Introduction to the control methods of rose powdery mildew

Rose powdery mildew mainly harms roses, Rosa spinosissima, and rugosa roses. This disease affects the leaves, young shoots, and flower buds of the plant, with an incidence rate of 50%—70%, and can cause flower buds to fail to open or flowers to become smaller. The pathogen is Sphaerotheca pannosa. The fungus overwinters as mycelium on the branches, buds, and fallen leaves of the host. In the following spring, if the temperature is suitable (18℃—25℃), it can produce large numbers of conidia that spread and infect. After overwintering through summer, in September—October, it infects again with the help of wind. Under conditions of a temperature of 20℃ and humidity of 97%, spores germinate in 2—4 hours, and powdery spores can form after 3 days. Generally, lower nighttime temperatures (15℃—16℃) and higher humidity (90%—99%) favor spore germination and infection. Higher daytime temperatures (23℃—27℃) and lower humidity (40%—70%) favor the formation and release of spores.

In early spring, the pathogen covers both the upper and lower surfaces of unfolded leaves with a white powdery layer. The leaves become wrinkled and curled back, thicken and turn purplish green, and gradually dry up and die. When leaves are infected during the growing season, small white powdery spots first appear, gradually expanding into round or irregular powdery spots. In severe cases, the powdery spots connect with each other into patches, the internodes shorten, the leaves shrivel and dry up, and the branches develop abnormally. The white powdery layer on the leaves, petioles, and prickles is very thick and difficult to peel off. The flower buds are covered with a full layer of white powder, shrivel and dry up, and the flowers are small and few, or even fail to bloom or bloom abnormally.

For control, first create a good environment and strengthen cultivation management: ensure sufficient sunlight, good ventilation, and watering during the day. Avoid applying too much nitrogen fertilizer, appropriately increase potassium fertilizer, and keep the soil well drained to improve disease resistance. During the dormant period, spray 2—3 degrees lime sulfur mixture to eliminate overwintering mycelium in diseased buds or cleistothecia in diseased parts. During the disease period, thoroughly prune out diseased branches, leaves, and buds, and remove surrounding rotted diseased leaves to reduce sources of infection. In combination with pruning diseased branches, alternately spray 70% thiophanate-methyl wettable powder at 700—800 times dilution, or 50% carbendazim solution.

The white powdery substance on the surface of diseased plants is very easily spread by wind. Failure to control it promptly, incomplete cleanup of diseased branches, or a windy environment favors the spread of the pathogen and reinfection, making it difficult to control. Therefore, observe frequently, detect it early, and spray promptly for control to achieve good results.