Aphid-Transmissibility Variants of Citrus Tristeza Virus in Infected Citrus Trees

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Abstract
Each of 4 orange trees infected with citrus tristeza virus (CTV) contained several variants of the virus that differed in aphid transmissibility. To ascertain the presence of these variants, small pieces of budwood of 2 trees, from which an overall low rate of transmission (7.5%) had been obtained, were grafted on 2 yr old orange plants. When those plants were used in aphid-transmissions experiments, highly (> 30%), intermediate (5-20%) and poorly (< 5%) aphid transmissibility variants were obtained. One of these trees came from an orchard in which no natural spread had been observed for 2 decades, although recently natural spreading had become apparent, and the 2nd tree was grated with budwood originating from this orchard. Using the same procedure with 2 trees from a region where natural transmission was evident during the last decade and overall transmission rates reached 25%, poorly transmissible variants also were obtained. Aphids apparently transmit simultaneously more than 1 isolate, as a spectrum of variants was observation from aphid-inoculated seedlings. It is suggested that tristeza-infected trees may harbor more than 1 variant; and that trees from a location where only limited natural spread is observed could contain CTV variants that are highly transmissible, but which are quantitatively suppressed by a dominating poorly transmissible variant.

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