THE PATTERN OF FIELD INFECTION OF POTATO BY THE BEET RINGSPOT STRAIN OF TOMATO BLACK-RING, A SOIL-BORNE VIRUS

The potential importance of the beet ringspot strain of tomato black-ring, a soilborne virus, was assessed by growing stocks of Kerr's Pink potato for I year on infested land and subsequently on uninfested land. The incidence of infection in two stocks was 39 and 8 % in the first year on uninfested land, and 29 and 5 after 2 years. The virus was usually restricted to the roots of plants in the first year of infection, but a few plants showed black rings and spots in their leaves. In the second year, 20-55 % of the plants grown from tubers set by symptomless, but infected, mother plants were infected: many of these showed leaf necrosis, others had stunted shoots, and cupped and distorted leaves; some were symptomless although systemically infected. In the third and fourth years, most of the progeny from plants which had symptoms or which were symptomless but systemically infected, contained the virus: nearly all such infected plants were stunted and distorted or were symptomless. Infection decreased the weight of tubers produced by plants with severe necrotic spotting but not the yield of plants with less necrosis. The number and weight of tubers per plant were decreased by 15 and 20% respectively, in symptomless systemically infected plants, and by 20 and 30% in stunted plants.