Powdery scab

Spongospora subterranea

Symptoms

• Starts as small dark brown spots on the stolons, lenticels, eyes and growth cracks
• Skin cracks open in a star shape releasing brown-black spores
• After sporulation, crater shaped lesions become visible
• After growth of the tuber the lesions become larger but more superficial
• A typical wall-like growth can be seen around the craters
• Anti-sprouting products often increase symptoms as no cork layer is formed

Infection and spread

• Survives as latent spores in the soil
• Particularly on peat and sandy soils, but can occur on clays
• Motile spores infect tubers
• Cool, humid conditions encourage infection
• Lesions occur at high temperature and in dry conditions
• Further spread via seed tubers, manure and dispersion of contaminated soil
• Contamination possible during tuber growth, but also during storage
• Vector of potato mop-top virus (PMTV) and associated with spraing on tubers

Possible confusion with

• Common scab (Streptomyces scabies)
• Wart disease: early infestation
• Root-knot nematode
• Slug damage