The nature of the causative agent of virus diseases is obscure. A great deal of information on the reactions of the agent and on the symptomatology of these diseases has, however, been accumulated, with a view to bringing to the solution of the main problem the largest available amount of detail. While there is no irrefutable argument in support of either the organismal or the non-organismal hypothesis, it has become clear, as the investigations have progressed, that the viruses of both plant and of animal diseases possess many of the qualities usually associated with organisms. The existence of strains in the animal viruses is now generally accepted, though less information has been available regarding this aspect of the plant viruses. The purpose of the present paper is to direct attention to the presence of strains in what was previously thought to be a single virus, and to the probability of similar conditions obtaining in other viruses. The interactions of these strains, one with another, and with other viruses have been examined and are also described and discussed. It is shown that four types of interaction between different plant viruses can be recognized and these types are examined in some detail.
Special Issue: “Plant Virus Pathogenesis and Disease Control”