Late Blight Information

Late Blight Information

By Katie Torpe, Garden Guides Contributor
blight on a leaf

About Late Blight

Late blight is a fungus-like pathogen. It reproduces asexually through a spore-like device. In rainy or windy weather, the disease can spread very quickly from one plant to another as the spores are blown onto other plants. For this reason, symptoms are most visible after a rain. Black lesions appear on the leaves. As they dry, the lesions are often surrounded by grayish fuzz. The fruit of a tomato will have a hard, greasy black lesion on it. Potatoes will have a purple lesion and will be dry and rotten if you cut into it.

Prevention and Control

To help prevent blight, buy "certified" seed potatoes. These seeds have passed tests to show they were disease-free. However, the plants can still get late blight if spores hit it. One idea is to build a mount about three inches tall around a potato tuber. It puts just that much more soil between the tuber and any spores that may land on the soil above it. Make sure you spray a fungicide onto the plants at least biweekly. You should also carefully examine your plants for signs of lesions or rot regularly and remove them. There are tomato varieties that are especially resistant to blight,but none are immune.


Late blight infects every part of tomato and potato plants and can devastate entire crops if gone unchecked. In fact, late blight was responsible for the Irish potato famine in the 1800's.

Other Methods of Control

* Mix fungicide into the soil when planting.
* Plant your crop in a spot with a lot of sun, where they are more likely to dry out after a rain.
* Monitor your plants, especially after a rain, and remove any infected plants immediately.

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