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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