Disease Note.

First Report of Powdery Scab of Potatoes in Alaska. D. E. Carling, Agricultural and Forestry Experiment Station, University of Alaska Fairbanks, 533 East Fireweed, Palmer 99645. Plant Dis. 80:1208. Accepted for publication 26 July 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-1208B.

Powdery scab, caused by the fungus Spongospora subterranea (Wallr.) Lagerh. f. sp. subterranea J. A. Tomlinson, was found on potato (Solanum tuberosum L.) tubers in 1994 and again in 1995, in a vegetable garden in the Matanuska Valley near Wasilla, AK. Aerial portions of plants showed no indication of disease-related stress, but at harvest surfaces of many tubers were moderately to heavily damaged with up to 40% of some tuber surfaces covered with typical powdery scab lesions. Powdery scab lesions on tubers were always raised and wartlike and very pale to white in color at the time tubers were removed from the soil. After tubers were removed from the soil, the color of the scab lesions gradually darkened to a medium brown and the dry circular lesions that remained were surrounded by the characteristic broken remnants of burst periderm and filled with powdery masses of spore balls. Discrete lesions generally were circular in shape and ranged from 2 to 4 mm in diameter. Resting spores were the typical polyhedral shape and ranged from 3 to 4 µm in diameter. Primary zoospores were observed emerging from masses of resting spores placed on potato dextrose agar. Five cultivars of potato were grown in this garden: Shepody, Yukon Gold, IditaRed, an undesignated banana-shaped, yellow-fleshed cultivar, and an undesignated purple-skinned cultivar. Tubers of the banana-shaped cultivar always had fewer scab lesions than the other four cultivars. Most tubers of the four other cultivars were moderately to heavily damaged. Powdery scab has not been observed on any other potatoes from Matanuska Valley commercial potato fields or gardens or on samples of potato tubers grown in other parts of Alaska. This is believed to be the first report of (he occurrence of powdery scab disease of potato in Alaska.