By Blake Jackson
Wisconsin grape growers are entering one of the most important periods for disease management, stretching from just before bloom until about three to four weeks afterward.
According to Leslie Holland, Assistant Professor and Extension Specialist at the University of Wisconsin-Madison, this stage is when leaves, blossoms, and young fruit are especially vulnerable to fungal diseases such as black rot, Phomopsis cane and leaf spot, powdery mildew, downy mildew, and Botrytis gray mold.
Fungal infections that occur during this period can remain hidden for weeks before symptoms appear later in the growing season or near harvest.
Some pathogens infect flowers and developing fruit before becoming dormant, while others establish infections that continue spreading throughout the season.
Because of this, maintaining a consistent fungicide program is essential, and skipping applications or extending spray intervals can increase disease risk.
During the immediate pre-bloom and early bloom stages, growers should prioritize protection against black rot, Phomopsis, and powdery mildew. Black rot remains a major economic concern, particularly in vineyards with a history of the disease or susceptible grape varieties.
Phomopsis can also infect fruit stems, creating problems that are difficult to manage later. Powdery mildew control is equally important, especially for susceptible cultivars such as Brianna, Frontenac, Frontenac Gris, and Marquette.
Downy mildew should also be monitored when warm temperatures, rainfall, extended leaf wetness, and humid nights create favorable conditions. In these situations, fungicides specifically labeled for downy mildew may be needed alongside broader-spectrum products.
Experts recommend thorough spray coverage around developing fruit clusters and rotating fungicide modes of action to reduce resistance.
Protectant products like mancozeb or captan combined with sterol inhibitor fungicides can provide broad protection, while pre-mixed fungicides may simplify spray programs.
Growers should also carefully review product labels, as fungicides containing difenoconazole may cause injury to certain non-vinifera grape cultivars and hybrids.
Photo Credit: istock-jhvephoto
Categories: Wisconsin, Crops, Fruits and Vegetables