Abstract
SUMMARYBlack‐currant yellows virus restricted the growth of bushes var. Baldwin and Wellington XXX and decreased the total weight of fruit harvested in six seasons by 70%. The size and number of fruits were decreased and in one season ripening was retarded. Virus spread naturally to only four of 250 other bushes in the trial.A strain of reversion virus which caused transitory vein‐pattern symptoms did not restrict vegetative growth, yet decreased the number and size of fruits. In contrast to the more severe effects of prevalent virulent strains, the total crops from the Wellington XXX and Baldwin bushes were decreased by 50 and 33% respectively. The virus spread to fifteen of the 136 healthy bushes and to at least six of the 108 bushes previously infected with yellows virus.The black‐currant gall mite (Phytoptus ribis Nal.) increased more rapidly on the Wellington XXX bushes than on those of Baldwin. In each variety the infestation was much greater on reverted than on other bushes, apparently because infection with reversion virus decreased natural resistance to mites.
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