As you begin harvesting your potatoes, you might notice your potatoes have blemishes. Potato scab is a common disease in potatoes that causes symptoms including dark brown, pithy patches that may be raised and appear “warty.” These lesions might affect just a small portion of the tuber surface, or may completely cover it. Potato scab is caused by a bacteria referred to as S. scabies. This however is a common bacteria found in most soil. The conditions when growing potatoes or other root crops can determine if the bacteria will affect your potatoes each year.
Soil acidity is a huge factor in potato scab. Often soil that has been recently treated with fresh manure, calcium, and potassium nitrate will have an increase in soil pH levels. This is often the cause of potato scab. Soil pH below a 5.2 has a reduced chance of potato scab. Although most vegetables prefer a pH around 6.5. This means lowering your pH below 5.2 isn’t ideal for vegetable production. However, keeping it higher than a 7 will greatly increase your chance for potato scab. Avoid treating your soil with manure or other sources high in nitrogen that tend to increase soil pH before planting or after you plant your potatoes. Amend your soil in advance and allow the pH to adjust before planting.
Soil moisture levels during early tuber development has an effect on common scab infection. You will need to keep the soil damp for 2-6 weeks during tuber development. This technique is effective because high soil moisture promotes growth of a bacteria that can crowd out S. scabies on the potato surface. It is important to not overwater your growing potatoes. Just keep the soil damp while providing adequate drainage.
Another common practice that can help keep potato scab down is to rotate your potato crops if possible. This is always a good defense for plant pests and diseases. Rotate potato crops with a crop that is not effected by potato scab (which includes other root based crops). Ultimately potato scab infected potatoes are still edible. Cut around infected areas and be aware that they will not store long. As always, use certified inspected seed potatoes when growing potatoes and never use scab infected potatoes as your seed potatoes.
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