As we approach veraison, Botrytis becomes a concern in susceptible varieties, especially tight-clustered grapes. However, continued dry, warm weather will also keep Botrytis at bay; the concern is if the weather changes and cool, wet conditions prevail. If needed, good control can be achieved with the reduced-risk fungicides Vangard and Elevate. These fungicides can be alternated for fungicide resistance management. A spray at veraison and one to two weeks prior to harvest is recommended.

Rainy weather in the weeks before harvest will increase chances of sour rot. Work by MSU’s Paolo Sabbatini has shown that berries can take up water rapidly through their skins, swell up and burst, leading to injuries that allow acetic acid bacteria and yeasts to enter the fruit and cause sour rot. For both Botrytis and sour rot management, it is not yet too late to pull leaves around fruit clusters, which will help to reduce disease pressure. However, be careful not to fully expose previously shaded clusters as strong radiation and high temperatures can cause severe scalding of berries.

With respect to fungicide use during hot, dry conditions, remember that in the absence of rain, high temperatures and solar radiation can still break down fungicides on the plant surface and, even though fungicide residues may still be visible, the active ingredient may have been reduced to non-effective levels. In addition, be careful with phosphites (Phostrol, ProPhyt) as leaf burning may occur when applied to drought – and heat-stressed vines and at high temperatures. Sulfur also should not be applied when temperatures are at or anticipated to be over 85 F. Remember that leaf temperatures can be substantially higher than air temperatures during the day.

The best times to apply systemic fungicides during this period is at night or in the early morning, especially when the soil is moist (i.e., after a rain event) and the cuticle is swelled up and permeable to fungicides. Avoid applying systemic fungicides during hot, dry conditions as they will not be taken up by the leavess and can be lost to evaporation and UV breakdown. Protectant fungicides, on the other hand, are OK to be applied during warm, dry conditions, as we want them to quickly dry onto and adhere to the leaves. If a tank-mix is applied, choose the conditions that favor uptake of the systemic component.