Late-season control of powdery mildew in grapes: focus on inoculum management

At this time of year, the focus of powdery mildew management should be on reducing inoculum production for next year, which typically peaks in mid-September. Eradicant sprays are best applied to visible colonies in early September.

Powdery mildew colonies are starting to become more apparent on grape leaves in a number of vineyards. While dry weather is generally considered favorable for powdery mildew, the heat and high solar radiation of the 2012 growing season have actually been detrimental to powdery mildew development. Temperatures of 95 degrees Fahrenheit and above are lethal to powdery mildew colonies. In this case, the temperature of the leaf surface is the most important to consider as this represents the immediate environment to which the powdery mildew colony is exposed. Leaf temperatures can be considerably higher than the surrounding air during sunny days.

Powdery mildew on grape leaf Powdery mildew on grape leaf
Recently observed powdery mildew infection on grape leaves. Photo credit: Annemiek Schilder, MSU

Powdery mildew colonies
Powdery mildew colonies. Photo credit: Annemiek Schilder, MSU

The most common approach to grapevine powdery mildew management is to apply preventative fungicide sprays. A more sustainable and cost-effective management approach is to aggressively protect the fruit during its most susceptible period (from bloom until five to six weeks after bloom) and then take a more relaxed approach to protecting the foliage in order to keep the leaves functional during fruit ripening. Frequent scouting will be needed as well as eradicative sprays once powdery mildew colonies start to appear.

At this time of year, the focus of powdery mildew management should be on reducing inoculum production for next year. In August and September, the fungus switches to producing overwintering structures called cleistothecia. They can be seen as small, yellow, brown and black specks on the surface of the powdery mildew colonies. Even if substantial powdery mildew occurred in a vineyard during the growing season, there is a window during which cleistothecium production can be nipped in the bud as most cleistothecia are produced in the first half of September in Michigan. Conversely, even if you had decent control of powdery mildew during the growing season, putting the sprayer away soon can allow the fungus to make a late comeback and still produce considerable numbers of cleistothecia before the leaves fall off. Lower numbers of cleistothecia going into the winter result in a delay in the onset of powdery mildew epidemics and overall lower disease pressure the following season.

Cleistothecia on leaf vein
Powdery mildew cleistothecia on vein of grape leaf. Photo credit: Laura Miles

While we have many effective fungicides for preventative and curative control of powdery mildew in grapes, not many fungicides can eradicate existing colonies. In fact, spraying systemic fungicides on raging infections is not very effective and can encourage fungicide resistance development. At most you can expect to suppress sporulation while you keep new infections from taking place on healthy tissues. Small plot efficacy trials in grapes have shown that JMS Stylet Oil, Sulforix and Kaligreen have good eradicative properties and can reduce the number of visible colonies (see graphs). To reduce the number of cleistothecia, Sulforix, Elite and Kaligreen appeared most effective (Elite did not eliminate existing infections, but prevented new infections). Do not apply Sulforix on sulfur-sensitive varieties or close to harvest, as sulfide residues may interfere with the fermentation process. More than two applications of Stylet Oil are thought to suppress brix accumulation but we are evaluating this premise in Michigan in 2012.

Effect of post-infection (Sept. 19) fungicide spray on powdery
mildew severity on Pinot noir leaves in Traverse City, Mich.

Effect of post-infection graph

Effect of a single post-infection spray on cleistothecium formation

Cleistothecia graph

A few things to remember when applying eradicative sprays.

  • Apply treatments as soon as possible after symptoms are seen (regular and careful scouting is important).
  • For cleistothecia prevention, apply an effective eradicant on visible powdery mildew colonies between now and early September.
  • Ensure thorough coverage of leaves and bunches, which means increasing spray volume (50 to 100 gal/acre), driving slower and spraying every row.
  • Since coverage is so important, waiting a little longer to ensure good spray conditions is better than spraying immediately under poor spray conditions.
  • If needed, ensure forward protection of healthy plant parts by including fungicides with good protective activity in the spray mixture. Fungicides that have broad-spectrum activity can also protect against late season downy mildew and Botrytis bunch rot development.
  • Always read the label for the pre-harvest and restricted entry intervals, incompatibility with other products, and other restrictions.
  • Scout again to see if your treatment was effective.

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

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