Spraying fungicides when there is a light breeze, like two to six miles per hour at nozzle level, is actually better than spraying during still conditions, because even without wind there may be updrafts and eddies caused by the spray equipment leading to fungicide off-target movement. A little wind will aid deposition; you will know where the spray is going and can adjust your spray configuration accordingly. If conditions are not good for spraying, it is better to wait an extra day for better conditions than to lose most of the product to drift.

Another important timing consideration is the time of day, particularly as it relates to temperature and humidity relationships. Both temperature and humidity can affect fungicide drift. The higher the temperature and lower the relative humidity, the greater the opportunity for fungicide evaporation or volatilization. This can be avoided by spraying early in the morning when temperatures are lower and the relative humidity is higher. In addition to the reduced drift hazard from fungicide volatilization early in the morning, drift may also be minimized in the morning hours due to calmer winds and lower convective air turbulence.

If you are not getting the control you used to get with a particular product, do consider the possibility of fungicide resistance. This could be an issue with pathogens that have a high reproductive potential (powdery mildews, downy mildews and rusts) that have been exposed to repeated sprays of systemic fungicides, such as the strobilurins (Abound, Cabrio, Flint, Pristine), sterol inhibitors (Rally, Elite, Procure, Indar, etc.), benzimidazoles (Topsin M), phenylamides (Ridomil) and dicarboximides (iprodione). In grape powdery mildew in Michigan, strobilurin resistance has been demonstrated, and sterol inhibitor resistance is suspected. Furthermore, we also suspect strobilurin resistance in grape downy mildew. However, you must first rule out poor spray timing and coverage as possible causes of poor fungicide performance. Ways to reduce the risk of fungicide resistance include alternating or tank-mixing fungicides with different modes of action and avoiding spraying systemic fungicides at below-label rates. Also, try to avoid applying systemic fungicides on heavily sporulating colonies as the probability of selecting mutants increases with higher pathogen population densities. Use a contact fungicide (JMS Stylet Oil, Oxidate, Kaligreen) first or in the tank-mix if appropriate to kill spores.

Tank-mixing certain fungicides can increase the risk of phytotoxicity. For instance, do not tank-mix sulfur and oil-containing products nor spray them within two weeks of each other. Do not apply sulfur to sensitive crop cultivars or at temperatures above 80°F. Do not apply copper under cool, slow-drying conditions. Also, do not tank-mix phosphorous acids (Prophyt, Phostrol) with copper, as copper can become more available and phytotoxic in acidic solutions. Also, do not apply phosphorous acids to plants that are stressed (i.e., due to heat or drought stress). Tank-mixing products containing difenoconazole (Revus Top, Inspire Super) with non-ionic surfactants or foliar fertilizers can increase the risk of phytotoxicity, particularly on succulent, fast-growing plant tissues. Some crop cultivars are sensitive to strobilurins – ‘Concord’ grapes are sensitive to Pristine and Flint. Captan can become phytotoxic when applied with oil or within four days of an oil spray.