Powdery Mildew
by Pete Anderson

 

 

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