A carlavirus (code name RLSV and subsequently named potato latent carlavirus, PotLV) with serological and biological characteristics different from those of potato M carlavirus (PVM) and potato S carlavirus (PVS) was detected in the potato (Solanum tuberosum) cultivar Red LaSoda by the Scottish Agricultural Science Agency in 1992. During a routine electron microscope testing of accessions in the Vancouver Collection of Virus-Free Potatoes growing in the California winter test in 1993, a filamentous rod-shaped virus similar to PVS and PVM was found in a Red LaSoda clone from Nebraska. The virus was isolated and purified. The monoclonal antibody, MAb 4E12, which is highly specific to the PotLV virus, was developed. From 1994 to 1998, the accessions in the Vancouver Collection of Virus-Free Potatoes were assayed by triple antibody sandwich enzyme-linked immunosorbent assay (TAS-ELISA) using the 4E12 MAb. Seven accessions tested from 1994 to 1996 were infected with PotLV None of the 270 and 267 accessions in this collection tested positive for this virus in 1997 and 1998, respectively. In 1997 and 1998, the 137 accessions in the U.S. National Varietal Collection maintained at Presque Isle, Maine, were also assayed using the 4E12 MAb. The cultivars High Plains, Platte, and Red LaSoda were the only accessions that tested positive for PotLV. Nicotiana benthamiana, N. megalosiphon, and N. occidentalis are new systemic hosts for PotLV. TAS-ELISA with the 4E12 MAb is now part of the standardized test for PotLV in Canada.

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.

Heat therapy and meristem tip propagation were used to develop two or three virus-free clones of 10 cultivars from the following sources: Epicure from British Columbia and Scotland; Katahdin, Kennebec, Keswick, Red Pontiac and Sebago from British Columbia and New Brunswick; Norchip from Alberta, Quebec and Washington; Norland from Alberta, British Columbia and North Dakota; Red La Soda from British Columbia, Nebraska and North Dakota; and White Rose from British Columbia and North Dakota. Tests prior to treatment showed that all but one of the stocks were infected with potato virus X and/or potato virus S but in only five of the ten cultivars were all clones infected with the same virus or viruses. Field trials with the virus-free clones, which extended over four years, showed that one of the Norchip clones produced fewer tubers per plant than two other clones in three years of the four years, but no consistent differences in yield or specific gravity of tubers were found among the clones of any of the 10 cultivars.

Numbers and specific gravity of Netted Gem potato tubers were unaffected by mottle and latent strains of potato virus X (PVX) and by potato virus S (PVS). Neither latent PVX nor PVS affected yield but mottle PVX alone and latent PVX combined with PVS reduced tuber size and yield (P = 0.05).

Virus-free clones were developed from meristem tips of five selections of Netted Gem (Russet Burbank) potato which had been grown separately for at least 40 years. During three years of comparative trials, plant and tuber characteristics of the five clones were identical. There were no significant differences in total yield. Differences in marketable yield and, tuber set were few and inconsistent.

A diagnostic procedure for potato spindle tuber disease is described which allows detection of the causal viroid in small amounts of potato and tomato tissue. The method involves extraction of cellular nucleic acids, their separation by polyacrylamide gel electrophoresis and staining with toluidine blue O. The procedure has been used to index greenhouse and field potatoes for mild and severe strains of the pathogen

When Netted Gem potato plants were inoculated with potato virus X 7 to 31 days before defoliation the tubers were not uniformly infected. Testing of a single germinating eye chosen at random from each tuber detected fewer than half of the tubers from which stem-end, mid, or apical-end sections produced infected plants. The number of completely and partially infected tubers increased as the length of time between inoculation and defoliation was increased. Infection was not correlated with tuber size. Partially infected tubers produced equal numbers of infected plants from stem-end and apical-end sections.

Potato virus X in sap from potato leaves was applied by atomizer to surfaces commonly contacted during potato planting, growth, harvesting, grading and storage. The virus retained its infectivity toGomphrena globosa L. or Netted Gem potato for 10 seconds on leather, 3 hours on unpainted wood, iron, rubber and human skin, 6 hours on painted wood, jute and cotton, and 24 hours on soil.

Potato disease research projects in Canada and United States are arranged according to causal agent and objective. Names and addresses of investigators are given.