Leafroll virus disease is associated with several related viruses of the closterovirus type. The viruses cause varying degrees of disease in different grape varieties. Susceptible varieties exhibit variable symptoms. Generally, susceptible black and red-fruited varieties exhibit a red or purplish leaf color in late summer, often, but not always, accompanied by a downward rolling of leaf margins. All or only some leaves on a shoot and some shoots on a vine may exhibit symptoms. Once symptoms show up, they recur every year. White-fruited varieties exhibit a light green to yellowish color, again sometimes accompanied by marginal leaf rolling. The disease slowly weakens the vine and early symptoms include delayed fruit maturity. Diseased vines eventually weaken to the point of being uneconomical to farm.
Some varieties can be infected with the virus and suffer no apparent negative effects. However, when the infected variety is grafted to a rootstock that is susceptible to leafroll virus, disease develops in the rootstock, eventually killing the vine.
The viruses responsible for leafroll are thought to be moved only through propagation practices. Viruses can be eliminated from plant materials with laboratory techniques. Some states have grapevine certification programs that provide clean starting material for nurseries, a protocol for propagating clean vines, and a testing program to verify that plants are clean of specific viruses. Purchasing grapevines that are certified free of damaging viruses is the surest way to avoid this disease.
Vine symptoms are not a reliable indicator of the presence of leafroll virus because other factors may lead to similar symptoms. Laboratory analytical techniques and/or the time-consuming (2-3 years) method of grafting to indicator vines, must be used to positively identify the virus in grapevines. Several commercial laboratories use the ELISA method to test for leafroll and other grapevine viruses. The Foundation Plant Materials Service (FPMS), a service department of the University of California, Davis, offers both the ELISA method and polymerase chain reaction (PCR) technology for virus-testing. The PCR test for leafroll virus is considered to be more sensitive than the ELISA method, although both tests have limitations.
For more information on virus-testing laboratories, visit the links below. Listing of these analytical laboratories is not intended to be an endorsement to the exclusion of other laboratories which may offer similar services.
Known susceptibilities of Vitidaceae (grape family)
Plant Viruses Online
Grape Virus Diseases
Oregon State University