Topical susceptibility of plants to viruses

Abstract

Beans were more susceptible to each of three viruses when inoculated on the upper surfaces of their true primary leaves than when inoculated on their lower leaf surfaces; sunflowers were more susceptible when inoculated on their lower leaf surfaces. Inoculation of the stems of beans or cowpeas with the viruses of peach yellow bud mosaic, peach ring spot, the cowpea strain of tobacco mosaic, spotted wilt, or clover mosaic caused systemic symptoms of greater severity, of more rapid appearance, and in a greater proportion of cases than inoculation of leaves, hypocotyls, or roots. Inoculation of bean or cowpea hypocotyls with peach yellow bud mosaic virus commonly resulted in root invasion, but with no symptoms in the roots or tops, and no apparent reduction in plant growth until long after inoculation on the stems had caused death of the growing point. When tobacco roots were inoculated with tobacco mosaic virus, the virus remained in the roots for several weeks, did not cause any apparent reduction in the growth of the tops, and reached a lower concentration in the roots than when plants were inoculated via the leaves. When beans or cowpeas were infected with tobacco necrosis virus via the leaves, the reduction in plant growth was greater than when infected via the petioles, stems, hypocotyls, or roots. Local lesions of tobacco necrosis virus were larger on bean stems and petioles than on bean hypocotyls. Differences in topical susceptibility were also demonstrated with peach yellow bud mosaic virus and peach ring spot virus in cucumber, and with peach yellow bud mosaic virus in sunflower.