White, wart-like outgrowths on roots and stolons – root galls – and blisters and pustules on tubers (lesions) are characteristic symptoms of the potato powdery scab disease caused by Spongospora subterranea (Wallr.) f. sp. subterranea. In Sri Lanka, potato is a major cash crop primarily in two agroecological zones of higher altitude, Nuwara Eliya and Badulla. Approximately 50% of the seed is produced nationally and the other half is imported from the Netherlands, France, Germany, and recently the United States (mainly high quality seed). During the 2002–2003 cultivation season, galls and lesions were observed on certified seed lots of potato cvs. Dura, Desiree, Roko, Cykoda, and Delawae imported from the Netherlands and planted in fields at the Seed Certification Service, Site Eliya near Nuwara Eliya, SriLanka after first inspection. Since then, similar symptoms were also observed on tubers and roots of cv. Granola at seed potato production sites in 2006 at Diagama, in 2007 at Bopathalawa, and in 2007 through 2010 at Pedru. In 2009 and 2010, blister-like lesions on tubers and root galls were again observed on seed of cvs. Calwhite, Keuka Gold, Red la Soda, and Chieftain imported from the United States and planted at the Agricultural Research Station, Site Eliya. In February 2004, a bioassay was carried out using healthy tubers of cvs. Roko, Cykoda, Delawae, and Isna to evaluate the potential contamination of field soils. Clay pots filled with soil samples collected from the suspected contaminated fields were planted with seed of the above cultivars. The pots were arranged in a complete random design with three replicates per cultivar in a place isolated from any potato-production location. Seventy-five days after planting, all varieties were recorded with white root galls but no lesions on the tubers. With light microscopy, suspected root and tuber tissue was examined to confirm the presence of the characteristic sporosori of the soilborne pathogen S. subterranea (Wallr.) f. sp. subterranea with their unique sponge-like structure (1). Additionally, two root galls and three tuber lesions were prepared for diagnostics by ELISA using antiserum produced against S. subterranea (Wallr.) f. sp. subterranea (3) and reactions were positive. Furthermore, in a lab-based bioassay (2), sporosori obtained from a single root gall and a single tuber lesion were used to inoculate tomato bait plants, cv. Marmande. Eight days postinoculation, zoosporangia were observed microscopically in roots at frequencies of 10 of 12 and 11 of 12 plants for the root gall and the tuber lesion, respectively. To our knowledge, the results presented here are the first confirmed report of the presence of S. subterranea (Wallr.) f. sp. subterranea, the causal agent of potato powdery scab in Sri Lanka. Powdery scab currently occurs at several places in Sri Lanka. Therefore, more attention should be paid to this disease in the seed certification process and seed import regulations concerning powdery scab should be strictly enforced.
References: (1) C. H. Lawrence and A. R. McKenzie. Powdery scab. Page 35 in: Compendium of Potato Diseases. The American Phytopathological Society, St, Paul, MN, 1981. (2) U. Merz et al. Eur. J. Plant Pathol. 110:71, 2004. (3) U. Merz et al. Eur. J. Plant Pathol. 111:171, 2005.