Biological Characterization of Naturally Occurring Citrus tristeza virus Strains in California Citrus

Abstract

The Citrus tristeza virus (CTV) collection housed at the Central California Tristeza Eradication Agency contains 339 isolates including representatives from the entire state of Califor-nia, with the majority of the isolates having been collected in the San Joaquin Valley. Isolates have been biologically characterized using the standard host indicators. Individual symptoms are rated on a scale of 0 to 5, 0 being no reaction and 5 being most severe. For each host plant variety, the symptoms are rated individually, then summed. A total rating is then assigned to each isolate tested by summing the ratings for each variety. Isolates can be compared at three levels according to individual symptoms, host indicator variety, or overall severity. This enables researchers to select particular isolates for study and to determine subtle changes in an isolate due to factors such as aphid transmission. Further, this rating system provides a means to identify whether an isolate is exotic to California. The majority of California CTV isolates are mild; however, some have the potential to cause severe symptoms in some host varieties. Quick decline and stem pitting of grapefruit were the symptoms most frequently rated as severe or moderately severe.