STUDIES OF THE MOSAIC DISEASES OF CASSAVA

SUMMARY The symptoms of mosaic disease in cassava, although generally typical of the mosaic group, show wide variations, due in part to the varietal reaction of the plant, to its stage of development and to the environment. The most important cause of variations however is differences in the strains of the virus, of which we have recognized, through the study of symptoms, two groups of severe and mild strains. The viruses are transmitted across a graft, but we have failed to obtain mechanical transmission by needle or hypodermic injection. A Bemisia sp. can transmit both groups of strains. It can inoculate the plant only through immature leaves, less than about one‐quarter of their full length. The virus so inoculated does not pass out of the leaf until about 8 days have elapsed. On the basis of this knowledge a convenient and reliable single‐leaf cage technique has been developed. After the virus has entered the stem it passes rapidly to the base of this stem, but odp slowly into side branches from it or into other stems arising from the same original cutting. Infection of a plant with a mild strain of virus failed entirely to confer immunity from infection by severe strains introduced by grafting. If the severe strains were inoculated by insects there was an indication of some conferred resistance but insufficient to make the procedure practically useful in control.