Thoughts from a Wine Enthusiast

Grapevine Leafroll Disease

WHAT IS GRAPEVINE LEAFROLL DISEASE???

Pathogen: Family: Closteroviridae. Genus: Ampelovirus

There are 9 different strains of the virus.

(GLRaV-3)

Large, flexuous rod-shaped particles with size range of 1250-2200 nm in length.

Category of Pathogen: Virus

Name of the disease caused by the pathogen: Grapevine Leafroll Disease

Symptoms of infection: The leaves on the infected vine roll downward, starting with the basal leaf on the cane. Areas between the major veins turn yellow or red, depending on if the variety produces red or white grapes.

Primary host (s): Vitis vinifera; Grapevines: red and white varietals.

Other host(s): None specifically for the GLRaV’s 1-9, but there are other leafroll viruses in potato plants (PLRV’s) and cherry trees (CLRV’s)

Route of transmission: Grape mealybugs (transmission of GLRaV-3 and GLRaV-5), other scale insects (transmission of GLRaV-1), and by grafting (transmission of any of the strains). We know that GLRaV’s may be transmitted in a circulative manner and also semi-persistently. GLRaV’s are spread more efficiently by baby mealybugs than by adults, and parent mealybugs that have the virus can not transmit it to the baby mealybugs.

What we do not know:

-Transmission Biology of GLRaV’s;

-Ecological aspects of GLRaV’s transmission;

-How the Virus-Vector Interaction occurs at the molecular and cellular levels. (Rodrigo Almeida)

The first picture shows instar nymphs, or crawlers, feeding on grapevine leaves. The second picture shows an adult male grape mealybug.

*****Click Here for the Washington State University Article on Grapevine Leafroll Disease by Naidu Rayapati, Sally O’Neal, and Douglas Walsh

R0: This is the reproductive ratio. This depends on if the infected vine is removed or not. If it is not removed, then it will infect the whole vineyard and Ro > 1, being an epidemic disease. If the infected vine is removed, then the disease dies out and Ro < 1. If vines are infected, removed, infected, and removed continuously then Ro = 1 and the disease remains at a steady level.

Prevalence: This disease is prevalent in all major grape-growing regions of the world. Some vineyards have had anywhere from 15% to 40% of GLRV infected vines. If an infected vine is not found, it will spread the disease to the rest of the vineyard. Late summer and early fall are the most important times to look for GLRaV’s.

Generation Time: It only takes 1 hour for a mealybug to transmit the virus from one vine to another. Once the vine has the disease, it remains infected.

Mortality Rate: Infected vines do not die; they only produce less fruit and will always produce small amounts. Mealybugs do not die from the virus, and are ineffective 4 days after receiving it (if they do not encounter it again from a viral plant).

Morbidity Rate: This depends on if the disease is diagnosed early on. Again, the disease can be found in any vineyard of the world. If a mealybug with GLRV feeds on a healthy vine for one hour, the vine will become infected.

Is it preventable? How?: Early detection in Vitis vinifera L. using vitro micrografting. Removal of infected vines is critical so that the virus does not spread throughout the entire vineyard. Tests for the virus need to occur in late summer, fall and winter. Petiole tissue should be tested in the late summer and fall; shoots and canes for cambium scrapings should be tested in the fall and winter. Polymerase Chain Reaction (PCR) tests can be done in late summer, fall and winter. Enzyme-Linked Immunosorbent Assay (ELISA) tests can be done in late summer and fall.

Does the disease trigger long lasting immunity?: No, the vines must be removed and replaced. Baby mealybugs are immune to their infected parents, but not to infected vines.

When was the pathogen first described and is there evidence that it was around much earlier than that?: The disease was documented in Europe in the early 19th century. Transmission of the disease by grafting was demonstrated in 1937. In the 1950′s, the Foundation Plant Materials Service (FPMS) clean stock program for grapes was created, because everyone in the viticulture business assumed that leafroll disease was spread by grafting healthy stock within infected stock. ELISA, a fairly new leafroll virus test, allowed researchers in the Foundation vineyard at UC Davis to discover leafroll disease transmission through vectors in 1992.

What is the economic impact of the disease?: This disease is one of the principle contributors to production loss in grapes. Some studies estimate yield losses of as much as 30% to 40%. (Golino) The cost of vineyard maintenance is increasing due to more frequent vine replacements.

The coolest thing about the disease (in your opinion):Hmmmmmm……. It does not kill the vine or the mealybugs. The fact that the mealybugs become ineffective after 4 days of getting the virus is interesting. I am interested in figuring out how to reverse the disease if your vine gets infected- why do they not become ineffective after a certain amount of days, like the mealybugs? One more cool thing……..a grape on an infected vine will not transmit any sort of disease to humans because it is the vine that is infected, not the fruit. The worst possible thing to humans, from GLRV, is that their grapes will not be as big, sweet and juicy compared to a grape that is on a healthy vine.

SUMMARY: Click Here for more info on Transmission Biology and overview of GLRV by Rodrigo Almeida