Occurrence and Control of Powdery Mildew on Japanese Euonymus

Symptoms: The disease affects leaves and twigs. The lesions are mostly distributed on the upper surface of the leaves. In the early stage of the disease, many small white round spots are scattered on the infected leaves. As the lesions gradually expand and merge with each other, they become irregular large spots. When the disease occurs severely, the infection rate of new shoots can reach 10%. White powder appears on the entire surface of the leaves and new shoots. The diseased leaves gradually lose green color, turn yellow, and finally turn brown-black. The diseased parts are deformed, the diseased leaves shrink, and the diseased shoots are twisted and stunted.

Pathogen: The pathogen is Oidium euonymi (Are.) Sacc., belonging to the Deuteromycotina, Hyphomycetes, Moniliales, Oidium fungus.

Occurrence pattern: The pathogen overwinters in the diseased plant body as mycelium. In the spring of the following year, the pathogen produces a large number of conidia, which spread by air currents and cause infection. The conidia can be produced repeatedly for re-infection. The pathogen parasitizes the surface of the host branches and leaves and produces haustoria that penetrate deep into the epidermal cells to absorb nutrients. High temperatures in summer are not conducive to the development of the disease. Plants planted under tree shade are severely affected, while adjacent plants facing the sun are less affected or not affected. Young leaves and new shoots are severely affected, while old leaves are mildly affected.

Control: ① Reduce infection sources: In winter, remove fallen leaves and diseased shoots and destroy them centrally. ② Chemical control: In the early stage of the disease, spray 25% Triadimefon wettable powder at 1500-2000 times dilution, or 70% Thiophanate wettable powder at 800-1000 times dilution, or 0.3-0.5 degree Baume lime sulfur mixture, or 50% Tuzet wettable powder at 800 times dilution, once every 10-15 days, spraying 2-3 times.