If you have questions about diseases or pests, your local cooperative extension service agent can help you. Here are brief descriptions of a few of the common potato diseases. The fungus that causes common scab lives in the soil for many years. It's not active, though, when the soil pH is below 5.4, so if you have a serious scab problem, take a soil pH test. You may want to lower the pH by not liming or adding wood ashes to the potato section of the garden.
Early blight injures foliage and reduces overall yields.
Affected leaves develop small, dark brown spots that often grow in size, and which eventually kill the leaves. Gardens in central, southern and eastern states are most susceptible. Planting certified seed and mulching with hay can prevent this disease. You'll notice the disease first by water-soaked areas on the leaves that turn brown and black as the leaf dies. The disease strikes often during cool, wet weather and may spread rapidly if the weather warms up.