Field experiments on the spread of black currant reversion virus and its gall mite vector (Phytoptus ribis Nal.)

In two experiments the spread of reversion virus from a row of systemi-cally infected black currant bushes heavily infested by the gall mite vector (Phytoptus ribis Nal.) was predominantly in the direction of the winds prevailing during the dispersal period. On each side of the sources there was a curvilinear decrease of galled buds and of virus infection as distance increased. In another experiment a central source of mites and virus was surrounded by concentric hexagons comprising alternate rows of healthy and virus-infected bushes. At leaf-fall, galls were forty times more numerous on virus-infected than on healthy bushes; plants in the sector downwind developed the most galls and those upwind the least. On both healthy and virus-infected bushes in each sector, the incidence of galls decreased with increasing distance from the source. The gradients of infestation were steeper on healthy than on virus-infected bushes, especially in sectors upwind from the source. In some sectors the infestation gradients were distorted because many of the virus-infected bushes were so heavily infested that most of the buds became galled. The spread of virus to initially healthy plants decreased from 100 to 75% near the source, to zero at the periphery. More bushes became infected downwind from the source than upwind. In each experiment more bushes developed galls than later produced symptoms of virus infection, the incidence of which was positively correlated with the number of galls recorded the previous winter.