Powdery scab

Powdery scab has increased in occurrence over the past 30 years in many nations, including France, Pakistan, Japan, Australia, New Zealand, Costa Rica, the United States, Colombia, Korea and China.

Biology

Sss persists in the soil as dormant spores that can stay active for an extended period. When conditions are right, these dormant spores awaken to produce zoospores that seek out the host plant’s roots. These zoospores then invade the roots, causing swelling and scab formation on the tubers, which are packed with sporosori. As the affected tubers break down, these sporosori are liberated into the soil, thus perpetuating the life cycle. Sss is seedborne and can be transmitted through infected seed potatoes.

Symptoms

Early stages of the disease are small lesions that appear on the tubers. These lesions develop into raised pustules containing a powdery mass. Sss also infects roots, leading to gall formation, which can serve as reservoirs for the pathogen, contributing to the spread and persistence of the disease in the soil.

Damage

Powdery scab primarily affects the appearance of potato tubers, leading to market rejection due to deep-pitted lesions that require extra peeling, though overall yield remains unaffected. However, in some russet-skinned processing varieties, root galls can inhibit growth, causing yield losses of 2 to 5 tons per acre and reducing tuber size, which impacts contract incentives. Additionally, Spongospora subterranea (Sss), the causal agent, serves as a vector for potato mop-top virus (PMTV), further increasing the disease's economic significance. PMTV causes spraing, which appears as internal necrotic arcs, rings or flecks in potato tubers (Figure 1).

Management

Effective powdery scab management involves crop rotation with non-susceptible crops for at least six years and using disease-free seed potatoes to prevent pathogen introduction. Planting in well-drained soils and avoiding overwatering help reduce disease risk, while long rotations of three or more years are recommended in infested soils. Russet varieties are more resistant than white or red varieties. Additionally, controlling weeds like nightshades, which serve as alternate hosts, further limits disease spread. Soil testing helps manage powdery scab by detecting pathogen levels, guiding crop rotation, variety selection and soil management to reduce disease risk and prevent PMTV spread.

Further readings

• Powdery scab of potato: A diagnostic guide
• Powdery scab of potato, BUL 1024
• Potato mop-top virus: Biology and disease management, BUL 1017
• Compendium of potato diseases, 2nd edition

Pesticide warning

Always read and follow the instructions printed on the pesticide label. The pesticide recommendations in this University of Idaho webpage do not substitute for instructions on the label. Pesticide laws and labels change frequently and may have changed since this publication was written. Some pesticides may have been withdrawn or had certain uses prohibited. Use pesticides with care. Do not use a pesticide unless the specific plant, animal or other application site is specifically listed on the label. Store pesticides in their original containers and keep them out of the reach of children, pets and livestock.

Trade names — To simplify information, trade names have been used. No endorsement of named products is intended nor is criticism implied of similar products not mentioned.

Groundwater — To protect groundwater, when there is a choice of pesticides, the applicator should use the product least likely to leach.