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Tomato Plant Leaves Have White Spots

by Pat

Q. Our tomato plants began having white spots on the leaves which eventually overtook the entire leaf that became all white, withered and died. We live in the humid islands of Hawaii. Sometimes we have had a good amount of rain each day. We water on the days we don't get rain. I haven't noticed any bugs around the plants and none fly around when I touch the leaf. Can you tell me what is wrong and what I should do?

A. Sounds like powdery mildew. White, powdery spots appear on leaf surfaces, spreading to cover leaves and even stems. They eventually die and fall off, and can leave fruit exposed to sunscald.

Treat affected plants with horticultural oils or neem oil.

Do not apply oils during a drought, when temperatures are above 90º F, or within two weeks of treating plants with a sulfur product. Thoroughly cover plant with treatments. Repeat every 7-10 days or after rain. Once mildew is present and progresses, it becomes more resistant to treatment, so begin spraying as soon as you see traces of mildew.

Next year, treat preventatively with sulfur dusts and sprays, such as Safer Garden Fungicide. But be sure to apply the sulfur product on a still day, when temperatures are well below 90º F. Apply in morning or evening, because sulfur can burn tomato plants in the direct sunlight. If you’ve applied oils to tomato plants, wait at least two weeks before using a sulfur product.

You can also treat tomato plants preventatively with a biofungicide like Serenade. Made up of specially-formulated microorganisms that destroy fungi, Serenade can help prevent mildew from infecting tomato plants. Follow label instructions.

Learn more about powdery mildew on tomatoes.

Good luck and happy gardening!
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