Septoria leaf spot, early blight, and anthracnose are common tomato diseases in Illinois. You probably recognize these fungal names because related species occur on many other landscape plants. Septoria leaf spot and early blight diseases defoliate the plants, exposing the tomato fruit to sunscald, which contributes to the development of anthracnose on the fruit. Anthracnose is another fungal disease, and all three fungi can be controlled using the same practices.
Septoria leaf spot is a more common disease for Illinois home gardeners. The disease appears initially as small, water-soaked spots on the lower leaves. These spots soon become circular to angular, with dark margins and grayish white centers that often bear one or more tiny black specks called pycnidia. The individual lesions are about 1/8 inch in diameter, but they are easy to spot because the leaf quickly turns yellow and drops from the plant. Defoliation starts at the base of the plant and can be severe during prolonged periods of warm, wet weather. When a grower complains of tomato plants dying from the bottom upward, we usually look first for Septoria leaf spot.
Early blight appears on tomatoes as they start to set fruit. High humidity levels and persistent dews are favorable for early blight, and cool temperatures may favor disease development. This fungal foliar disease is caused by Alternaria. It is characterized by small brown leaf spots with a targetlike series of concentric rings within each lesion. As with Septoria leaf spot, lower leaves show symptoms first.
Early blight can cause economic loss, but sprays are not usually initiated until spotting occurs. Generally, sprays are started at first bloom; however, some of the newer tomato varieties may be more susceptible to this fungal blight, and sprays may be needed earlier on those varieties. Scout for this disease regularly, especially in wet weather. Dry weather is not favorable for development of early blight, but we still see it in drought periods when growers use frequent overhead irrigation.
Anthracnose ripe rot causes lesions about 1/2 inch in diameter on the ripened fruit of the tomato. Concentric rings may appear within these lesions. Although fruit lesions are the most common symptom, this disease may appear on other plant parts as well.
Commercial growers must often rely on chemical control of these diseases. Chemical control can be obtained with Bravo, mancozeb, or Quadris, applied at seven- to ten-day intervals, after the first sign of disease, or after the first fruit forms. In areas with high rainfall, growers should stay with the shortened intervals. Home growers should concentrate on keeping all ripe fruit picked off the plants, improving air circulation in the garden, mulching to avoid fruit rots, and removing tomato vines and unharvestable fruit at the end of the season. A two- or three-year crop rotation is also suggested to reduce losses from these diseases. A soil-surface spray of mancozeb after the last cultivation improves anthracnose control in commercial plantings. Chemical options for home growers are listed in the Illinois Homeowner’s Guide to Pest Management. Varieties such as Floramerica, Jetstar, Manlucie, Roma VF, and Supersonic have some tolerance to early blight. For more information, consult RPD No. 908.