Early Blight of Potato and Tomato

Randall C. Rowe, Sally A. Miller, Richard M. Riedel, Ohio State University Extension Service~

Early blight is a very common disease of both potato and tomato. It causes leaf spots and tuber blight on potato, and leaf spots, fruit rot and stem lesions on tomato. The disease can occur over a wide range of climatic conditions and can be very destructive if left uncontrolled, often resulting in complete defoliation of plants. In contrast to the name, it rarely develops early, but usually appears on mature foliage.

Potatolateblight1 2 Alternariafrucht1 Alternariablatt2 Tomaten



On leaves of both crops, the first symptoms usually appear on older leaves and consist of small, irregular, dark brown to black, dead spots ranging in size from a pinpoint to 1/2 inch in diameter. As the spots enlarge, concentric rings may form as a result of irregular growth patterns by the organism in the leaf tissue. This gives the lesion a characteristic "target-spot" or "bull's eye" appearance. There is often a narrow, yellow halo around each spot and lesions are usually bordered by veins. When spots are numerous, they may grow together, causing infected leaves to turn yellow and die. Usually the oldest leaves become infected first and they dry up and drop from the plant as the disease progresses up the main stem.
On tomato, stem infections can occur at any age resulting in small, dark, slightly sunken areas that enlarge to form circular or elongated spots with lighter-coloured centers. Concentric markings, similar to those on leaves, often develop on stem lesions. If infested seed are used to start tomato transplants, seedlings may damp off soon after emergence. When large lesions develop at the ground line on stems of transplants or seedlings, the plants may become girdled, a condition known as "collar rot." Such plants may die when set in the field or, if stems are weakened, may break over early in the season. Some plants may survive with reduced root systems if portions of stems above the canker develop roots where they contact the soil. Such plants, however, usually produce few or no fruits. Stem lesions are much less common and destructive on potato.

Blossom drop and spotting of fruit stems, along with loss of young fruit, may occur when early blight attacks tomatoes in the flowering stage. On older fruits, early blight causes dark, leathery sunken spots, usually at the point of stem attachment. These spots may enlarge to involve the entire upper portion of the fruit, often showing concentric markings like those on leaves. Affected areas may be covered with velvety black masses of spores. Fruits can also be infected in the green or ripe stage through growth cracks and other wounds. Infected fruits often drop before they reach maturity.

On potato tubers, early blight results in surface lesions that appear a little darker than adjacent healthy skin. Lesions are usually slightly sunken, circular or irregular, and vary in size up to 3/4 inch in diameter. There is usually a well defined and sometimes slightly raised margin between healthy and diseased tissue. Internally, the tissue shows a brown to black corky, dry rot, usually not more than 1/4 to 3/8 inch deep. Deep cracks may form in older lesions. Tuber infection is uncommon under Ohio conditions.


Early blight is caused by the fungus, Alternaria solani, which survives in infected leaf or stem tissues on or in the soil. This fungus is universally present in fields where these crops have been grown. It can also be carried on tomato seed and in potato tubers. Spores form on infested plant debris at the soil surface or on active lesions over a fairly wide temperature range, especially under alternating wet and dry conditions. They are easily carried by air currents, windblown soil, splashing rain, and irrigation water. Infection of susceptible leaf or stem tissues occurs in warm, humid weather with heavy dews or rain. Early blight can develop quite rapidly in mid to late season and is more severe when plants are stressed by poor nutrition, drought, or other pests. Infection of potato tubers occurs through natural openings on the skin or through injuries. Tubers may come in contact with spores during harvest and lesions may continue to develop in storage.



TOMCAST (TOMato disease foreCASTing) is a computer model based on field data that attempts to predict fungal disease development, namely Early Blight, Septoria Leaf Spot and Anthracnose on tomatoes. Field placed data loggers are recording hourly leaf wetness and temperature data. This data where analysed over a 24 hour period and may result in the formation of a Disease Severity Value(DSV); essentially an increment of disease development. As DSV accumulate, disease pressure continues to build on the crop. When the number of accumulated DSV exceed the spray interval, a fungicide application is recommended to relieve the disease pressure.


Timing fungicide applications for early blight, Septoria leaf spot, and Anthracnose

A system of weather-based disease forecasting called TOMCAST, developed by Dr. Ron Pitblado at the Ridgetown College of Agricultural Technology in Ontario, Canada, can be used to time fungicide applications for three fungal diseases; early blight (caused by Alternaria solani), Septoria leaf spot (caused by Septoria lycopersici), and fruit anthracnose (caused by Colletotrichum coccodes). If late blight is present in your county or adjacent counties, or conditions are present for movement of spores into your area, use the Simcast late blight forecast system to time fungicide applications.


TOMCAST is not useful on farms that have a history of bacterial diseases. If you commonly have problems with bacterial spot, speck, or canker in your tomatoes, you should not use TOMCAST because the recommended spray intervals will not be sufficient for control of bacterial diseases if you're tank-mixing copper with your fungicide applications. Find information about reducing bacterial diseases here: http://extension.psu.edu/plants/vegetable-fruit/news/2015/farming-like-you-expect-bacterial-diseases

TOMCAST uses leaf wetness and temperature data to calculate disease severity values (DSV's) as shown in Table 1.

TOMCAST is derived from the original F.A.S.T. (Forecasting Alternaria solani on Tomatoes) model developed by Drs. Madden, Pennypacker, and MacNab ? at Pennsylvania State University (PSU). The PSU F.A.S.T. model was further modified by Dr. Pitblado at the Ridgetown College in Ontario into what we now recognize as the TOMCAST model used by Ohio State University Extension.

DSV are A Disease Severity Value (DSV) is the unit of measure given to a specific increment of disease (early blight) development. In other words, a DSV is a numerical representation of how fast or slow disease (early blight) is accumulating in a tomato field. The DSV is determined by two factors; leaf wetness and temperature during the "leaf wet" hours. As the number of leaf wet hours and temperature increases, DSV accumulate at a faster rate. See the Disease Severity Value Chart below.
Conversely, when there are fewer leaf wet hours and the temperature is lower, DSV accumulate slowly if at all. When the total number of accumulated DSV exceeds a preset limit, called the spray interval or threshold, a fungicide spray is recommended to protect the foliage and fruit from disease development.


The spray interval (which determines when you should spray) can range between 15-20 DSV. The exact DSV a grower should use is usually supplied by the processor and depends on the fruit quality and end use of the tomatoes. Following a 15 DSV spray interval is a conservative use of the TOMCAST system, meaning you will spray more often than a grower who uses a 19 DSV spray interval with the TOMCAST system. The tradeoff is in the number of sprays applied during the season and the potential for difference in fruit quality. USING TOMCAST: Tomatoes grown within 10 miles of a reporting station should benefit from the disease management function of TOMCAST to help forecast early blight, Septoria, and Anthracnose. If you decide to try TOMCAST this season please keep in mind three very important concepts.

One: If this is your first time using the system, it is recommended that only part of your acreage be put into the program to see how it fits with your quality standards and operational style.

Two: Use TOMCAST as a guide to help better time fungicide applications, realizing in some seasons you may actually apply more product than a set schedule program might require.

Three: The further a tomato field is from a reporting site increases the likelihood of distortion in the DSV accumulation, i.e., the reported value may be a few DSV higher or lower than that experienced by the field location. This should be taken into consideration when application of fungicides is likely a few days away. Listen to the DSV reports of nearby stations and triangulate to your own location as the best way to roughly estimate your DSV accumulation.

FIRST SPRAY USING TOMCAST: There has been some discussion over the years regarding the application of the first spray when following TOMCAST. The rule stated in the 1997 Vegetable Production Guide centers around the planting date.

Tomato plants that enter the field before May 20 should have the first spray applied when DSV for that area exceed 25 or when a fail safe date of June 15 arrives. The fail safe is used only if you have not treated since May 20, and is a means to eliminate initial disease inoculum. After the first spray, these tomatoes are subsequently treated when the chosen spray interval (range 15-20 DSV) is exceeded.

Tomatoes planted after May 20 are treated when they exceed the chosen spray interval (range 15-20 DSV) or when they have not been treated by the fail safe date of June 15. Therefore, it is critical to compare the tomato planting date to the date DSV reporting began in that area to guide the spray decision process.