Virus-strain interference in relation to symptoms of psorosis disease of citrus

From the studies reported in this paper a hypothesis is advanced that psorosis A infected sweet orange trees carry two virus strains or components, one of which (component a) is capable of producing bark lesions and the other (component b) is not capable of producing lesions. In non-lesions bark, component a is the dominant part of the mixture and prevents component b from increasing sufficiently to bring about the injury that leads to lesion development. Bark lesions are initiated when component b by chance invades one or more plant cells ahead of component a. In the absence of a, component b increases in concentration and can then invade adjacent tissues and overcome the defense offered by the former. Such a procedure would explain the delayed appearance and slow enlargement of spontaneously developing lesions on orchard trees which have been systemically invaded by psorosis virus for many years. Some experimental evidence is presented to support this hypothesis.