Use an appropriate fungicide for the diseases you want to control.Regularly scout vineyards to know what diseases are present and if you are not sure what the problem is, send a sample to MSU Diagnostic Services. Use disease models if they are available (i.e., in the Enviro-weather system) to estimate disease risk so as to optimize application timing. For best post-infection activity, apply fungicides as soon as possible after an infection period, weather permitting.

During rainy periods, systemic fungicides perform better than protectant or contact fungicides since they are less susceptible to wash-off by rain (although eventually, a portion of systemic fungicides also gets washed off by rain). To get the most curative (post-infection) action from a systemic fungicide, apply the highest labeled rate since activity is concentration dependent. The same goes for preventive action; usually a higher rate often extends the residual period. You may need to reapply protectant fungicides after 1 to 2 inches of rain. Systemic fungicides can last a bit longer.

Add an adjuvant (surfactant, sticker, penetrant) if recommended on the label. Some adjuvants have ultraviolet (UV) light blockers that slow UV degradation of pesticides. However, many modern fungicides are formulated in such a way that adjuvants are not necessary. In fact, some fungicide labels prohibit the use of adjuvants due to the increased potential for phytotoxicity. In our experience, most biocontrol agents (e.g., Serenade) do benefit from the use of stickers or extenders, such as Nu-Film P. In some cases, adjuvants have a disease-suppressive effect because they are detrimental to pathogens; i.e., surfactants zoospores of downy mildew pathogens to explode.

Apply protectant fungicides (e.g., Captan, Ziram, Manzate) during sunny, dry conditions to allow for quick drying onto the leaves. In fact, it appears that protectant fungicides become better adsorbed to the plant surface and more rainfast over several days after application. Any dew formation at night will help redistribute product over the plant surface. While it is better to have protectant fungicide applications on before a rain or heavy dew event which could represent an infection period, avoid putting on protectant fungicides within several hours before a rainstorm as you may lose much of it to wash-off.

Apply systemic fungicides (e.g., Elite, Abound, ProPhyt, etc.) under humid, cloudy conditions when the soil is moist. That way, the cuticle, or the waxy layer covering the plant surface, will be swelled up and allow the active ingredients to quickly pass through. Under extended hot, dry conditions, the cuticle becomes flattened and less permeable; any product that is not absorbed may remain on the plant surface and break down due to UV light or microbes or get washed off by rain.