Vineyard sites and/or canopies subject to poor air circulation and
increased microclimate humidity, and seasons
with frequent rainfalls, provide a significantly
greater risk for PM development than their drier
counterparts. Thick canopies and rainy seasons are
not only associated with high humidity but — even
more important — they are also associated with
limited sunlight exposure, which greatly increases
the risk of disease development (see below).
3) Our research shows that berries are extremely
susceptible to infections initiated between the
immediate pre-bloom period and fruit set, then
become highly resistant to immune about four
weeks later. European workers have found the same
thing, but this concept reportedly has not held
as well in California. Nevertheless, conventional
wisdom has always been that berries are particularly
susceptible to infection when they are young, and
this is when we recommend that growers use their most
effective PM fungicides and do not cut corners in terms
of spray interval and application technique.
4) Powdery mildew is a unique disease in that the causal
fungus lives almost entirely on the surface of infected
tissues, sending little “sinkers” (haustoria) just one
cell deep to feed. This makes it subject to control by
any number of “alternative” spray materials
 
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(oils,
bicarbonate and monopotassium
phosphate salts, hydrogen peroxide, etc.) that have little to no effect on other disease-causing fungi, which live down inside the infected tissues.
Two primary limitations to this group of products need to be considered if you want to use them effectively: a) they work by direct physical contact, so can only be as effective as the coverage you provide; and b) they work primarily in a post-infection/curative mode by killing the fungus right after they hit it, providing only modest (horticultural oil) to no (potassium salts) protection against new infections that might occur after the application. This means that they need fairly frequent re-application, or should be tank-mixed with something that provides good protective (forward) activity in order to allow longer spray intervals.
The superficial growth habit of PM fungi also makes them uniquely vulnerable to the harmful ultraviolet (UV) rays of the sun, as detailed below.
Effect of sunlight exposure
“It has long been known” that PM is most severe in shaded regions of the vineyard (canopy centers, near trees, etc.), but until recently there was very little work done to determine either the magnitude or cause(s) of this effect. However, Cornell graduate student Craig Austin recently completed a thorough study of the phenomenon, and the results were quite striking.
One of his first experiments was conducted in a New York Chardonnay vineyard where a small portion of the easternmost row was bordered by a group of 50-foot-tall pine trees. The trees completely shaded these vines each day until the sun crested over their tops just before noon. Craig Austin inoculated leaves on either the a) outer, exposed or b) inner, shaded portions of two groups of vines, which were located either (i) immediately next to or (ii) 200 feet away from the trees, thereby providing a total of four levels of natural shade.
The resulting disease severity increased substantially with each increasing level of shade, becoming
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8 to 40 times more severe on the most heavily shaded leaves (interior of vines next to the trees) compared to unshaded leaves on the exterior of vines away from the trees (Figure 1).
Although shading could potentially change air temperature or relative humidity within the vine canopy, our measurements did not show this. However, they did show that UV radiation levels and leaf temperatures were dramatically different among the different treatments.
Within the shaded regions, UV levels were (as one would expect) a mere fraction of those in the sun, and temperatures of leaves in the sun were anywhere from 2° to 23°F higher than those of leaves in the shade (the average was around 10°F higher). As we later found out, both elevated leaf temperature and UV radiation are responsible for the inhibitory effects of sunlight on PM development.
Sunlight characteristics influencing powdery mildew development
UV radiation from the sun can damage the cellular structure of virtually all forms of life. However, as noted above, powdery mildew is uniquely vulnerable to such damage due to its superficial growth habit. Additionally, the PM fungus is white — it has no pigment (“suntan”) to protect against the radiation that it is exposed to.
Furthermore, the additional heat of sunlight-exposed leaves and berries lead to suppression or even death of PM colonies on those tissues while colonies on cooler tissues in the shade thrive. Recall that mildew grows best at temperatures around 80°F, but stops growing at temperatures above 90°F and will start to die at temperatures above 95°F, depending on how hot it is and for how long.
On a hypothetical day (or portion thereof) above 80°F, temperatures of shaded leaves and clusters will remain near that of the air — i.e., at or near optimal for PM development. However, nearby tissues that are exposed to sunlight can often have temperatures elevated to a point that is detrimental or even lethal to the fungus.
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