Focus on powdery mildew control in wine and juice grapes

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Some powdery mildew has been seen on ‘Concord’ berries and on leaves of unsprayed Chardonel grapes. At this point, the colonies are still very small and may be hard to see with the naked eye. We have had multiple occasions for primary ascospore release this spring: 0.1 inch of rain at a temperature of 50ºF or more. When the cleistothecia overwintering in cracks in the bark are sufficiently wetted, infectious ascospores are discharged within four to eight hours and are carried by wind to susceptible plant tissues. The more of these events occur in the time period from before bloom until four weeks after bloom, the higher the risk of fruit infection. Remember that grape berries are highly susceptible to powdery mildew infection in the first two to three weeks after bloom. A spray missed during this period can result in a season-long battle against powdery mildew on the clusters.

Once the fungus gets established, it does not need water or rain for infection – in fact, heavy rain is detrimental because it washes the spores from the leaves and causes them to burst. The fungus grows as circular colonies on the plant surface and produces secondary spores (conidia) that are windborne and cause new infections. Under optimal conditions, the disease can spread rapidly, as the time from infection to production of conidia can be as short as seven days. Although infections can occur at temperatures from 59 to 90ºF, temperatures between 68 and 77ºF are optimal for disease development. Temperatures above 95ºF inhibit spore germination, and the fungus may be killed at temperatures above 104ºF.

Berry age has a marked effect on susceptibility to powdery mildew. Researchers in New York showed that when clusters of ‘Chardonnay,’ ‘Riesling,’ ‘Gewürtztraminer,’ and ‘Pinot Noir’ were inoculated from pre-bloom to six weeks post-bloom, only fruit inoculated within two weeks of bloom developed severe powdery mildew. Berries became substantially resistant to infection by three to four weeks after bloom, resulting in diffuse, non-sporulating colonies on berries, and were virtually immune at six to eight weeks after bloom. Therefore, early sprays (from immediate pre-bloom until three to four weeks after bloom) are critical for preventing powdery mildew on the clusters. This usually coincides with critical sprays for black rot. For wine grapes, control of late and barely visible infections is also important as these can predispose the grapes Botrytis bunch rot and sour rot later in the season.

Sulfur remains an effective and inexpensive protectant fungicide for powdery mildew control in non-sulfur-sensitive grape varieties. The most effective systemic fungicides for powdery mildew control are the sterol inhibitors (Nova, Elite, Vintage, etc.) and the strobilurin fungicides (Pristine, Sovran, Abound and Flint). New fungicide options that provide good to excellent control of powdery mildew are Quintec, Endura, and Adament (mixture of Flint + Elite). Do not apply Pristine, Flint or Adament to ‘Concord’ grapes, as crop injury may result. JMS Stylet Oil and Sulforix are good materials to apply after infection has started to show up. Applying these materials in sufficient water is important to get excellent coverage because these compounds have to contact the fungal colonies to be effective. Do not apply Sulforix to sulfur-sensitive grapes.

Luckily, we do not have any reports of fungicide resistance to strobilurins in the grape powdery mildew fungus in Michigan. However, sterol inhibitors appear to be less effective than they used to be in vineyards where they have been heavily used for many years. It would be best to not solely rely on SI’s during the most critical period for fruit infection (immediate pre-bloom until three weeks after bloom), but alternate or tankmix with other effective fungicides. Over the last two years, we have noticed that Ziram as a tank-mix partner with Elite did improve control of powdery mildew over Elite by itself.  

Dr. Schilder’s work is funded in part by MSU‘s AgBioResearch.

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