Severe pruning and cutting back of plants to promote new growth actually favors powdery mildew infection. The fungus is spread by conidia that are dispersed by air currents or splashing water in greatest number during the middle of the day. Powdery mildew is actually caused by numerous different fungi, although symptoms of infection are identical. Control for powdery mildew is the same regardless of fungal species present. Since the disease is favored by high humidity and poor air circulation, increase plant spacing, avoid watering late in the day which can increase humidity levels and reduce humidity by increasing air circulation and venting the greenhouse late in the day to exchange warm, wet air with drier air from the outside to reduce infection.
Fungicides are important and useful in preventing and controlling powdery mildew.
Several systemic fungicides including fenarimol, myclobutanil, propiconizole, thiophanate methyl, triadimefon, and triforine provide good powdery mildew control. Fungicide resistance to some of these products has been identified in powdery mildew fungi. So if there is no improvement in disease development, resistance may be a factor and use a different product. For the most part, fungicides act as a preventative. They prevent infections from occurring, they do not get rid of existing infections.