Potato scab is a condition caused by bacteria called Streptomyces scabies. You won't see signs of the problem above ground. Instead, you see the evidence on the skin of the potatoes (Solanum tuberosum). The lesions vary in appearance and might include slightly raised scabs, pits, cracking, darkening or a corklike feeling to areas of the potato. The growing conditions often affect the occurrence of potato scab. By improving the soil and care of the potato plants, you reduce the risk for potato scab.
Choose a scab-resistant potato variety. Options include "Juliette," "King Edward," "Pentland Javelin" and "Golden Wonder." Check out the tubers you plan to use to look for signs of scab.
Rotate the location where you plant potatoes with four or five years between. Alternate with corn (Zea mays), grasses (Poa annua), soybeans (Glycine max) or small grains, which seem to reduce the risk of scab affecting future crops of potatoes. Don't use red clover (Trifolium pratense) as a rotation crop if scab was a problem in the garden area in the past. The clover might increase the risk of scab to future potato crops.
Work the soil well with a tiller so the soil isn't compacted. Till in 3 to 4 inches of compost to improve the soil structure. The improved soil quality may help reduce potato scab. It also helps the soil retain moisture, which is key in preventing scab.
Test the soil to check the pH level, which should be between 5.0 and 5.2 to create the acidity that reduces the risk of potato scab. Test each year to check the levels.
Apply ammonium sulfate to the potato hills during planting. Apply the fertilizer around the seed potato without the the ammonium sulfate actually touching the potato. The standard application rate for vegetable gardens is 1 pound of ammonium sulfate per 100 square feet. The ammonium sulfate increases the acidity of the soil to get closer to the 5.0 to 5.2 range. It may take several years of application to get to an ideal acidity level.
Irrigate the potato plants regularly, especially for the four to six weeks when the tubers develop. Dry soil can contribute to scab development. Avoid overwatering, which can cause the potatoes to rot.