Pacific Pests, Pathogens & Weeds - Mini Fact Sheet Edition

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Potato powdery scab (294)

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Photo 1. Potato tuber with raised brown "scabs" of powdery scab, Spongospora subterranea.

Photo 2. Close-up of Photo 1: potato tuber with symptoms of powdery scab, Spongospora subterranea.

Photo 3. Sporeballs or sporosori of powdery scab, Spongospora subterranea, which germinate to produce swimming zoospores.

Diagram. Life-cycle of powdery scab, Spongospora subterranea.

Summary

• Worldwide distribution. On potato, but also tomato, capsicum, and weeds in the potato family. It is related to the club root of brassicas (see Fact Sheet no. 283). An important disease.
• Resting spores in the soil produce swimming spores which infect all underground parts. On tubers, wart-like, powdery, roughly circular "scabs" 10 mm diam. on the surface, merging and containing resting spores. Dark galls 1-10 mm diam. on roots.
• Disease reduces quality (and price), affects processing, and transfer a serious virus.
• Spread occurs in soil on machinery, shoes, and via manure if cattle fed diseased tubers. Long disrance spread on tubers for seed and trade.
• Cultural control: disease free (certified) tubers; improve drainage; avoid fields where disease known to occur (spores last up to 10 years); do not use manure from animals fed diseased tubers; avoid excessive amounts of N; crop rotation for >3 years.
• Chemical control: treat tubers with mancozeb..

Common Name

Powdery scab

Scientific Name

Spongospora subterranea f.sp. subterranea. It is a member of the family Plasmodiophoraceae, the same family as that of Plasmodiophora brassicae, the cause of club root of brassicas (see Fact Sheet no. 283). Like Plasmodiophora brassicae, powdery scab belongs to a group called rhizaria in the Kingdom Protista; they are related to slime moulds.

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