First Report of Potato Powdery Scab, Caused by Spongospora subterranea f. sp. subterranea, in North Dakota.

Powdery scab is a serious disease of potatoes (Solanum tuberosum L.) that can cause extensive surface defects on susceptible potato cultivars. The pathogen, Spongospora subterranea (Wallr.) Lagerh. f. sp. subterranea J. A. Tomlinson, is an important concern in seed potato production because current seed certification standards in North Dakota have a zero tolerance for this pathogen. Powdery scab had not previously been identified in North Dakota. Until recently this disease was not thought to be present in U.S. commercial potato-producing areas. Powdery scab has now been shown to be more widely distributed than previously thought (1), having been identified in several locations in eastern and western North America. However, environmental conditions in North Dakota, including high soil pH, low rainfall, and high temperatures, are not considered favorable for powdery scab development. Diseased potatoes were found in a field from Griggs County, ND, in the fall of 1994 and characteristic cystosori were present in erumpent lesions on infected tubers. The field was an irrigated circle planted to three different cultivars. The cultivars Goldrush and Red Norland were symptomless, while cv. Red La-Soda was severely affected. Up to 30% of the tuber surfaces were diseased and disease incidence approached 25%. The infected crop was destroyed. Pathogenicity was confirmed by planting pieces of diseased tuber tissue adjacent to healthy seed pieces of Russet Burbank. Transmission of the obligate parasite occurred naturally by simulating conditions that were favorable for infection, acidic potting mix (Jiffy-Mix) maintained at a high water potential. Progeny plants were found to be infected on both tubers and roots after 60 days. Control plants that were not exposed to diseased tuber pieces were unaffected. North Dakota, a major seed-potato-producing state, ships seed to every other potato-producing area in the U.S. and much of Canada. Infected seed shipped to areas with a favorable environment for disease development may result in crops becoming diseased and unmarketable. The cooler temperatures and higher than normal rainfall during the 1992 to 1995 seasons may account for powdery scab development in North Dakota. Recent increases in irrigated agriculture and potato production in North Dakota could lead to the infestation of new areas, restricting the value of that land for potato production. Reference: (1) B. Christ et al. Am. Potato J. 65:583, 1988.