Separation and interference of strains from a citrus tristeza virus isolate evidenced by biological activity and double‐stranded RNA (dsRNA) analysis

Separation of strains of citrus tristeza virus (CTV), differentiated by their double‐stranded RNA (dsRNA) profiles, was obtained by graft‐inoculating citron plants from a Mexican lime that had been recently aphid‐ or graft‐inoculated with a mild CTV isolate (T‐385). Up to 24 sub‐isolates with differing dsRNA profiles were obtained from the aphid‐inoculated lime. Some of these sub‐isolates induced stronger symptoms in several citrus species than the original T‐385 isolate. One sub‐isolate, T‐385‐33, was mild in Mexican lime, but induced stem pitting on sweet orange. Inoculation of this isolate on Mexican lime, sour orange and Eureka lemon induced mild or no symptoms when inoculum was taken from citron, but very severe symptoms when the inoculum was from sweet orange. Mexican lime and sweet orange plants co‐inoculated with T‐385‐33 from sweet orange in combination with the other 23 sub‐isolates showed mild symptoms. The results obtained suggest that there is natural cross‐protection among sub‐isolates in the original T‐385 isolate.