|
WHAT IS IT?
It is caused by the fungus Uncinula
necator. According to UC Davis,
"it is without a doubt, the most enduring and
persistent disease problem faced by grape producers, especially among
California Vitis vinifera."
SYMPTOMS
UC Pest Management Guidelines describe the symptoms as: "red blotchy
areas appear on dormant canes. On leaves, initial symptoms appear as chlorotic spots on the upper leaf surface. Signs of the
pathogen appear a short time later as white, webby mycelium. As spores area
produced, the colony takes on a white, powdery appearance. On fruit and
rachises the pathogen appears as white, powdery masses that may colonize the
entire berry surface." There is relatively little Downy Mildew in California; it exists mainly in the northeastern part
of the United States
where it develops on vitis labrusca
plants. The symptoms are similar , but the upper surface of the leaf will
have spots with a translucent oily appearance which subsequently turn yellow;
lower surface will have a white downy growth.
Some varietals are
more susceptible than others; carignane is one of
the most susceptible. In areas of high infection, the carignane
will produce new shoots that already show the signs of major infection. On
most varietals, early symptoms will appear as small yellow-green blotches on
the upper surface of leaves with web-like fungus growths opposite on the
lower surface. A dusty film covers the leaf which gradually darkens as the
leaf cells die; leafs will mottle, crinkle and fall off.
Symptoms on the
berries will appear early as web-like blemishes before ash-like growth; the
berry growth will be retarded and may split. Severely infected bunches are
more prone to result in bunch rot.
INFECTION
AND SUBSEQUENT RISK ASSESSMENT
The fungus overwinters as
dormant fungal strands (mycelium) in buds which can produce "flag
shoots" in the Spring; and/or fruiting structures (cleistothecia)
sheltered under the bark or in leaf litter which can produce ascospores in the Spring after rain or wetting by
irrigation. Conditions for infection are: 0.1 inch of rain or irrigation is
followed by 13 hours of leaf wetness when temperatures are between 50 and 80
degrees F.
*** DORMANT SPRAY IS
CRITICAL -
A combined spray of Stylet Oil and Lime-Sulfur -
drenching the entire dormant vine and any leaf litter near the vine is highly
recommended. Do not use a combined oil and sulfur spray if there is leaf
growth on the vine as it will cause severe burning of the leaf.
Once powdery mildew
infection occurs, the ideal temperature for growth of the fungus
is between 70 and 85 degrees F which is the normal temperature for most of us
in the San Diego
County area.
Temperatures above 95 degrees F for 12 hours or longer can stop the fungus
growth. UC Davis has developed a Risk Assessment Index to help manage the
treatment of the disease. This RAI can be downloaded from the UC Davis Pest
Management website. Except for those areas having temperatures in excess of
95 degrees F for at least 15 minutes and fewer than 6 continuous hours of
temperatures between 70 and 85 degrees F, the remainder of the county area
will continually be in the highest disease pressure zone and should maintain
a persistent spray schedule based upon the degree of infection.
DISEASE
MANAGEMENT Below is
a spray program for both backyard vineyards and larger vineyards which have
pest control license to apply controllable chemicals, but for the best
comprehensive guidelines please go to the UC Davis Integrated Pest Management
site: www.ipm.ucdavis.edu/PMG.
BACKYARD
VINEYARD
DORMANCY
Following pruning and
before budbreak, apply a lime-sulfur and Stylet oil combined spray - drench the entire vine.
BUDBREAK -
LEAF GROWTH
Apply a wettable sulfur or liquid lime-sulfur spray
every 14 -21 days depending on infection history and current assessment. Can
alternate with Stylet oil or Kaligreen
(potassium bicarbonate). Continue until either signs of infection persist or
stage of veraison is reached.
INFECTION
When signs of infection persist, the wettable
sulfur/lime-sulfur, even sterol inhibitors such as Elite, Rubigan,
Rally, etc. will not eradicate the disease - you will need to apply Stylet Oil (Ultra Fine Oil, etc.) . After eradication,
you can go back to the regular spray program which is a preventative action.
CLUSTER SET
(BB-SIZED BERRIES)
UC Davis Dr. Doug Gubler recommends LEAF REMOVAL
one node above and below the cluster to open the canopy and increase
ventilation which is one of the best controls for both powdery mildew and
bunch rot. An added benefit of this action is that it opens the fruit zone to
allow more effective spray applications. Leaf removal should not be performed
until after cluster set, since the leaves near the cluster are necessary for
berry development up to this stage.
VERAISON
When the berries soften and change color, STOP THE SULFUR SPRAY. If sulfur is
continued, there is a good possibility elemental sulfur will remain on the
peel of the berry or in the rachis which can produce hydrogen sulfide (H2S)
during fermentation (based on personal experience). If there still exists a
need for additional sprays, either preventative or to eradicate, use the Stylet oil or Kaligreen
substances. When the berry brix reaches 12 - 15
degrees, the powdery mildew sporulation generally
will cease in the cluster. However, if the bunch rot fungus had developed
prior to the preclose stage of the cluster, it will
start to appear since it feeds on the heightened sugar content of the berry
usually starting at around 8 degrees brix.
LARGER
VINEYARDS
The control program will follow the same phenological
stages of berry development outlined above, but with the Pest Control License
these vineyardists will be able to substitute
sterol inhibitors (Elite, Rubigan, Rally, etc) and strobilurins (Abound, Flint, Sovran, etc) for the sulfur, kaligreen and oil sprays. In particular, they can apply Vangard ,which is a spray specifically for Botrytis cineria fungus, that causes the bunch rot. If you can
apply these sprays, it is important to alternate the use with a different
chemical in order to prevent a resistance developing in the plant. Also, it
is advised not to exceed two or three applications per season. There is a new
product named Pristine produced by BASF that went through bench trials a
couple of years ago . The trials were on the Carignane
vine in one of the highest disease pressure areas in California - it reportedly out performed
all other controls. This has been approved for use in California - I have not used this product,
but it might be worth exploring.
PROGRAM
SELECTION
Which treatment program works best for you will depend on several factors:
vine varietal type; macroclimate; canopy management. Based on my experience,
it is advisable to experiment with different treatment programs to get the
one that works best for you. This is also supported by the professional
knowledge shared with me by Bill Mosby of Mosby Winery in Buelton,
Ca and Dick Hoenisch of UC Davis Vineyards:
Peter I start the
year with thiosulfate at 8 in. of growth. last year
I had great control using rally rubigan and abound
but always with 3# / acre of thiosulfate and
(something new for me) 5# / acre of mono potassium phosphate. I cut the thiosulfate from the program the end of may. I also used auxigro which is an amino acid for 2 applications. this
regimen allowed me to cut my entrance into the vineyard to every 20 to 30
days. I will be checking out this procedure again this year to verify its
worth. good luck bill
Dear Peter,
We use Kocide just before leaf fall to kill any
overwintering fungal or bacterial bodies. We found it works well especially
against bunch rots.Doug Gubler's
researchers have found that PM mycelium can overwinter in the buds. Therefore
at bud break, the infection is off to a perfect start. When I took over the
management here at the end of 1997, I found a vineyard in which PM was almost
endemic. Within the next year (El Nino) we brought it under control.The first sulfur spray should be right at bud
break. We have so many different varieties, both early, mid, and late, that
we spray every week for 3weeks until we have all the varieties covered. This
nips the infection "in the bud" (pun intended)!!!! After that we
spray every 2 weeks with Thiolux, then, as I said
before, with Abound as soon as the canopy is fully grown. Abound is a bit
pricey (around $225 per gallon), but one uses very little and the effect is
next to miraculous . We are also very careful to limit the canopy by careful
and rigorous pruning, removing all the small, weak canes and limiting the
number of spurs to3 on each side of the cordon. Our new plantings are 6x10,
double cordon with VSP trellises. Happy 2002, Dick
|