Occurrence and Control Methods of Brown Spot Disease in Rhododendrons

Brown spot disease is a common leaf spot disease of rhododendrons. It occurs fairly commonly throughout the country, and is especially severe in Guangzhou, Suzhou, Nanjing, Lianyungang, Wuhan, Hefei, Shenyang and other places. Besides harming rhododendrons, this disease also harms other plants of the genus Rhododendron.

Symptom characteristics: Brown spot disease mainly occurs on the leaves. Initially, small reddish-brown spots appear on the leaf surface. They then gradually expand into nearly circular lesions, or polygonal lesions due to restriction by the leaf veins. The lesions are 2-5 mm in diameter and dark brown; in the later stage, the center of the lesion turns yellowish-white to grayish-white, with a dark brown edge (varying by variety). Grayish-brown mold clusters grow on the surface of the lesions. In severe cases, the lesions join together into patches, and the leaves turn yellow, wither and fall off. Disease pattern: The disease is caused by infection with the fungus Cercospora rhododendri. The pathogen overwinters on diseased leaves or diseased plant debris. The following spring, as the temperature rises, it spreads with the wind. Generally, the disease begins in April-May and stops spreading by the end of December. In the Guangzhou area, rhododendron brown spot disease is most severe from April to July. Potted rhododendrons in greenhouses can develop the disease all year round. High temperature and high humidity conditions favor the occurrence of the disease. Rhododendrons prefer acidic soil; in areas with slightly alkaline and poorly drained clay soil, rhododendrons grow weakly and are prone to the disease. Different rhododendron varieties differ somewhat in disease resistance. Generally, Western rhododendrons are more susceptible to the disease than Spring rhododendrons. According to a report by the Fujian Academy of Agricultural Sciences, varieties such as Wubaozhu and Huantiangxidi grown locally are relatively resistant to the disease.

Control methods: (1) Strengthen cultivation management; appropriately apply additional ferrous sulfate, especially for slightly alkaline soil. Keep the soil moist and loose, and avoid waterlogging. Promptly remove diseased leaves and clear away diseased fallen leaves in the potting soil or on the ground, and burn them in a centralized manner. Keep the plants well ventilated and exposed to light to reduce humidity, thereby eliminating conditions favorable to the disease. (2) Chemical control: During the disease period, spraying a 1000-fold dilution of 70% thiophanate-methyl has a good control effect; other fungicides may also be applied. (3) Select resistant varieties: Choose varieties with relatively strong disease resistance suitable for local cultivation, such as Nanjing and Baiyuxingjin grown in Nanjing. Varieties such as Wubaozhu and Huantiangxidi planted in Fujian are all relatively resistant to the disease.