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Undestanding Late Blight Disease of Tomatoes and Potatoes


Late Blight

Late Blight Spots on Tomato Leaves

Late blight is a disease that affects mainly tomatoes and potatoes. Once it takes hold, the spores spread rapidly and great distances. Cool, wet weather encourages the development of the fungus.

Symptoms include greasy looking, irregularly shaped gray spots on the leaves. A ring of white mold can develop around the spots, especially in wet weather. The spots eventually turn dry and papery. Blackened areas may appear on the stems. Tomato fruits also develop large, irregularly shaped, greasy gray spots.

The Late Blight fungus can overwinter in frost free areas. Since it spreads to potatoes, it also overwinters in potato debris and seed, even in colder areas. Remove all debris and don't save seed potatoes.

There are three major problems with controlling late blight.

  1. Once it’s detected, it’s usually too late to save the plant. Late blight develops quickly.

  2. The spores are air born and can travel hundreds of miles. An infected plant in a neighboring town can mean the ruin of crops for that whole region.

  3. Late blight can over winter in plant debris and seed. You should dispose off all plant debris by the end of the season. Do not compost it.

    Late blight can even over winter in potatoes, so if you saved some to plant for next year or even if you simply missed one in the ground and it sprouts next year, you could be in for another season of trouble.

If you suspect you have Late Blight, contact your Cooperative Extension Service for definite ID. And if late blight is detected in your area, here are some ways to prevent late blight in your garden.

Read about more tomato problems:
Early Blight
Spetoria Leaf Spot
Fusarium Wilt

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