Pathotypic variation in turnip mosaic virus with special reference to European isolates

A collection of 124 isolates of turnip mosaic virus was gathered from around the world, principally from European countries, and characterized by inoculation to four differential lines of Brassica napus (oilseed rape and swede). Three symptom phenotypes were induced—apparent immunity, local infection only, or systemic infection. Twelve distinct patterns, i.e. pathotypes, were observed. Three pathotypes were predominant in the collection; pathotype 1 isolates, which were the most common, did not overcome any of the most extreme sources of resistance in the differential lines. Of the other two, pathotype 3 isolates overcame one of the major sources of resistance and pathotype 4 isolates overcame all sources of resistance. The distribution of pathotypes within Europe was examined. No pathotype was confined to any geographical area, although pathotype 4 isolates were not found in southern Europe or Asia. Most isolates (90) originated from Brassica hosts, while others were from other cruciferae genera (19) or non-crucifers (5). The species of plant that the isolates originated from was not clearly related to the pathotype of the isolates. Resistance to pathotype 1 isolates is controlled by a dominant allele in one of the differential lines, and resistance sources are being examined in the other lines. Isolates belonging to pathotype 1 appeared to be able to mutate readily to overcome the resistance in one of the rape differential lines, but no isolates appeared to mutate to overcome the other major source of resistance in the differentials. The implications of the results for disease control strategies are discussed.