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Growing Good Food at Home

How to Spot, Treat, and Prevent Diseases in Potatoes and Sweet Potatoes

Diseased, rotted potatoes.

Diseased, rotted potatoes.

Potato

The best way to prevent potato diseases is to avoid creating conditions to let infections in. It really begins with getting certified seed potatoes, then monitoring your plants for signs of disease, and maintaining good soil moisture.

Any plants that have yellow or shriveled leaves before the end of the season, appear smaller than the others, or just look different should be removed and discarded. Always discard diseased plants in the trash, not the compost.

These are the common diseases in potatoes and how to address them.

Common Scab

Symptoms: Small brown lesions on the potato skin.

Control:       Crop rotation (see the Getting Ready to Grow section for details).

Common scab is a cosmetic disease; it doesn’t affect the quality of the potato. However, it does mean you’ll need to have your paring knife ready to cut those pieces away. You won’t see this until harvest, of course. One of the best ways to avoid it? Rotate your crops.

Early Blight

Symptoms: Lesions that look like targets, starting on the lower, maturing leaves of the plant.

Control:       Fungicide can help treat this; prevent plant stress and maintain proper water and fertilizer levels.

Potatoes whose leaves have been stricken by early blight are still edible.

Late Blight

Symptoms: Dark, greasy-looking lesions on the leaves. Upper leaves may have a yellow “halo” around the lesions. The underside of the leaves may have lesions ringed with white fungus.

Control:       Preventive fungicide is the only option. Once late blight afflicts a plant, remove it and discard it in the trashnever in the compost. Consider planting potatoes with some resistance to late blight, such as Kennebec.

Late blight is a death knell for potatoes. The tubers become discolored and break down. You must discard diseased plants immediately. This is a fungal infection and it can be spread by the wind to other areas of your garden—or your neighbor’s garden. The spores can travel for miles and affect commercial production miles away. It’s essential to monitor the health of your plants.

Pink Rot

Symptoms: Swollen, waterlogged tubers, partially or completely rotted. If cut or broken open, they turn salmon pink within a half hour.

Control:       Water management, especially late in the growing season.

Commonly called water rot (killed with kindness?), pink rot is the result of overwatering.

Black Scurf

Symptoms: Small, irregular black patches on the potato skin that won’t wash off.

Control:       Buy certified disease-free seed.

As creepy as it sounds, black scurf is a strictly cosmetic disease. But it’s probably worth getting disease-free seed to avoid it.

Sweet Potato

Sweet potatoes tend to be less susceptible to disease than their [not really] cousins, the potato. There are enough ailments for sweet potatoes to fill a small book, but these tubers usually tend to be fairly tough—as long as you take care of them.

 Bacterial Stem and Root Rot

Symptoms: Dark lesions on the stems and petioles (the stem that supports the leaf); wilted plants; lesions on the root or rot inside the root.

Control:       Buy certified disease-free plants/slips. Rotate crops regularly. Before planting, turn soil over and allow it to dry; repeat. Destroy diseased plants.

This soil-borne disease can lie dormant in the soil for years. Another preventive measure is to cover soil with black plastic to heat up the soil.

Bacterial Wilt

Symptoms: Wilted plants; yellow to brown lesions at the base of the plant stem; harvested roots may have brown streaks or develop a distinctive odor.

Control:       Buy certified disease-free plants/slips. Rotate crops regularly. Before planting, turn soil over and allow it to dry; repeat. Destroy diseased plants.

This soil-borne disease can lie dormant in the soil for years. Another preventive measure is to cover soil with black plastic to heat up the soil.

Leaf Spot

Symptoms: Whitish, tan, or brown lesions on the upper and lower surfaces of the leaves. The lesions usually have a dark brown or purple edge.

Control:       None known. Destroy and discard damaged plant material.

This is a fungus that can spread through the air, from one plant to another, and through splashing water. Leaf spot is very common around the world. It doesn’t affect the root, but it can damage the leaves and vines of the plant.

Fusarium Wilt

Symptoms: Dull, yellowing leaves; wilting vine.

Control:       Buy disease-resistant slips. Destroy and discard damaged plant material. Test and treat soil as needed.

This is a soil-borne fungus that specifically targets sweet potatoes. It can survive in the soil—and in the affected plant matter—for years. Always sterilize your gardening tools to avoid spreading this fungus.

Have you had to deal with diseases attacking your potato or sweet potato plants? What types of problems do you regularly face? Please tell us how you treat and prevent diseases from destroying your potato or sweet potato crop.

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