Potato blight is a disease caused by the pathogen Phytophthora infestans. Blight is typically divided into two categories: early blight and late blight. Early blight can happen every year, and it damages foliage on mature potato and tomato plants. Late blight is a more destructive disease, as it can infect a crop at any time during its development and destroy the foliage, stems, fruits and tubers at a rapid pace. Though late blight epidemics are rare and require certain climate conditions, you should always keep an eye out for symptoms of blight.

Recognizing Blight

1. Inspect the leaves of your potato crop, paying special attention to those lowest to the ground. Look for small, light to dark green water-soaked spots near the edges of the leaves. These are the telltale symptoms of blight onset.
2. Look for large brown or black lesions on the leaves if you are performing inspection in cool, moist weather. Humidity can cause the green lesions to spread and turn dark rapidly. They typically appear with a light yellow border surrounding them.
3. Check the undersides of the leaves for white mildew growth. This is a major symptom of sporulation, or that the blight has begun to spread to other parts of the potato plant.
4. Inspect the outside of the potato tubers for brown to purple patches that are slightly depressed into the skin. Beneath these patches, a granular rot usually occurs and extends less than 1/2 inch or so into the tuber. In some cases, you may see the white mildewy growth mentioned in Step 3 on the surface.

Preventing Blight

1. Plant only disease-free seed tubers to limit the chances of blight infestation. When shopping for seed potatoes, choose only batches that have been certified by your state’s certification agency.
2. Destroy all damaged and volunteer potatoes, as these can help the disease survive through winter and continue its spread to new crops.
3. Avoid frequent watering, or watering from above just before nightfall. Frequent moisture, and especially sitting moisture on leaf surfaces, encourages the spread of the disease.
4. Harvest potatoes only when the vines have died. Harvesting while infected foliage is still alive will increase the probability of spread to the tuber.

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