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. Orchard evaluation of selected isolates (three were evaluated as mild, with 0-20 pits per square centimeter; two as intermediate. with 20-50 pits per square centimeter; and two as severe, with more than 50 pits per square centimeter) over 9 yr confirmed the glasshouse results: that growth cannot be used to differentiate isolates. No significant differences occurred in cumulative fruit production over a 5-yr period among trees planted virus-free, those inoculated with the mild isolates, and those inoculated with the intermediate isolates. Trees inoculated with two isolates selected as severe had a significantly reduced production similar to that of a known severe i solate. Trees planted virus-free had more small fruit than any of the trees inoculated with the mild or intermediate isolates, indicating that protection against natural severe strains was provided by the isolates. Fruit size of the trees infected with the severe isolates was commercially unacceptable.