This disease is characterized by the dense, persistent and grayish patches of fungal growth on leaves, stems, inflorescences and fruits. Over time, the powdery coating causes the distortion and discoloration of leaves and premature leaf fall in severe cases. Buds often fail to unfold normally or be destroyed. Twigs may show a stunted growth. Fruits develop discoloration and lesions that prevents further normal development. In case of severe infections, the skin eventually turns dark brown, with the surface becoming leathery and hard, often subjected to cracks. Hosts include apricot, sour cherries and peach.
The disease is caused on roses and stone fruits by the fungus Podosphaera pannosa, which survives the winter in a circular, spore-producing structure. Under favorable conditions in spring, this structure breaks open and releases spores that are later spread by wind, insects, and rain. When they land on a susceptible host and germinates, they start producing the characteristic “powdery” appearance on leaves or fruits. Optimal conditions for powdery mildew of rose are 16-27 °C, with the optimal temperature for fungal growth at 23 °C. Shaded areas are also preferred since the prolonged exposure to light reduces its growth. Moreover, these fungi do not need moisture to germinate and infect their hosts. In fact, prolonged leaf moisture inhibits rather than promotes fungal growth. The use of silicon as part of a fertilization program can also reduce the occurrence of powdery mildew.
Products based on the antagonist fungus Ampelomyces quisqualis and the bacteria Bacillus subtilis have been used as biological agents to control Podosphaera pannosa. Several organic fungicides are also available in the form of horticultural oils, neem oil, jojoba oil, potassium bicarbonate and bicarbonate of soda.
Always consider an integrated approach with preventive measures and biological treatments if available. Formulations based on thiophanate-methyl, propiconazole and sulfur fungicides have been recommended for powdery mildews in general, including powdery mildew of rose. Other products that have been used with some success include dinitrophenols and compounds based on pyridine and pyrimidine.