Management of powdery mildew on grape clusters

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.    

This summer, powdery mildew, caused by the fungus Uncinula necator, has appeared relatively early on clusters of susceptible grape varieties in Michigan, e.g., Chardonnay, Chardonnel, Seyval, Aurore and even Concord grapes. In some vineyards, over 75% of the clusters have powdery mildew on them with over 25% of the berries infected. Such high levels of disease are of concern, as severe powdery mildew infections can cause splitting, rotting and dehydration of berries, resulting in lower juice yield. Grapes with powdery mildew generally mature earlier and are smaller and lighter than healthy grapes. Powdery mildew can also affect wine quality by imparting off-flavors and other undesirable sensory qualities. Even inconspicuous (late-season) infections barely visible can compromise the integrity of the berry skin by creating small dead spots, which can provide entry points for pathogens that cause Botrytis and sour bunch rots.

Symptoms

Symptoms of powdery mildew on clusters include a white to gray powdery coating on the surface of the berry. The coating is made up of fungal threads (mycelium) and spores. On severely infected berries, the skin underneath is gray to brown discolored due to the formation of dead spots where the fungus penetrates the berry skin. Later in the season, small brown-to-black specks become visible on the berries. These are cleistothecia, the overwintering fruiting bodies that cause new infections next spring. The rachis (cluster stem) can also be infected. In some cases, powdery mildew may be mistaken for downy mildew. However, downy mildew tends to be fluffier and less dense than powdery mildew and is never gray. It also tends to be more unevenly spread over the berry and cluster than powdery mildew. ‘Chancellor’ grapes are the poster child for downy mildew infection of the clusters.

Conditions favoring infection

The likely reason for the early onset and severity of the disease this year is the long rainy period that occurred in May, which would have been beneficial for spore release by the overwintering cleistothecia lodged in the bark, followed by extended dry, warm weather in June and July, which favored further disease development. Ascospore discharge is initiated in the spring if 0.10 inch or rain occurs at an average temperature of 50°F. Most mature ascospores are discharged within 4 to 8 hours after the onset of wetting and are carried by wind to susceptible plant tissues. They can infect any green surface on the developing vine resulting in primary infections. The fungus then grows on the plant surface and produces a second type of spore (conidia) under high relative humidity. These conidia are windborne and cause secondary 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. Unlike other grape pathogens, the fungus does not need free water for infection; moderate to high relative humidity (40 to 100%) is sufficient for germination of conidia. In fact, rainfall is detrimental to survival of conidia as they tend to burst in water. Although infections can occur at temperatures from 59° to 90°F, temperatures between 68° and 77°F are optimal for disease development. Temperatures above 95°F inhibit spore germination, and the fungus may be killed at temperatures above 104°F.

Effect of berry age on susceptibility

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. Also, rachises of ‘Chardonnay’ and ‘Riesling’ fruit clusters developed severe powdery mildew when inoculated at bloom, whereas rachises inoculated 31 days after bloom developed only trace levels of powdery mildew. Therefore, early sprays (from immediate pre-bloom until three to four weeks after bloom) are critical for preventing powdery mildew on the clusters. This coincides with critical sprays for black rot. When timing fungicide sprays, it is important to remember that all clusters in the vineyard may not be of the same age. If there is much variation in cluster development, the critical period for applying fungicides should be extended until the youngest clusters have caught up.

Effect of powdery mildew on wine quality

Recent studies have shed more light on the effects of powdery mildew on wine quality. In a study done in Australia, grapes were selected in different infection categories: 0%, 1-5%, 10-30% and 31-100% of the bunch covered with sporulating powdery mildew. Titratable acidity, total phenolic content, hydroxycinnamates, flavonoids and brown pigments in juice and wine increased with increasing infection. Even small amounts of infection (1 to 5% of the bunch infected) resulted in increased oily/viscous mouth feel characters, which was correlated with phenolic content (grapes produce phenolic compounds in response to infection by fungal pathogens).

Wine made from grapes with higher levels of infection (particularly the 31 to 100% category) were also perceived as having fungal, earthy and cooked tomato attributes compared to the control. In another study done in Ontario, grapes were selected based on percent of the berry surface with scarring due to powdery mildew infection: none (0%), Low (1 to 25% of the surface scarred), moderate (26 to 75% of the surface scarred) and severe (100% of the surface scarred). Visually, the pressed juice became darker and more turbid as severity of powdery mildew infection increased. The research showed that a low infection severity (1 to 25% of the berry surface scarred) did not result in detectable differences in wine quality versus the control. However, wines made from moderately to severely infected berries (26 to 100% scarring) had a higher pH and titratable acidity as well as reduced citrus aroma and tropical flavor. In addition, an earthy aroma and flavor and caramel flavor, higher viscosity and bitterness were detected by tasters.

Management

Powdery mildew on the clusters is best controlled by maintaining an open canopy and applying effective fungicides during the critical period (immediate pre-bloom until three to four weeks after bloom), which has now passed. For infection prevention, good fungicide options include sulfur, sterol inhibitors (Nova, Elite, Procure, Rubigan, Bayleton), strobilurins (Pristine, Sovran, Abound, Flint), Endura and Quintec. Remember that some grape varieties are sensitive to sulfur, Pristine or Flint, and that fungicides differ in their pre-harvest intervals. Also, sulfur applied late in the season may interfere with wine-making so is not advised beyond veraison.

There is some concern about potential fungicide resistance to the sterol inhibitor fungicides as some growers have seen a lack of control. One would first have to rule out poor timing, poor coverage and excessive disease pressure before considering fungicide resistance, but in vineyards that have received sterol inhibitor sprays for many years, resistance is a real possibility. In addition, there appears to be a link between resistance to sterol inhibitors and strobilurins, which is of concern. Alternating fungicides with different modes of action is therefore important. Quintec (quinoxifen), Endura (boscalid) and Sulfur (sulfur) are especially useful in this regard since they have unique chemistries different from the sterol inhibitors or stobilurins (just as a reminder, boscalid is one of the two active ingredients in Pristine). If powdery mildew is already present on the clusters, there are several possible eradicants available: JMS Stylet Oil (paraffinic oil); Armicarb, Kaligreen and MilStop (all potassium bicarbonate salts); and Oxidate (hydrogen peroxide). None of these compounds has been tested specifically for eradicative activity in Michigan, although previous trials have showed JMS Stylet Oil to be more effective than Armicarb or Oxidate for control of powdery mildew when applied on a preventive schedule. Prev-Am (boric acid/citrus extract) may also be an option but, although it is a good surfactant, has not been tested for efficacy as a powdery mildew eradicant. Sulfur can also kill colonies, but would have to be applied at high rates to be effective.

Whatever product is used, thorough coverage of the clusters will be critical, which means using higher spray volumes (at least 50 to 100 gallons per acre) while spraying every row. One concern with JMS Stylet Oil is that it can delay Brix accumulation, so it is best not to use it after veraison. Also, do not apply oil and sulfur within 14 days of each other. While most berries may already have become naturally resistant to infection, a protective fungicide such as Quintec or Pristine may still help protect younger clusters as well as leaves from infection. At the high labeled rate, Quintec provides up to three weeks of protection of sprayed foliage (but not new foliage). Removing leaves in the fruiting zone can also help reduce powdery mildew severity by increasing airflow, light penetration and fungicide penetration and is also advised for control of bunch rots.

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