Practical Winery
65 Mitchell Blvd, San Rafael, CA 94903
phone: 415-453-9700 ext 102
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Bud mites cause damage from the outside in. They typically first appear inside the outer green layers of the bud, eventually working their way into the inforescence and vegetative tip in the middle. One bud may have hundreds of mites, but they cannot be seen without a good microscope.
Damage to the outer layers is of no consequence; thus many buds may be infested without damaging the crop if mites reach the middle, they may start to damage the surface of the inflorescence or the vegetative tip. The damage map appear as bubbly protrusions or as hard brown scars. Damaged inflorescences may emerge and form flower clusters, and the grower may not know that those clusters would have been larger without the bud mites.
The worst infestations will destroy flower clusters and prevent emergence of the shoot. Infestations with 5% of inflorescences damaged or destroyed are not uncommon. The most serious populations can damage or destroy 50% or more of the inflorescences.
Shoots sprouting from infested buds may have short internodes and a zig-zag growth pattern. South African researchers have posted pictures.9
Some grape varieties aremore susceptible to budmites, and the problemmight be more serious in some growing districts. We have seen few if any budmites in winegrapes grown in Napa, Sonoma, and Lodi districts, among others.
We have seen serious infestations in table grapes in Kern, Tulare, Fresno, and Imperial counties. Highly susceptible varieties, in our experience, include Flame Seedless,Autumn Royal, Black Seedless, Crimson Seedless, and Princess Seedless. Thompson Seedless is moderately susceptible, and we have seen only a few bud mites on Red Globes.
Predatory mites (Phytoseiidae) do eat Erineum mites, but biological control of bud mites is not well-understood. 7 Bud mites are exposed to predators during the same short period as they are exposed to sprays, right after bud break. In addition, bud mites hide under new leaf bracts for a few weeks while they wait for new buds to form. Predatory mites can probably reach some under the leaf bracts, and on the outer layers of the new buds.
Some materials are harmful to predatory mites, including fungicides in the dithiocarbamate class (Dithane, Ziram, Maneb, etc.), Benlate, lime sulfur, Lorsban, pyrethroids, and some other insecticides.7 Many of these materials are typically applied just before and after bud break to control Phomopsis, mealybugs, or other pests. Possibly this could contribute to increased bud mite problems. On the other hand, we have observed reductions of about 50% in bud mite infestations in the year following pre-bud break application of Lorsban, compared to nearby unsprayed vine rows.
In one study, some bud mites were found under the stipular scales of next year’s bud primordia oneweek after bud burst. By four weeks, 50% of the population was protected under the scales, and by 10 weeks, 100% was in the buds.
The Australians have studied control of bud mites with sulfur. The best timing was during the week following bud break. The rate was 200g per 100 liters of 80% wettable sulfur, using a “high volume,” (about 1.75 lbs. per 100 gals). The researchers did not experiment with different rates or volumes.7
Benefiting from Grape Bud Analysis
ud fruitfulness may be predicted before pruning by using grape bud analysis (GBAnalysis) — dissection of the buds under a microscope to count flower cluster primordia. Some analyzing services also rate the sizes of the primordia. The main purpose of GBA is to help growers decide how many canes or spurs to retain on the vine and how many buds to leave per spur.
In many years, GBAnalysis will merely confirm the typical fruitfulness of the vineyard and not change pruning decisions. But when fruitfulness is higher or lower than normal, a GBAnalysis can give a grower a warning in time to make changes.
By using GBAnalysis, growers can be confident they do not need to leave extra spurs as insurance against low fruitfulness. In a normal year, this may save on thinning costs. But when fruitfulness is in low, growers will be forewarned to consider tactics to increase the amount of fruit, such as leaving more or longer spurs or adding one or more “kicker canes.”
Sometimes a GBAnalysis can provide clues about the cause of low fruitfulness. If many buds are healthy but have no inflorescence, the low fruitfulness was caused by failure to differentiate early in the previous year. This means conditions in early spring were to blame. If many buds are dead, the cause was necrosis, and conditions in late spring or summer were responsible.
In one case involving a block of Red Globe table grapes, a GBAnalysis found very
low fruitfulness in the first five buds but good fruitfulness in more distal buds. Not only was the grower warned in time to leave some canes to obtain a crop, but he was able to trace the probable cause to a change in sprays at bloom.
In 2006, growers throughout California saw low fruitfulness in their vineyards. Those who had a GBAnalysis were forewarned and able to modify their pruning to obtain the most from a sparse crop.
Accuracy of a GBAnalysis depends mainly on the grower providing a sample of spurs or canes that represent what will actually be left on the vines after pruning. If the sample is biased towards either good or bad spurs, so will be the results.
A typical sample size is 20 spurs or canes per block. Usually, analysts look at four buds on each cane or all the buds on a spur. This sample size is adequate to track fruitfulness from year to year and to judge whether the crop load is normal, high, or low. Larger samples would be needed to discern modest differences between blocks or to calculate a precise number of expected clusters per vine. Precision depends on the number of samples, not the number of acres. Samples can be combined from two or more blocks of the same variety with similar soil and growing conditions to give more precise results.
More suggestions on taking samples are available on the website of Advanced Agricultural Services.
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