Transmission of Plant Viruses by Insects

Abstract

Insects, as a general rule, play an essential part in the survival of the viruses that cause plant disease. It is true that a virus will pass from scion to stock, or the reverse, across a graft in those species of plants where organic union is possible. I t is also true that by vegetative reproduction of a diseased plant the virus is perpetuated in its progeny. But every living plant owes its origin ultimately to sexual reproduction; and the real problem, from a theoretical and a practical point of view, is how a virus can become established in a sexually produced plant. In only a few rare instances is it known that a virus can be transferred through the true seed from one or other of the parents (16, 21, 57, 58). Some form of secondary transfer from plant to plant must be possible. Many viruses, but not all, can be experimentally so transferred by mechanical inoculation of juice. It is doubtful, however, whether such a method of transmission can operate generally without human intervention, although the natural spread of one virus has been explained by juice transfer during the rubbing of leaves in the wind (49). A few viruses can be disseminated, in some manner not certainly known, through the air (70) or soil (90). Nevertheless, so far as our present knowledge goes, we believe that the great majority of viruses are transferred in nature from one plant unit, i.e., from the product of a single true seed, to another only by insects; and without the insect the virus would suffer extinction with the death of the plant that had been its host.