Practical Winery
65 Mitchell Blvd, San Rafael, CA 94903
phone: 415-453-9700 ext 102
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Each mixture has a different name, depending on the “partner” fungicide employed: Revus Top, which also includes a downy mildew-specific material and is the cheapest of the three by far in the eastern region; Inspire Super, which at its top rate of 20 fl oz per acre also provides the active ingredient in Vangard, at a dose equivalent to 7 oz per acre of that material; and Quadris Top, which at its top rate of 14 fl oz per acre also provides the active ingredient from Abound at a rate of 11 fl oz per acre of that product. Not surprisingly, these three DFZ products have different attributes and per-acre costs.
In the eight trials where we have looked at it (two trials each in the past four growing seasons), DFZ has provided excellent to outstanding powdery mildew control, far superior to that provided by traditional DMI materials such as Rally, Vintage/Rubigan, Procure, and Elite/tebuconazole generics.
As most growers are aware, performance of the standard DMI products has been “slipping” in a number of locations over the years, throughout the world. For example, in a 2010 Chardonnay trial in New York, a seasonal program applying Rally at its maximum label rate of 5 oz per acre provided virtually ZERO control of the disease on clusters, i.e., they were completely destroyed by mildew. In stark contrast, two different DFZ products (Revus Top, Inspire Super) provided almost complete control (Table I).
Why? As noted in an accompanying text (page 15), resistance to the DMI fungicides is a “shades of gray” phenomenon, dependent on the rate and activity of the material being used. Lab studies show that the dose of myclobutanil (the active ingredient in Rally) required to provide a given level of control of the mildew fungus is many times greater than the dose of DFZ necessary to give the same control, yet they are used at comparable rates of active ingredient in the field.
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The bottom line is that simply by utilizing a VSP training system and basal leaf removal at fruit set, fruit disease severity was reduced by 35% relative to UK-trained vines with no leaf removal.
Exposure of fruit to sunlight and pesticides
Canopy management practices that increase sunlight penetration into the fruiting zone obviously should increase the penetration of sprays applied to the clusters as well. With the assistance of Dr. Andrew Landers, we were able to quantify the effect that Chardonnay canopy density can have on spray coverage with a Berthoud air blast sprayer delivering 50 gpa shortly after fruit set. The bottom line is that when the average number of leaf layers between the cluster and the sprayer was increased from 0.5 to 1.5, the amount of spray deposited on the clusters was reduced by 50%.
“Cold shock”
Cornell graduate student Michelle Moyer (now Extension Viticulturist at Washington State University), working in the lab of Dr. David Gadoury and Dr. Bob Seem, recently completed her thesis research examining some other aspects of powdery mildew biology. One interesting finding was that cold nights (below 40°F) set PM back significantly.
After as little as two hours at 36°F, portions of existing colonies are killed, and new colonies that do form take longer to develop and are reduced in size. Thus, cold nights during the period between early shoot growth and bloom have the potential to restrict the ability of the PM fungus to produce new spores capable of infecting young, highly susceptible berries.
Or seen another way, lack of such nights — typical in many regions during certain Spring seasons — can give the disease a running start relative to a “normal” year. Note especially that prolonged cloudy conditions that otherwise favor PM by increasing humidity and limiting exposure to direct sunlight also limit those really chilly spring evenings that can suppress mildew. Control programs might need to be adjusted should such conditions occur.
Fungicides: an update
1. New(er) products
a. The difenoconazoles — Difenoconazole (DFZ) is an important “new” (to the U.S.) sterol demethylation inhibitor (DMI) fungicide, registered for use on grapes under three different trade names. This apparent confusion is because DFZ is labeled on grapes only in mixtures pre-packaged with another fungicide in the parent company’s product line.