Mid-summer grape disease update

Carefully scout for grape diseases, especially downy mildew, powdery mildew and black rot.

Grape disease symptoms are becoming more apparent now, although well managed vineyards continue to look very clean. Downy mildew has been active for about a month due to abundant spring rains. The first cluster infections were first seen in ‘Chancellor’ grapes around mid-June, and unsprayed Niagara and Concord vineyards are showing foliar infections.

The first powdery mildew colony was sighted on a berry in the unsprayed sentinel plot in southwest Michigan late last week (July 3-9). Phomopsis symptoms have been present on leaves and canes since May and new growth may be obscuring older infections. We should be nearing the end of Phomopsis cluster infections. Anthracnose symptoms are increasing in severity in table grapes and some wine grapes. The pathogen is able to spread rapidly through rain splash and wind-driven rain and reddish spots are now visible on young berries. Fungicide applications are recommended to protect clusters and shoots from further infections.

Black rot symptoms are coming on strong in unsprayed or poorly sprayed vineyard blocks, and low levels may also occur in some sprayed vineyards due to suboptimal spray timing or fungicide wash-off by rain. Symptom development is relatively early this year, despite hot and dry weather over the last few weeks. Black rot is always a bit confusing as symptoms that are showing up now are the result of infections that happened during rain events (infection periods) two to three weeks ago. Applying a fungicide now may not do much to control infections that are becoming visible at this time. However, depending on the age of the berries, fungicides applied now may still be able to prevent new infections.

Grape berries are most susceptible to black rot infection for the first two to three weeks after bloom and become progressively more resistant as they develop, finally becoming highly resistant about five to eight weeks after bloom, depending on the variety. In general, 'Concord' berries are resistant to infection about four to five weeks after bloom, while some V. vinifera cultivars don’t become fully resistant until eight weeks after bloom. Thus, the key to control black rot in juice grapes is preventing the establishment of early berry infections during the first few weeks after bloom. This means that most disease control was provided by the first and second post-bloom fungicide sprays. See MSU Extension Bulletin E-154, Michigan Fruit Management Guide, for fungicide recommendations.

Continue careful scouting for all grape diseases, including virus diseases. We have seen symptoms of peach rosette mosaic virus in juice grapes, and tobacco ringspot virus and grapevine leafroll virus in wine grapes. Herbicide injury symptoms are also commonly seen on foliage, including RoundUp, Gramoxone, and Stinger injury.

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

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