It’s not like we really have a firm understanding why some plants typically produce brilliant red foliage before dropping their leaves and others do not. It has been suggested that anthrocyanins are produced as a sun screen as chlorophyll is pulled back, but this seems a little shaky as it is unclear why soon to be abandoned leaves would warrant this protection and it does not explain why a large number of plants, including grapes, don’t bother.
At least they are not supposed to. It has been well known for some time that certain mineral deficiencies, certain viruses, and a girdling beetle prompt red varietals to produce red leaves worthy of an autumn centerpiece, but a new player, GRBaV, has burst on to the scene in the last five years and the associated disease has progressed rapidly enough that it is becoming harder and harder to find red varietal vineyards in Napa and Sonoma counties that do not exhibit symptoms. Infected white varietals do not show any red leaf color but have the same low brix and high acidity as affected reds.
And now it is in the wild grape, vitis California. In late August we first noticed symptoms in wild grape. These vines climb high in trees and are often protected by formidable sentries of poison oak. When we finally found an accessible sample we took it to the UC Davis Oakville station. This small sample did not test positive but subsequent testing of symptomatic wild grape did test positive for GRBaV. (Dr. Sudarshana, personal communication).
Symptoms like these have proliferated since August in the wild grape.
It is easy to get the feeling that people just don’t have their arms around the magnitude of this problem. One sees symptomatic vineyards being ripped out and replanted as if there were a meaningful way to prevent reintroduction of the disease. Some vineyards replanted in 2010 are already completely reinfected.
We see the disease beginning in vines with red anthrocyanins being produced around the loci of insect damage. Mites and leafhoppers are prime suspects.
In this picture one can see typical mite stippling that has led to typical rust colored “bronzing” and then a progression to more scarlet coloration and vein reddening characteristic of red blotch.
We see a second stage where leaves develop red veins but remain yellow and fall off prematurely.
We observe a discoloration of the cane wood typical of beetle girdling but ending at nodes which beetles avoid. This suggests to us progression of the disease down the shoots from initial infection in the leaves. A downward progression is also consistent with observed “one armed” infections of cordons.
We see the final stage as the development of gross symptoms. These symptoms of maroon leaves are associated with a unique tissue profile strongly deficient in Phosphorus and extremely elevated in Iron. Prior to symptoms the tissue profile is normal. This would seem to indicate a blockage of P uptake and Fe circulation.
Dr. Sudarshana at Davis has observed a one year latency between detection of the virus by assay and the onset of symptoms and Dr. Fuchs at Cornell has found an average two year latency. Part of the discrepancy may be in the definition of “onset of symptoms” but two strains of the virus have been identified and it could be that the western version moves more quickly to symptoms. It seems the western variety may spread more efficiently as well.
We suspect a two or three year progression between initial and gross symptoms and see a strong association of extreme gross symptoms with the 44-53 rootstock. At the gross symptom stage the plants seem to hold on to their leaves longer than in the initial stages.
If there is any good news here it is that symptomatic plants can still produce bountiful crops of grapes. The symptoms do not appear until very late in the growing season and this year they have been progressive, even accelerating after harvest.
It is clear to us that it is far too late to contain this disease. We will need to adjust viticultural and winemaking techniques and probably winemaking styles until resistant stock is found.
It seems to us that current research is bogged down with insect vectors. This would be interesting to know but current IPM and sustainable techniques forbid the complete elimination of insect populations even if this were possible. We should be aggressively looking for resistant stock, but to the best of our knowledge there is no current research on this front.
We may be witnessing the transition of red varietals to plants that do turn red in the fall.
Maple. Photo credit KEF.