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Beginner

Scientific Name
Spongospora subterranea

Identification
Tubers : Initial symptoms on tubers are purplish-brown, sunken lesions. Later, brown, raised pustules develop. They are filled with spores. Powdery scab lesions look very similar to common scab.

Pustules enlarge to about 5 mm in diameter. As they mature, pustules release the spores.

Roots and stolons: On roots and stolons, small necrotic spots develop into white galls 1–10 mm in diameter. Galls become dark brown when mature, and then gradually break down, releasing powdery masses of spores into the soil.

Often Confused With
Common scab

Period of Activity
The resting spores of S. subterranea survive in the soil in a dormant state and release swimming spores (zoospores) only in response to potato roots. The fungus is an obligate parasite, and it becomes active only in the presence of a host.

Scouting Notes

• There are no above-ground symptoms.
• To assess powdery scab incidence, dig tubers at random from mid to late season. Check low spots and areas of poor drainage.

Advanced

Scientific Name
Spongospora subterranea

Identification
Tubers: Initial symptoms on tubers are purplish-brown, sunken lesions. Later, brown, raised pustules develop. They are filled with spores. Powdery scab lesions look very similar to common scab.

Pustules enlarge to about 5 mm in diameter. As they mature, pustules release the spores.

Roots and stolons: On roots and stolons, small necrotic spots develop into white galls 1–10 mm in diameter. Galls become dark brown when mature, and then gradually break down, releasing powdery masses of spores into the soil.

Often Confused With
Common scab

Biology
The powdery scab fungus may survive many years in the soil as resting spores. It attacks roots, stolons, young shoots and tubers. The fungus is spread by planting infected seed tubers, by moving infested soil or by spreading contaminated cattle manure.

Potato roots stimulate resting spores to germinate. Resting spores release swimming spores that infect roots, root hairs, stolons, young shoots or tubers. Galls are formed in the infected tissue. A fungal mass grows inside infected tissues, then releases secondary swimming spores that spread the disease even more. Under favourable conditions, several generations of secondary spores will be produced and released.

Powdery scab is most common in cool, wet soils. During warm, dry weather, the disease can develop in low-lying or shaded areas of fields heavily infested with spores.

Period of Activity
The resting spores of S. subterranea survive in the soil in a dormant state and release swimming spores (zoospores) only in response to potato roots. The fungus is an obligate parasite, and it becomes active only in the presence of a host.

Scouting Notes

• There are no above-ground symptoms.
• To assess powdery scab incidence, dig tubers at random from mid to late season. Check low spots and areas of poor drainage.

Thresholds
There are no thresholds established.  It is not a common disease in Ontario.

Management Notes

• Plant powdery scab-free seed.
• Maintain optimum soil moisture. Do not over irrigate.
• Do not use manure of farm animals that have fed on powdery scab infected tubers.
• Avoid moving soil on farm equipment from contaminated to healthy fields.