Potato spindle tuber viroid (PSTVd) causes a serious disease of potato, affecting yield and tuber quality. To control the disease, the Canadian seed certification program maintains a zero tolerance for the disease and a requirement that all nuclear stock, the micropropagated plantlets from which each lot of seed potatoes is initiated, is tested using reverse polyacrylamide gel electrophoresis (rPAGE) to ensure freedom from PSTVd. Moreover, seed potato fields are visually inspected during two or more annual field inspections for the presence of PSTVd and viruses. Symptoms of PSTVd have not been observed during field inspections for at least the last 25 years. Prior to 1989, seed potato stocks in the provinces of Prince Edward Island and New Brunswick were tested using rPAGE and nucleic acid dot blot hybridization for the presence of the viroid, and no infections were found (1). Similar surveys for PSTVd in Canada's western provinces of British Columbia, Alberta, and Saskatchewan also failed to detect the viroid (2). During 2000--2004, the PSTVd survey was extended to the provinces of Manitoba, Ontario, Quebec, Nova Scotia, and Newfoundland in which 211, 188, 95, 6, and 10 samples, respectively, were collected. Each sample consisted of 400 randomly selected leaves from selected potato fields representing seed lots registered in one of the four Elite seed classes or in the Foundation and Certified classes, except for a small number of samples (11%) that were from commercial nonseed fields. Leaves were tested using the dot blot procedure in composites of 50 leaves as described (2). Approximately 10% of the samples were retested using rPAGE followed by northern blotting to confirm dot blot results. All dot blot and rPAGE/northern blot results were negative for PSTVd. The cumulative results of the PSTVd surveys in all 10 Canadian provinces and the absence of the disease in the field as determined by annual visual inspection meets the International Standards of Phytosanitary Measures for the Requirements for the Establishment of Pest Free Areas (3). Hence, Canada declares that PSTVd is absent within its potato industry. A similar declaration was made by the United States recently on the basis of similar field inspection and survey data (4).
References: (1) D. Coates-Milne. FAO Plant Prot. Bull. 37:130, 1989. (2) S. H. De Boer et al. Can. J. Plant Pathol. 24:372, 2002. (3) FAO. ISPM Pub. No. 4, 1996. (4) M. Sun et al. Am. J. Potato Res. 81:227, 2004.
