Transmission of grapevine leafroll associated viruses

1995 FINAL REPORT – Evidence is strong that leafroll disease has spread from infected vines to uninfected vines in the disease-tested, true-to-variety collection of grapevines at the Foundation Plant Material Service facility in Davis. The collection is the core of the California Grapevine Registration and Certification Program. This disease spread has serious implications for the grape clean stock program and commercial vineyards in California. Reports do exist of field transmission of viruses in the leafroll group by mealybugs in other areas of the world. Due to the success of early transmission experiments in the Golino laboratory with mealybugs, this aspect of the research has recieved additional attention in 1995. Four populations of mealybugs were collected, identified and increased: one of the obscure mealybug, Pseudococcus affinis, a mealybug species commonly found in California; one of the citrus mealybug, Planococcus citri; and two cultures of the long tail mealybug, P. longispinus, also common in California vineyards. In the last year, experiments were done using the obscure mealybug to transmit leafroll types I, II, m (isolates 109 and 101), IV and corky bark isolate 100. Isolate 109 is a very severe strain of leafroll type III, the most important leafroll type in California. By 8 months after inoculation 30%of inoculated plants were infected. We are in the process of repeating these experiments to determine whether this rate of transmission is consistent. This is the first evidence that the obscure mealybug can act as a vector for grapevine leafroll virus. The obscure mealybug did not transmit leafroll type II virus in any of three experiments involving 87 Cabernet Franc and LN-33 test plants. Preliminary results indicate the the obscure mealybug transmits grapevine corky bark virus, a related closterovirus, at a low rate; tests with LR type I are in progress. ELISA tests have been successful in detecting leafroll type III in the bodies and honeydew of the obscure mealybug . Initial results with the citrus mealybug indicate it cannot transmit leafroll III. Preliminary results with the long tail mealybug indicate it can transmit corky bark. Future studies will concentrate on the long tail mealybug.