How to Prevent and Treat Powdery Mildew on Zinnias? A Complete Guide to Practical Methods

Powdery mildew on zinnias occurs in all parts of China and is one of the main diseases of zinnias. On severely affected plants, the leaves turn yellow and wither, and the flowers wither as soon as they bloom. This seriously affects the ornamental value of the plants.

Symptoms: The disease mainly affects the leaves, with symptoms occurring primarily on mature leaves. In the early stage of the disease, scattered white powdery spots appear on the surface of infected leaves. As the disease develops, the leaf surface becomes covered with a grayish-white powdery substance, and in autumn small black dots are scattered within the white powder; these are the cleistothecia of the pathogen. In the late stage of the disease, the affected leaves turn yellow, shrivel and die.

Pathogen: The pathogen is a fungus of the genus Sphaerotheca (Sphaerotheca fuliginea Schlecht), belonging to the Ascomycota, Pyrenomycetes, Erysiphales.

Disease pattern: The pathogen overwinters as mycelium and cleistothecia on diseased plant residues. When the temperature is suitable the following year, it produces conidia and ascospores, and the spores spread by wind and rain and begin to infect plants. The disease generally can occur from May to October. In particular, August to September, when the temperature is high and humidity is great, is the peak period of the disease.

Prevention and treatment: 1. Reduce sources of infection: promptly pull out diseased plants and destroy them. 2. Choose a site with good ventilation and drainage for planting. Do not plant too densely. 3. Chemical control: in the early stage of the disease, spray 15% triadimefon wettable powder at 1000 times dilution, or 70% thiophanate-methyl wettable powder at 1000 times dilution. At the same time, attention should also be paid to powdery mildew on other nearby plants to prevent cross-infection.