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 As a class, fungicides make up the largest percentage of pesticides applied to grapes. Of pesticides sold in dry formulations, fungicides make up 98% of the volume in Sonoma County and 96% in Napa County. Of pesticides sold in liquid formulations, fungicides make up 53% of the volume in Sonoma County and 31% In Napa County.
Fungicides kill molds, rust, mildew, and bacteria above the ground -- on trunks, branches, leaves and fruit -- as well as in the soil. Fungicides applied above ground include sulfur, benomyl, mancozeb, myclobutanil, propargite, triadimefon, captan, fenarimol, and copper hydroxide. Chemicals applied in the soil include methyl bromide, chloropicrin, metam sodium and tetrathiocarbonate.
Fungus that grows on above ground vine parts, primarily on the fruit, can suddenly appear, spread quickly, and damage or destroy grapes. Soil fungi, on the other hand, attack roots and can cause stunting or, occasionally, death to the entire plant. Following are descriptions of promising methods of reducing the need for fungicides applied to above ground plant parts. Alternatives to methyl bromide and other soil fungicides are described in the "Replacing Methyl Bromide" section of this report.
Fungicides which are dusted or sprayed onto above-ground plant structures are applied routinely, as a preventative, before fungal diseases occur. Fungicide applications commonly are scheduled for as often as once a week or every ten days during summer months. But this approach promotes over-use of the chemicals because it is driven not by need but by habit.
One promising alternative to clockwork application schedules combines highly refined scientific understanding of the effects of weather on the life-cycle of the pest with regular weather monitoring at a specific locale.
U.C. Davis plant pathologist Doug Gubler developed such a system for powdery mildew, the most common above soil fungus in Sonoma and Napa, and is developing systems for other fungi and for insects.
Marketed by Adcom Telemetry, the powdery mildew system measures humidity and temperature through a network of weather stations set up in vineyards and a centralized computer which receives data at 15-minute intervals for evaluation in a highly specialized, computerized pest-risk model. The result is a constantly changing risk index number which guides the grower in determining when conditions demand fungicide application.
If, for example, temperatures of 70-85 degrees Fahrenheit occur for 6 hours per day for three consecutive days, the risk skyrockets that powdery mildew will grow and spread. Temperature or other weather changes can reduce pest risk significantly, which is reflected in the risk index produced by the computer. Consequently, participating growers know whether their vines are very likely to begin to support fungal growth unless fungicides are applied, or if conditions are unlikely to advance fungal growth.
By utilizing such well-tuned understanding of pest life-cycles in relation to climate changes, growers can extend intervals between fungicide applications and thus apply significantly less fungicide. According to Gubler, grape growers using this system also were able to limit the rate of infected grape clusters to 5% compared to about 25% statewide.
Almost three dozen Adcom weather stations have been set up in Napa Valley vineyards, and Gubler says that growers have been very satisfied with the results. But the weather stations have not caught on in Sonoma County. One concern is that growers who pay about $3,300 to install a weather system resent sharing with growers who can access the information by buying software for just $400.
Less high-tech but still highly scientific is the method of reducing the risk of fungal disease by selective removal of grape leaves. Because it is also performed to influence grape sugars and speed ripening, grapevine leaf removal has been intensely researched and developed.
Opening the vineyard canopy improves air circulation in the vines and reduces humidity favorable to fungal growth. Leaf removal also enhances the distribution of sulfur and other fungicides during application so less can be applied to achieve adequate coverage.
Leaf removal is labor-intensive, involving careful hand picking by trained workers. It's also expensive-- though the price can be reduced significantly. Central Coast grape grower Joe Carrari says he used to pay $150 per acre for manual leaf removal, but reduced the expense to $7 to $10 per acre by using a propane flame thrower, a technique he developed on his own.
Carrari found a specialized nozzle manufactured by Flameco and put together a propane tank and four nozzles on a three-point hitch which he drives slowly down rows of vines. The 2,000 degree Fahrenheit flame passes 18 inches away from the plants, singeing green leaves which later die and fall off the vine. The flame does not damage vine trunks, canes or wooden stakes.
He flames early in the day to avoid wind, and says that he carefully times leaf removal to avoid adversely affecting wine flavor. In fact, when some of the berries were scorched by the flames, "it actually gave the wines a little more flavor!" he said.
Fungicides under fire due to toxicology and environmental problems also are in trouble because pests have developed resistance to many of the poisons. Fortunately, sulfur is far less toxic and does not have the resistance problems associated with synthetic chemicals. Although sulfur is not without its problems toxicologically, it is an important alternative for viticulturists who otherwise use cancer-causing propargite or mancozeb and other hazardous fungicides.
Sulfur is allowed for organic growing and many non-organic growers in Sonoma and Napa rely entirely on sulfur for fungicidal protection without experiencing crop losses. Among these are viticulturists who have learned to refrain from routine application schedules and instead apply sulfur only when the level of risk requires it. They also manage the canopy so that less sulfur is required for thorough distribution. By keeping sulfur use down to the essential amount, growers can avoid toxicological problems associated with using large quantities of this useful fungicide.
Because fungicides comprise such a large portion of pesticide use in Sonoma-Napa wine country, alternative fungus-control practices are of paramount importance in reducing the amount of pesticides used in the region.
Wine Grape Report Index
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