Transmission and protective inoculation studies with viruses of the citrus tristeza complex

Abstract

An investigation since 1949 of viruses of the citrus tristeza complex indicated that the association of virulent, yellows, and tristeza-inducing viruses with orange and mandarin and that of avirulent types with grapefruit, lemon, and sour orange is a general but not absolute rule, as some exceptions were noted. Attempts to separate yellows and non-yellows components from a virulent isolate, by applying short and long feed techniques to the vector aphid, Toxoptera citricidus Kirk., were unsuccessful. A comparison of the vector efficiency of 12 clonal lines of T. citricidus aphids revealed different transfer characteristics between clones. However, transmission results from two experiments were inconclusive. Cross-protection experiments were conducted over an 11 year period with seedling trees of grapefruit and sweet and sour orange seedling combinations. In these, mild isolates from grapefruit and lemon differed considerably in initial protective ability. Over a long period, however, even a strain of low initial protective ability exerted a considerable influence, as determined by tree growth. The virulent challenge virus used in these experiments retained its yellows- and tristeza-inducing ability when it was the sole inoculant, but not in trees previously inoculated with a mild isolate. The results of transmission and cross-protection tests are interpreted as evidence that viruses of the tristeza complex are related entities, varying in virulence but rarely if ever existing as pure strains. An explanation of host-reaction phenomena in terms of the combined effect of host selectivity of strains and mutual interference between tristeza variants is discussed.