What Soil Condition Causes Potato Scab?

Symptoms

• Typically, potato scab appears as round raised lesions or cork-like marks on potatoes that are brownish in color. These lesions vary in size and may be pitted where they develop deep into the potato. Deeper lesions are often darker in color, and the tissue beneath the scabs is yellow-brown and almost transparent. Tubers are vulnerable to scab infections as soon as they form, and lesions can be seen on them within a few weeks of infection. However, potato scab isn't generally noticed until late in the season or at harvest. Once the tubers mature, they're no longer vulnerable to new lesions, but the existing ones will grow as the tubers grow; and as the growing season continues, the disease becomes more severe.

Dry Soil

• Potato scab develops primarily in warm, dry soils. Coarse soils that dry out rapidly are more prone to causing potato scab than finer soils with lots of moisture. This can be prevented by making sure the tubers have adequate moisture early in the growing season. During the first two weeks after the plants emerge from the soil, when tuber development is still in its early stages, keep the soil consistently moist but don't over-water. Soil that is too wet can cause root rot or poor growth.

Alkaline Soil

• A group of bacteria called actinomycetes, which are commonly found in soils, are the cause of potato scabs. Soils that have a pH above 5.5 may harbor Streptomyces scabies, caused by the actinomycetes bacteria. Keeping soil pH lower than 5.5 can prevent this bacteria from developing. Sulfur can be applied to lower your soil's pH and make it more acidic. Using fertilizers with ammonium sulfate and diammonium phosphate, which aid in making a more acidic soil, will reduce potato scab more effectively than ammonium nitrate fertilizers.

High Calcium

• Soils with high calcium to phosphorus ratios tend to be more vulnerable to the bacteria that cause potato scab. Potato scab thrives in calcium-rich soils. Phosphorous neutralizes calcium ions, which can help prevent potato scab from developing. Calcium- and potassium nitrate-rich fertilizers can raise the soil's pH as well. Adding fertilizers high in calcium, such as fresh manure, should be avoided.

Contaminated Soil

• Potato scab can contaminate soil for years. If you've had crops with potato scab in previous years, rotate potatoes with crops, such as rye, alfalfa or soybeans, for three years, after the problem potato crop, to rid the soil of the scab infection. Avoid planting fleshy crops, including radishes, turnips, beets and carrots, during these rotations. Often scab problems can accumulate when scab-vulnerable crops are planted frequently.