As many southern Wisconsin vineyards approach veraison, especially on early ripening varieties, this marks a critical window for grape disease management. Veraison signals the start of fruit ripening, when grapes begin to soften, change color, and accumulate sugars. Unfortunately, this also coincides with increased vulnerability to certain late-season grape diseases, particularly in wet and humid conditions like we’ve been experiencing recently.

After veraison, grape berries become increasingly susceptible to cluster rots such as Botrytis bunch rot and sour rot, while they develop ontogenic resistance to powdery mildew and downy mildew, typically around 6 to 8 weeks post-bloom. However, foliar tissues remain susceptible to both powdery and downy mildew for the entire season, meaning foliage protection must continue until harvest.

Botrytis (gray mold) is generally more problematic in tight-clustered, thin-skinned varieties, such as Vitis vinifera and many French hybrids. Though less common in Wisconsin cold-climate varieties, this year’s rainfall has raised concern. Key fungicide timings are at bloom, bunch closure, and veraison. Applications post-veraison tend to be less effective, so early action is warranted. Look for signs of Botrytis now, such as brown, water-soaked lesions or gray spore masses, especially in dense clusters or shaded areas.

Sour rot is the more common late-season cluster disease in cold-climate varieties and is heavily influenced by:

• Rain-related berry splitting
• Bird damage
• Insect feeding, particularly by Drosophila fruit flies

Sour rot generally appears after berries reach ~15° Brix, when sugar levels support rapid microbial activity. The disease is caused by a complex interaction of acetic acid bacteria, yeasts, and fruit flies. With current weather patterns (high humidity + warm temperatures), early symptoms may already be present in damaged clusters.

Integrated management for sour rot includes, timely insecticide applications targeting fruit flies, maintaining canopy openness, minimizing cluster injuries, and avoiding overly tight cluster formations (which may still be mitigated now with cluster thinning).

Rainy, humid weather creates ideal conditions for fungal growth. To help reduce disease pressure:

• Shoot thinning and strategic leaf removal around clusters can improve airflow and sunlight penetration.
• Cluster thinning at veraison can reduce cluster compaction and enhance fungicide coverage while improving fruit quality.
• Trellis management should continue to encourage vertical shoot positioning and discourage shaded microclimates.

Be aware: bird netting, while important to reduce bird-related berry damage (a sour rot entry point), can also impede fungicide coverage. Adjust spray volumes and nozzle configurations to compensate.

Fungicides at Veraison

At veraison, fungicide applications in cold-climate vineyards play a critical role in protecting grape clusters from several late-season diseases that can significantly impact fruit quality and yield. The primary disease concerns at this stage include Botrytis bunch rot, downy mildew, powdery mildew, and, under the right conditions, sour rot. Disease pressure varies depending on cultivar, canopy density, and weather, but humid or wet conditions at veraison can sharply increase risk, especially in tight-clustered varieties such as Marquette and Frontenac.

To manage Botrytis bunch rot, growers should consider applying a Botrytis-specific fungicide at veraison, particularly for cultivars with tight cluster architecture. Effective options include products such as Switch (FRAC 9 + 12), Luna Experience (FRAC 3 + 7), Vangard (FRAC 9), Elevate (FRAC 17), and Kenja (FRAC 7) or Scala (FRAC 9). These products should be rotated by FRAC code to help manage fungicide resistance. Broad-spectrum materials like Captan also offer some activity against Botrytis and may complement a rotation.

For downy mildew, the disease may still be active at veraison, especially under wet conditions or in dense canopies. Targeted fungicides include Zampro (FRAC 40 + 45), Revus (FRAC 40), and Ranman (FRAC 21). Phosphonate products like ProPhyt (FRAC 33) can also provide good control and can be included in a resistance management program. Copper products are another option, particularly for organic growers, although their activity is limited in high disease pressure situations.

Powdery mildew also requires attention at this stage to protect ripening fruit. Options include Vivando (FRAC 50), Cevya or other DMI fungicides (FRAC 3), and sulfur or stylet oils for growers looking for multi-site or contact materials. Sulfur should be used with caution in high temperatures, and oils should not be applied too close to sulfur to avoid phytotoxicity.

Finally, sour rot becomes a risk in warm, wet conditions and is most common when fruit begins to soften. Some management can be achieved using a combination of antimicrobials (e.g., Oxidate) and insecticides targeting fruit flies (e.g., Mustang Maxx), which are key vectors in sour rot outbreaks.

In addition to fungicide use, cultural practices such as cluster-zone leaf removal and canopy thinning can help reduce disease pressure by improving air flow and spray penetration, especially in highly vigorous cold-climate cultivars. As always, growers should tighten spray intervals during periods of active canopy growth or frequent rainfall and be sure to rotate fungicides by FRAC code to mitigate the development of resistance.

This article was posted in Disease, Grapes and tagged disease, Grapes, Leslie Holland, Veraison Disease Management.