Comprehensive Prevention and Control Methods and Techniques for Common Rhododendron Diseases

The main diseases of rhododendrons include root rot, brown spot, black spot, leaf blight, and iron deficiency chlorosis.

After a rhododendron contracts root rot, its growth weakens, the leaves wilt and dry up, and water-soaked brown patches appear on the root surface; in severe cases soft rot develops, with gradual decay and peeling of the bark, and the xylem turns black. This disease occurs most easily under conditions of high temperature and high humidity.

The control method is to strictly disinfect the potting soil before repotting, and to keep the soil loose and moist with good permeability, avoiding waterlogging. If an affected plant is found, promptly deal with the diseased plant and the potting soil. For treatment, the diseased plant can be washed with a 0.1% potassium permanganate aqueous solution or 2% ferrous sulfate, then rinsed with clean water and repotted. A solution made from 70% thiophanate-methyl wettable powder plus 1000 times water, sprayed onto the potting soil, can cure it.

Brown spot is one of the main diseases of rhododendrons. When the disease first appears, small brown spots appear on the leaf surface, gradually developing into irregular large spots; many small black or grayish-brown dots form on the lesions, causing the affected leaves to turn yellow and drop, which affects flowering that year and the development of flower buds the following year. This disease often occurs during the plum rain season when humidity is high.

The treatment method is to normally ensure the plant has ventilation and light, avoid excessive humidity, and apply more organic fertilizer and a mixed fertilizer of nitrogen, phosphorus, and potassium to enhance the plant's resistance to infection and its growth capacity. If diseased leaves are found, promptly remove them and burn them in a centralized manner. In the early stage of the disease, spray 0.5% Bordeaux mixture or 0.4 degrees Baume lime sulfur mixture, with 4% flour added to increase adhesion. Leaf spot and black spot can also be treated with the same method.

Iron deficiency chlorosis often occurs in areas where the soil is alkaline. In mild cases, only delayed greening of the plant appears; in severe cases, the leaf tissue can all turn yellow and the leaf margins become scorched and withered. When the disease occurs, it is most obvious on the leaves at the top shoots of the plant, and it is generally caused by internal iron deficiency.

The control method is to change the iron-deficient nature of the soil and reduce soil alkalinity. Apply more organic fertilizer to improve clayey soil. For iron-deficient plants, a 0.2%-0.3% ferrous sulfate solution can be sprayed directly. You can also use chopsticks to poke several holes about 15 cm deep in the soil around the plant, slowly inject a 1:30 ferrous sulfate aqueous solution, and fill the holes to increase soil acidity and reduce alkalinity.

The onset situation and environmental conditions of rhododendron black spot are similar to those of brown spot, and similar methods can be used for its control.