Brown spot of rhododendron

Brown spot is a common leaf spot disease of azalea. It is common all over the country, especially in Guangzhou, Suzhou, Nanjing, Lianyungang, Wuhan, Hefei, Shenyang and other places. The disease not only harms rhododendron, but also harms other rhododendron plants.

Symptoms: Brown spot disease mainly occurs on leaves. The leaves initially produce reddish brown dots. Then gradually expanded to nearly round or due to the limitation of veins were polygonal spots. The diameter of the lesion is 2 - 5 mm, it is black brown, and the center of the late lesion becomes yellow-white to gray-white, and the edge is dark brown (depending on the variety). The surface of the disease spots are gray brown mold clusters, and when the disease spots are serious, they are connected into pieces, and the leaves are yellow and fall off. Pathogenesis: The disease is caused by Cercospora rhododendron infection. Pathogens overwinter on diseased leaves or diseased bodies. The following spring, the temperature rises and spreads with the wind. Generally, the disease starts from April to May and stops spreading until the end of December. In Guangzhou area, azalea brown spot is most serious in April and July. Greenhouse potted rhododendrons can all year round disease. High temperature and wet conditions are conducive to disease occurrence. Rhododendron like acid soil, in the slightly alkaline and poor drainage clay soil areas, Rhododendron growth weak, easy to disease. There are some differences in disease resistance among different varieties of Rhododendron. Western cuckoos are usually more susceptible to disease than spring cuckoos. According to Fujian Province Academy of Agricultural Sciences reported in the local planting of Wubaozhu, Huantianxidi and other varieties of disease resistance.

Control methods: ① Strengthen cultivation management, should be appropriate to add ferrous sulfate, especially for alkaline soil. Keep the soil moist, loose and avoid ponding. Remove the diseased leaves in time and eliminate the diseased leaves in the basin soil or on the ground, and burn them intensively. Keep ventilation between plants, light transmission and reduce humidity to eliminate disease conditions.② Chemical control: During the disease period, spraying 70% thiophanate-methyl 1000 times solution has good control effect, and other fungicides can also be applied.③ Selection of disease-resistant varieties: select varieties with strong disease resistance suitable for local cultivation, such as Antarctic and Baiyu Xingjin planted in Nanjing. Wubaozhu, Huantianxidi and other varieties planted in Fujian were more resistant to disease.