A collection of citrus tristeza virus (CTV) isolates from different citrus-growing regions of the world was started in 1984 in a quarantine facility at Beltsville, MD. Isolates with diverse biological properties have been received through cooperation with scientists in and Venezuela. To date, 175 CTV isolates have been established and are being maintained. Symptom expression on five different indicator plants has been evaluated or is under evaluation for 114 isolates. Transmission of 28

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... by Aphis gossypii has been achieved, and 22 aphid transmitted subisolates also have been evaluated for symptomatology. Results confirm great biological diversity among CTV isolates. This diversity in reaction is independent of the effects of host, environment, and evaluation methods. Severity of reaction was not consistent in different hosts for some isolates. Severity of stem pitting in grapefruit and sweet orange was not necessarily correlated to the decline reaction observed in young, grafted sweet orange on sour orange plants. This collection is a valuable source of CTV isolates for research on characterization and identification of specific isolates and for further studies on mild strain cross protection.