Symptoms and Signs
Tuber symptoms of common scab vary in extent and appearance. Common scab lesions are usually circular and 0.25 to 0.33 inch (6 to 8 mm) in diameter, but they can be smaller in early stages of development and larger if they coalesce. Lesions typically possess a raised margin and slightly depressed center. Some characteristic symptoms have descriptive names: russet scab appears on tubers as superficial tan to brown corky lesions; pitted scab is characterized by lesions with depressions beneath the tuber surface; and raised scab appears as cushionlike, warty lesions. Common scab lesions can be confused with tuber lesions of powdery scab caused by Spongospora subterranea and patchy russeting caused by Rhizoctonia solani. In addition to tuber symptoms, Streptomyces spp. can cause brown stem and stolon lesions.
Comments on the Disease
Streptomyces spp. are widely distributed and persist in soil on decaying organic matter. Inoculum is also carried on infected seed tubers. Disease severity is usually increased by continuous cropping to potatoes. Tubers become susceptible to infection when they start forming. Scab lesions expand as the infected tubers grow. Mature tubers with well-developed skins are not susceptible. The fungus can also persist in un-composted manure from animals that have fed on infested tubers. When done properly, however, composting can apparently destroy infective inoculum of Streptomyces. Infection is favored by warm dry soils. A relatively low soil pH (5.5) inhibits scab, but S. acidiscabes can cause scab in soils at less than pH 5.5.
Maintaining high soil moisture (80–90% of available water storage) during tuber initiation and the 6 to 8 weeks that follow reduces the severity of scab and usually controls the disease adequately. Other practices that help reduce the incidence of scab include crop rotation with green manure crops such as rye, millet, and oats; whereas, rotations with carrots, beets, spinach, turnip, and radish are not advisable. Avoid soil application of animal wastes, since the pathogen survives passage through the digestive tract. Use certified seed tubers free from common scab. Contact your local farm advisor about relative resistance of varieties that are adapted to your area.
When applied before planting, some soil amendments such as sulfur and triple superphosphate help to increase soil pH, which makes the soil less favorable to disease development. Your local farm advisor can provide information on amounts that are appropriate for your soil conditions.