The Effect of Some Latent Virus Infections on the Growth and Cropping of Apples

Abstract

SUMMARYSeedlings of Newton Wonder apple multiplied vegetatively on Malus sikkimensis rootstocks and inoculated with a mixture of chlorotic leaf spot, scaly bark and dwarf viruses showed varying degrees of sensitivity to these infections. Although none of the seedlings showed clearly recognizable virus symptoms, many showed a very significant reduction in vigour; the girth differences between the infected and control trees were still increasing three years after inoculation.The infected trees showed signs of nitrogen deficiency, which were confirmed by foliar analysis ; leaf nitrogen as percentage of dry weight was 10% lower than that of the controls. Four years after inoculation 43% of the infected trees had cropped, compared with 58% of the controls.The results suggest that some selection of varieties and rootstocks for virus tolerance has taken place and that the evaluation of new seedlings on their own roots may not be a true indication of their performance on infected rootstocks.