Small Blisterlike Spots on Potatoes

Raised Bumps

• Powdery scab initially appears as small, raised bumps just beneath the skin of potato tubers. As the disease progresses, these bumps break open to release a dark mass of spores that looks powdery, giving the condition its name. Skin spot is caused by another fungus and does not appear on potatoes until they have been harvested and stored for about eight weeks. The spots are blue and raised above the surface of the skin.

Black Spots

• Black scurf affects only the surface of potatoes. The fungus creates small black spots that appear just on the skin of tubers. The black dot fungus looks similar to black scurf, but it affects roots and stems as well as the tubers themselves. Fungal diseases live in soil and often attack plants when conditions are moist. Wet foliage and standing water on soil attracts fungal growth. Water plants in the morning, giving them time to dry during the day, and amend soil to improve drainage, which will prevent standing water and wet conditions.

Other Fungal Blemishes

• Silver scurf is similar to black scurf and is very common in potatoes. Silver scurf spots are grayish with somewhat darker edges and grow larger over time. Common scab also creates large, asymmetrical spots on potatoes. Both silver scurf and common scab are very unattractive, making potatoes look sickly and unappetizing.

Blight

• Late blight is perhaps the most damaging fungal disease that strikes potatoes. Despite the name, late blight may appear early in the season or in the middle of the season. Late blight spreads quickly, attacking potato foliage, and will potentially kill the leaves of the entire crop. When late blight is present, potatoes may be affected both in the ground and after they've been harvested. Late blight is potentially fatal to potatoes. In early stages, it appears as greenish spots that look wet or watery, like blisters. The spots quickly become brown and black before they spread to stems and other parts of the plants. The tubers will develop brown or purplish spots, which are usually curved inward. If late blight is not treated, the tubers could die and they will certainly be unfit to eat. Use fungicide to prevent late blight, and promptly remove all potatoes that have been affected by the disease.