Septoria leaf spot and early blight are fungal leaf diseases that will soon be present in vegetable gardens around the state. Because these diseases defoliate the plants, the tomato fruit is exposed to sunscald, which contributes to the development of anthracnose, another fungal disease, on the fruit. All three fungi can be controlled with the same practices.
Septoria leaf spot is a common disease in Illinois home gardens. 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 (spore structures). The individual lesions are about 1/8 inch in diameter but 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.
Early blight appears on tomatoes as they start to set fruit; consequently, we have not seen much of this disease yet. High humidity levels and persistent dews are favorable for development of early blight. Cool temperatures may also favor disease development. This fungal foliar disease is caused by Alternaria and 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 the spotting occurs. Generally, sprays are started at first bloom; however, some of the newer tomato varieties may be more susceptible to this fungal blight, so sprays might be needed earlier on those varieties. Therefore, it is important to scout for this disease on a regular basis, especially in wet weather. Dry weather is not favorable for development of early blight.
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 anthracnose symptom, this disease may appear on other plant parts as well.
Commercial growers often need to rely on chemical control of these three diseases. Chemical control may be obtained with Bravo, mancozeb, or Quadris on a seven- to ten-day schedule after the first sign of disease or after the first fruits form. A soil-surface spray of mancozeb after the last cultivation improves anthracnose control. In areas of high rainfall, growers will need to stay with the shortened intervals. Home growers should concentrate on keeping all ripe fruit picked off 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. It is also suggested that a two- or three-year crop rotation will reduce losses from these diseases. Chemical options for home growers are listed in the Illinois Homeowner's Guide to Pest Management. For more information, consult Report on Plant Diseases No. 908.