Considering the poor juice grape crop and extended warm, dry weather, the need for fungicide sprays during the rest of this season in juice grapes is minimal, but wine grapes are still at risk of foliar and fruit diseases. However, the weather conditions have definitely helped to keep fungal diseases at bay, for which we can be grateful. The main concern is if the weather changes and we will have to deal with lots of rain in the weeks before harvest, which would promote Botrytis bunch rot and sour rot. Below are some considerations which may help in making a spray decisions for the rest of the season.
Powdery mildew is showing up on leaves here and there on susceptible varieties and in unsprayed plots. This disease does not need a lot of moisture and can continue its development despite the drought. However, since there was not a lot of rain to get the disease started in the spring and early summer, and high radiation and heat can kill young colonies, powdery mildew pressure has been light this year. High temperatures that do not harm the plant can harm the fungus; spores and mildew colonies can be killed after extended durations of temperatures above 91 degrees Fahrenheit. The fungus is killed completely when air temperatures rise above 95 degrees Celsius for 12 hours or more if colonies are directly exposed to UV light.
One thing to remember is that leaf temperatures are usually higher than air temperatures during the day. Protection against powdery mildew now is not necessary in Concords or Niagara, as many vineyards have excess foliage and little fruit. The vine can withstand a fair bit of disease anyway, especially with a reduced fruit load. At this point, most fruit clusters have developed resistance to powdery mildew infection, as berries are susceptible to powdery mildew infection for four to six weeks, with the time of highest susceptibility being from bloom to two to three weeks after bloom. Even wine grape growers can consider easing up on powdery mildew sprays and applying eradicative sprays (i.e., with JMS Stylet Oil or bicarbonate salts) only if powdery mildew were to show up on the leaves between now and harvest.
At this point, there is little risk of fruit infection unless there are berries out there that are still relatively young, which could be the case with uneven bloom after spring frost damage. An eradicative spray is recommended anyway in late August and early September to knock back overwintering inoculum for next year.
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