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Sunday, September 8, 2013

Grape Leafroll Associated Virus (GLRaV)

The Grape Leafroll Associated Virus (GLRaV) colonizes the phloem in the plant’s vascular system, cutting off distribution of nutrients to leaves, shoots and fruit. This delays maturity of the grapes, reduces fruit yield, and lowers accumulation of sugars (the Brix), affecting wine quality. Symptoms include reduced vigor, yellowing of the leaf margin and veins, and the characteristic under curl of the leaves. The symptoms are more noticeable in red varieties than white varieties. Symptoms are typically most evident in mid-August, but, in some cases, there are no visible symptoms. The virus is a group of ten strains, identified as GLRaV- followed by a number. Take a look at the symptoms on a variety of grape types:





According to the brochure, "Grapevine Leafroll – an Increasing Problem in the Finger Lakes, the US and the World", published by the Cornell University Cooperative Extension, this is a worldwide problem threatening many grape-growing regions. The virus has been found in both eastern and western United States grape growing regions, including California. Even the San Francisco Bay Area backyard gardener may encounter the virus.

The virus spreads through infected bud wood cuttings and graft unions. If the grafts are not clean, infected starter plants can be distributed unwittingly around the globe. European researchers discovered the virus is also spread through insect vector - Pseudococcus mealy bugs (grape, obscure, and longtailed) and soft scales (Parthenolecanium corni and Pulvinaria innumerabilis). The vectors hitch hike on farm workers, equipment, and plant debris, and can quickly spread from an infected field to a neighboring clean field.

Since vines cannot be cured once they are infected, the best strategy is preventing infection. Plant only certified clean root stock. Inspect the plants routinely, looking for symptoms and signs. Manage mealy bugs and soft scales using organic methods. Once infection is detected, replace the infected vines with certified clean stock at once. Testing is available for several strains of the Grape Leafroll associated viruses; see the U.C. Davis Integrated Pest Management (IPM) site for information.

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