The Q&A Archives: Blue Leaf on Tomatoes

Question: For the past few years, blue leaf has been killing my tomatoes and everything the nursery has told me to use, can't be used until the plants are

Answer: I'm not familiar with the term blue leaf. Blue mold - downey mildew - affects tobacco plants but not tomato plants; but blue mold can affect harvested tomatoes. So, I'm not sure what tomato disease you are dealing with. Late blight does affect tomato plants. It is caused by the fungus Phytophthora infestans and shows up late in the season. Late blight first appears on the leaves and stems, and later on the fruit. Lesions initially appear on the leaves as water soaked spots becoming light green to brown as the fungus kills the leaf tissue. When weather conditions are wet, a white, cottony growth forms on the lesion. Later, the lesion turns brown and dies. Lesions appear on the stems and petioles as they do on the leaves. On the tomato fruit, lesions are olive colored and greasy in appearance eventually covering the entire fruit.

The late blight fungus overwinters in plant debris and home gardens. During cool, moist weather conditions in the spring, the pathogen begins to grow and produce spores to infect new tomato plants. Spores of the late blight pathogen are produced at relative humilities of >90% and optimum temperatures of 60? to 70?F. In general, cool nights and warm days with moist weather are the best conditions for development of the disease.

Although the plants are affected early in the season, the symptoms don't always show up until later in the growing season.

If the growing season is wet, late blight spores will be present in many areas, and fungicides will be necessary to protect your plants from infection. The only fungicides that are effective against late blight are protectant materials, which means that they must be on the foliage before spores land on leaves that are wet from rain, irrigation, fog, or dew. Therefore, continuous fungicide coverage is necessary to protect plants from infection. Tomatoes are susceptible to late blight at any time during the growing season. Choose a fungicide that has maneb, mancozeb, chlorothalonil, or fixed copper as an active ingredient AND has tomato late blight on the label. And remember, all pesticides should be used in accordance with instructions on the label.

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