We are quickly approaching one of the most important disease management periods of the season: immediate pre-bloom through approximately 3 to 4 weeks post-bloom. During this window, leaves, flowers, and developing fruit are highly susceptible to infection by several important fungal pathogens, including black rot, Phomopsis cane and leaf spot, powdery mildew, downy mildew, and Botrytis gray mold.

The fungicide applications made during this period often determine disease pressure observed later in the season. Many disease symptoms that develop near harvest originate from infections that occurred weeks or even months earlier.

Some pathogens infect flowers and young fruit, remain dormant during fruit development, and only become apparent as fruit begin to mature. Other diseases establish early-season infections that serve as a source of inoculum for repeated cycles of disease throughout the growing season.

For these reasons, this is not the time to cut corners on application rates, stretch spray intervals, or skip fungicide applications. Protecting clusters during the pre-bloom to post-bloom period remains one of the best investments growers can make toward producing high-quality, disease-free fruit.

Disease Management Priorities at Pre-Bloom in Wisconsin Grapes

During the immediate pre-bloom to early bloom period (approximately E-L stages 17-19), inflorescences have emerged and flowering is about to happen. Fungicide applications made when shoots reach approximately 10-12 inches in length should focus on protecting against black rot and Phomopsis, while maintaining protection against powdery mildew.

Black Rot in Grapes

Black rot remains one of the most economically important diseases of grapes in Wisconsin. Vineyards with a history of black rot, susceptible cultivars, or visible disease symptoms from previous seasons should receive fungicide protection before bloom to prevent fruit infections.

Phomopsis in Grapes

Phomopsis management is also critical during this stage. While many growers associate Phomopsis with leaf and cane lesions, infections can also occur on rachises (fruit stems), resulting in fruit infections later in the season. Once rachis infections become established, management options are limited.

Powdery Mildew in Grapes

Continuous protection against powdery mildew is equally important. Fruit becomes highly susceptible during the bloom and post-bloom period, and infections established now can lead to fruit cracking, poor fruit quality, and inoculum buildup within the vineyard. This is particularly important for susceptible cultivars such as Brianna, Frontenac, Frontenac Gris, and Marquette, as well as vineyards where powdery mildew was problematic in previous years.

Downy Mildew in Grapes

Downy mildew should also be considered when environmental conditions are favorable. Infection risk increases when temperatures exceed 50°F and are accompanied by rainfall, prolonged leaf wetness, and high nighttime humidity. Growers anticipating downy mildew pressure should include a product specifically targeting downy mildew in their spray program. Keep in mind that many downy mildew fungicides provide limited activity against black rot and powdery mildew and should therefore be tank-mixed with products that provide broader disease control.

Fungicide Recommendations for Pre-Bloom and Early Bloom Grapes in Wisconsin

Good fungicide coverage remains essential, particularly within the fruiting zone where flowers and developing clusters are located. As shoots rapidly expand, newly emerging tissues quickly become vulnerable to infection. Thorough spray coverage and appropriate spray intervals are critical during periods of rapid vegetative growth.

A commonly recommended approach during this period is to combine a protectant fungicide, such as mancozeb or captan, with a sterol inhibitor fungicide (FRAC 3) including products such as Cevya, Mettle, Procure, or Rally. These combinations provide strong activity against several of the key pathogens active during the bloom period.

To reduce the risk of fungicide resistance development, growers should rotate fungicide modes of action throughout the season. Strobilurin fungicides (FRAC 11), including Abound, Flint Extra, and Sovran, can be incorporated into rotation programs to diversify chemistry and reduce selection pressure on any single fungicide class.

Several pre-mixed fungicides also provide broad-spectrum activity against multiple diseases. Examples include:

These products can be valuable tools for protecting fruit during the bloom period while simplifying fungicide programs.

Growers should be aware that products containing difenoconazole, including Inspire Super, Quadris Top, and Revus Top, may cause phytotoxicity on Vitis labrusca cultivars, V. labrusca hybrids, and other non-vinifera hybrids. Carefully review product labels and cultivar restrictions before application.

Wisconsin Grape Management In the Coming Weeks

Weather forecasts indicate warmer temperatures across Wisconsin during the coming weeks, with many regions expected to experience daytime highs in the upper 80s. Warm temperatures combined with rainfall, humidity, and rapid canopy growth can create ideal conditions for disease development.

The next fungicide applications will likely be among the most important of the season. Maintaining strong protection from immediate pre-bloom through approximately 3 to 4 weeks post-bloom will help protect clusters during their most vulnerable stage and establish the foundation for successful disease management throughout the remainder of the growing season.For product efficacy ratings and additional management recommendations, consult the current Midwest Fruit Pest Management Guide and utilize NEWA grape disease models to monitor disease risk in your region.