Evaluation of citrus tristeza virus isolates for cross protection of grapefruit in South Africa.

Citrus tristeza virus isolates were collected during 1976 and 1977 from healthy-appearing old grapefruit trees in different climatic areas of South Africa. Initial screening of citrus tristeza virus isolates under glasshouse conditions on West Indian lime (Citrus aurantifolia) hosts showed that growth was not a good criterion for differentiation among isolates; however, differences in stem pitting symptoms were more distinct. This varied greatly, from 2.5 to 76.0 pits per square centimeter. Significant differences occurred among isolates from the same orchard, indicating the presence of multiple strains within an orchard (.)