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
B
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.
