Assembly, quality control and use of a potato cultivar collection rendered virus-free by heat therapy and tissue culture

Tissue culture of the meristem tips of heat-treated plants has facilitated the eradication of potato virus X (PVX) and potato virus S (PVS) from the cultivars and seedlings which are important to the potato industry in Canada and the United States. The treated cultivars and seedlings comprise a field-grown collection which currently contains 239 accessions, all of which are tested twice each year to monitor for virus freedom. The collection, developed and still being expanded at the Vancouver Research Station, Agriculture Canada, has been utilized since 1968 by seed growers and potato improvement agencies in British Columbia, the other Canadian provinces, United States, Australia, New Zealand and several other countries. Surveys in British Columbia show that despite minor field spread on some farms, particularly in the first few years after replacement of infected stocks, successful control of PVX and PVS has been achieved on all but one of 22 Elite seed farms in the Pemberton and Cariboo seed potato control areas. The surveys, in which the field incidence of infection is expressed as Confidence Intervals (P=0.05), also show that crops in other districts grown for one or two years beyond the Elite 3 seed class usually have virus incidence no greater than the Elite seed, especially when the Elite seed had been given the lowest Confidence Interval (0.0–1.5%), indicative of no infected leaflets in samples from 250 to 1000 plants. An unusually high incidence of PVS in two fields was traced to the use of a contaminated seed tuber cutter.