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| Figure 1. Early blight of Tomato | Figure 2. Septoria leaf spot. |
DiseaseThere are several diseases of tomato which result in leaf spots and fruit rots. Typically, these diseases do not kill the plants, but result in significant loss in fruit quality and numbers. Two of the most common fungal leaf diseases of tomato are early blight and Septoria leaf spot. Both diseases may occur anytime during the growing season, but they generally become more severe after blossom-set. Both diseases result in the formation of leaf spots. These spots typically develop first on the older leaves nearest the ground. Under favorable conditions for disease development, these diseases can cause extensive defoliation, resulting in sunscalding of fruit and reduction in the numbers of fruit produced. Early blight and Septoria leaf spot are fairly easy to distinguish from one another in the field. Early blight results in the formation of irregular, brown leaf lesions or spots that range in size up to 2 inch in diameter. The most important diagnostic feature of early blight is the formation of dark, concentric rings within the lesion, giving the spots a target-like appearance. Often, several lesions coalesce, causing the leaf to turn yellow, dry up, and fall off the plant. Defoliation weakens the plant and exposes the fruit to sunscald injury. Although early blight primarily is a foliage disease, lesions may develop on both stems and fruit. Fruit lesions are tan to brown, leathery and typically originate at the stem end of the fruit. Symptoms of Septoria leaf spot first appear as small, water-soaked spots on the lower leaves. The leaf spots generally are smaller and more numerous than those resulting from early blight. Eventually the center portion of the Septoria lesion turns light tan or gray, while the margin remains dark. Small black fruiting structures (pycnidia) of the fungus, readily visible with a 10X hand lens, are formed in the center portion of the lesion. |
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Figure 3. Defoliation caused |
CausesBoth the early blight (Alternaria solani) and the Septoria leaf spot (Septoria lycopersici) fungi overwinter in plant debris, on seed, or on weeds such as nightshade and horse nettle. Spores of these fungi may be splashed or blown to tomato leaves. Disease development is favored by relatively warm temperatures, abundant rainfall, and a high relative humidity. ControlSanitation measures in the fall will reduce the amount of inoculum available for infection the following year. In the fall, till plants into the soil or remove them from the garden. This will help prevent the fungal inoculum from surviving through the winter. Avoid planting tomatoes in the same area year after year. Be sure to purchase clean seed or healthy transplants. When planting, stake or cage plants. This allows for better air movement. Place straw or plastic mulch around the base of each plant to reduce water splashing. Avoid overhead irrigation of plants. Instead, use, irrigation tapes, drip lines or soaker hoses. Currently, none of the tomato varieties have resistance to both early blight and Septoria leaf spot. Therefore, it may be necessary to control these diseases with routine fungicide applications For best results, begin applications between the time of flowering and first fruit set. Repeat applications at 7- to 21-day intervals. Several fungicides, including azoxystrobin, copper-based products, mancozeb, and chlorothalonil are recommended for the control of both early blight and Septoria leaf spot. |
| Fungicide | Typical Application Interval | Examples of Trade Names |
|---|---|---|
| azoxystrobin | 7 to 14 days, | Quadris |
| chlorothalonil | 7 to 14 days, | Daconil, Terranil, Echo, others
Homeowner: Daconil, Fungi-Gard, Liquid Fungicide, Encore, Monterey Bravo |
| copper products | 7 to 14 days, | Bordeaux, Kocide , Tenn-Cop
Others |
| mancozeb and maneb | 7 to 14 days | Dithane, Penncozeb, Manex Homeowner: Mancozeb, maneb |
| ziram | 7 to 14 days | Ziram |
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Web updated 9/01/06 |
