Identification and Control Methods of Common Fungal Diseases of Azaleas

Among the common azalea diseases, the main fungal diseases include azalea powdery mildew, rust, gray leaf spot, blight, and leaf gall, among others. These diseases mainly affect the leaves, young branches, buds and floral organs of azaleas, and when not controlled in time they often cause great losses.

(1) The main differences among the following diseases are: 1. Azalea powdery mildew: generally occurs in the middle and late stages of growth, initially appearing as chlorotic spots, then growing a white mycelial layer and producing large amounts of powdery conidial layers, turning grayish brown in the later stage 2. Azalea rust: in the early stage, transparent golden or light brown spore masses appear in spots and patches on the underside of leaves, and in the middle and late stages yellow-brown or red leaf spots appear on the leaves. 3. Azalea gray leaf spot (anthracnose): the leaf spots are relatively large, silvery gray on the upper surface, dark brown at the edges, light brown on the underside, with raised small black dots. It mostly occurs on areas affected by sunscald and frost damage. 4. Azalea blight: generally the base of the main root and stem turns brown and rots, and the terminal buds and leaves turn brown, curl and droop; young branches develop cankers and shrivel, often resulting in "dead top". After the stem base rots in a ring, the branches and leaves above wither. 5. Azalea leaf gall (leaf swelling disease): the affected young leaves, new shoots and flower buds become thick and swollen, often forming galls (10-20 mm in cross-section) that are pale white; in severe cases the leaves become deformed, new shoots cluster, buds become spherical, and flowers form waxy galls, with the surface of the diseased parts grayish white or pink.

(2) Control of azalea fungal diseases: the conditions for the occurrence of azalea fungal diseases are high humidity, poor ventilation, insufficient light, as well as frost injury and sunscald. Secondly, there is the year-after-year accumulation of inoculum from successive years of cultivation, as well as excessive application of nitrogen fertilizer and overly frequent watering. The main control methods in cultivation are: 1. Improve the ecological environment of azaleas, strengthen ventilation and light penetration, reduce wounds, and replace potting soil. Apply compound fertilizer. Promptly prune diseased and weakened branches. 2. Chemical control: for azalea powdery mildew and rust, use 25% triadimefon at 1500 times dilution; or use 70% thiophanate-methyl wettable powder at 1000 times dilution. For control of azalea gray leaf spot: use 70% thiophanate-methyl at 1000 times dilution; or use 75% chlorothalonil at 600 times dilution. For control of azalea blight: use 1% Bordeaux mixture; or spray with 0.1-0.2% copper sulfate solution. Metalaxyl at 600 times dilution can also be used. Chlorothalonil at 600 times dilution for control. For control of azalea leaf gall: spray with 100-150 times equal-amount Bordeaux mixture or 0.5 degree lime sulfur mixture.