In February 2013, severe symptoms of the powdery scab canker type were observed during harvesting at the Mallia area of Heraklio, Crete, Greece, on approximately 20% of the tubers of two commercial smooth skinned potato (Solanum tuberosum L.) field crops (cv. Spunta), of which the seeds had been imported from the Netherlands. Initially, small (up to 2 mm diameter), sunken, purple-brown lesions appeared before harvest on the tubers, followed by pimple-like swellings, which increased in size. As lesions matured, they broke through the tuber skin and developed into shallow depressions. Lesions often expanded in depth and width, forming deep pits and open cankers, eventually destroying considerable amounts of tuber tissue. Light microscopic preparations showed the presence of spongy spore balls with a honeycomb-like spore wall structure. Spore balls were golden brown to brown, oval, circular, elongate, pyriform or irregular in shape, and 54.2 ± 0.93 (36.3 to 79.9) × 47.8 ± 0.82 (31.5 to 72.6) μm. These were composed of aggregates of hundreds of spores that were circular to polygonal in shape, and 4.5 ± 0.06 (3.5 to 5.6) μm in diameter. The morphological characteristics closely resembled those reported for the powdery scab pathogen Spongospora subterranea (Wallr.) Lagerheim f. sp. subterranea Tomlinson. Pathogenicity was confirmed by a lab-based bioassay (3), where spore balls scraped from tuber scab lesions were used to inoculate 20 tomato (cv. Earlypak) bait seedlings, while 20 other seedlings were employed as non-inoculated controls. Zoosporangia of S. subterranea were observed microscopically in root hairs and epidermal cells of all inoculated seedlings but not of any control plants 2 weeks after inoculation. The identity of the pathogen was also confirmed in the original potato samples employing Sss AgriStrip (BIOREBA AG, Reinach, Switzerland), an immunological assay specific for S. subterranea resting spores. In addition, DNA was extracted from infected tubers and the region corresponding to the internal transcribed spacers (ITS) and the ribosomal DNA (rDNA) gene regions was amplified (1). PCR products were purified, direct sequenced, and deposited in GenBank (Accession No. KF208654). In a BLASTn search, the 18S (partial)-ITS1-5.8S-ITS2 (partial) sequence stretch showed 95% similarity to a 509-bp sequence (AF102820.1) of the corresponding S. subterranea gene in GenBank (query coverage 98% and E value 0.0). Based on morphological characteristics, pathogenicity test, immunological assay, and molecular data, it was concluded that the pathogen on potato in Crete is S. subterranea f. sp. subterranea. Powdery scab of potatoes by S. subterranea had been reported earlier in some areas of Greece (Argolida, Attica, Kefallinia, Messenia, and Salonika) (2,4), but without any data confirming the identity of the pathogen. To our knowledge, this is the first record in the island of Crete, and the first confirmed report of this pathogen in Greece. Because S. subterranea f. sp. subterranea is a destructive pathogen of potatoes, its presence in Crete could be of significant concern for commercial growers. Consequently seed import regulations should be strictly enforced.
References: (1) K. S. Bell et al. Eur. J. Plant Path. 105:905, 1999. (2) C. D. Holevas et al. Ann. Inst. Phytopath. Benaki 19:52, 2000. (3) U. Merz. Bull. OEPP 19:585, 1989. (4) M. E. Pantidou. Fungus-host index for Greece. Benaki Phytopath. Inst., Athens, 1973.