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Problem: Early Blight and Septoria Leaf Spot - Alternaria solani , Septoria lycopersici
Plant Host: Tomato
Description: Two of the most common 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 ½ inch 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. Septoria leafspot is generally more common in Kansas gardens than early blight.
Both fungi overwinter in plant debris, on seed, or on weeds such as nightshade and horsenettle. Spores of these fungi may be splashed or blown to tomato leaves. Disease development is favored by relatively warm temperatures, abundant rainfall and high relative humidity.
Recommendations: Sanitation measures in the fall will
reduce the amount of inoculum available for infection the following year. In the fall,
tomato plots should be deep-plowed to bury tomato debris, or dead plants should be removed
from the garden and destroyed. Avoid planting tomatoes in the same area of the garden year
after year. Clean seed and healthy transplants in the spring also will help control the
disease. Both diseases can be controlled effectively with fungicides.
Chlorothalonil is effective and has a 0 day waiting period from application to
harvest. Products that contain chlorothalonil include but are not limited
to Fertilome Broad Spectrum Fungicide, Ortho Garden Disease Control, GardenTech
Garden Disease Control, and PBI Gordon Multipurpose Fungicide. For best results,
applications should begin as soon as the symptoms first become apparent, generally, around
the time of the first blossom set. Applications should be made every 7 to 10 days.
Severely infested plants may not be able to be rescued with fungicides.
References:
1. Leaf and Fruit Diseases of Tomato. K-State Research and Extension, Publication L-721.
2. Early Blight and Septoria Leaf Spot of Tomato. K-State and Extension Electronic Publication
Last Update: 01/23/2004
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