Tuesday, August 5, 2008

Early Blight of Tomatoes

One of the first disease problems I have encountered in WNC is Early Blight on tomatoes. Early blight is caused by Alternaria solani, a fungal pathogen. Traditionally, symptoms of Early Blight appear as spots on the foliage. These spots have concentric circles and look like a "bull's eye" (image, right- click on image to enlarge). These symptoms first appear on older leaves of the tomato plant.

In addition, Early Blight can actually affect the stems of the tomato plants (image, below). This condition is known as "collar rot". Notice that the stem lesion begins where the plastic begins, not at the soil line. Concentric circles may also be seen in the stem lesions. Early Blight attacking stems can be an severe problem, especially in fields that are not fumigated, as plants will not be able to survive stem girdling.

In order to control A. solani and other important pathogens of tomato, prevention is key. Dr. Kelly Ivors and Dr. Frank Louws, both of the Department of Plant Pathology at NCSU have put together a comprehensive Foliar Fungicide Spray Guide for Tomatoes in NC. This is program is a holistic approach to tomato disease control and followed by many WNC tomato growers.

Other fungicides that are very effective for Early Blight on tomato are the strobilurin fungicides, azoxystrobin (Amistar, Quadris) and pyraclostrobin (Cabrio). *Strobilurin fungicides must be rotated with a fungicide that is not in FRAC group 11.* Other fungicides with good efficacy towards Early Blight are boscalid (Endura), mancozeb (Dithane, Manzate, Penncozeb, Manex II) and maneb (Manex, Maneb).
Efficacy data obtained from Southeastern Vegetable Extension Workers 2008 Vegetable Crop Handbook. Always follow application guidelines on the fungicide label!

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