becoming sunken later. The tissue is hard and dry, and somewhat leathery in texture. Ordinarily early blight lesions remain hard and dry, but wet rot will occur if the lesion is invaded by other fungi or bacteria. Tuber infections are infrequent in Utah.
Early blight is caused by the fungus Alternaria solani. The fungus overwinters on debris from previously diseased plants. Airborne spores that infect new tomato and potato plants are produced on such debris and on volunteer tomato and potato plants. The disease is more serious under wet or humid conditions. The first lesions occur in the lower part of the plant where humidity is highest.
Early blight control is best achieved by using several practices together. Some cultural control techniques are used to reduce the amount of initial inoculum (spores) that are present in the local environment; other practices keep plants healthy and vigorous.
- Use crop rotations. Avoid planting in the same area for 2 years.
- Use disease-free transplants or seed pieces.
- Destroy all volunteer tomatoes and potatoes.
- Plow down or destroy all debris immediately after harvest.
- Maintain plant vigor with proper nitrogen when plants flower to prevent nutrient depletion.
- Avoid sprinkler irrigation at times when the leaves would remain wet for long periods of time.
- If lesions are noted on the bottom third of the plants during the first part of August, a fungicide spray program should be initiated. Disease occurring later in the season will not cause enough damage to warrant a spray program.
The following fungicides are effective on the early blight fungus: Captan, Maneb, Mancozeb, Chlorothalonil, and Bordeaux.
***Always follow pesticide label instructions.***
Precautionary Statement: All pesticides have benefits and risks, however following the label will maximize the benefits and reduce risks. Pay attention to the directions for use and follow precautionary statements. Pesticide labels are considered legal documents containing instructions and limitations. Inconsistent use of the product or disregarding the label is a violation of both federal and state laws. The pesticide applicator is legally responsible for proper use. This publication is issued in furtherance of Cooperative Extension work. Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Vice President for Extension and Agriculture, Utah State University.