What are the control methods for azalea leaf spot?

1. Symptoms

Small reddish-brown spots first appear on diseased leaves, gradually expanding into nearly circular spots, or restricted by leaf veins into polygonal lesions, 1-5 mm in diameter, dark brown; sometimes grayish-white in the center with indistinct edges. The spots are darker on the upper surface and lighter on the lower surface of the leaf, and lesions at the leaf margin may merge with one another; when humid, grayish-black mold spots mostly form on the surface of the diseased areas.

2. Pathogen

The pathogen of azalea leaf spot is Cercospora rhododendri Ferraris, belonging to the order Hyphomycetales, class Hyphomycetes. The grayish-black mold spots on the lesions are the stroma and conidia of the pathogen. The conidia are light brown with 1-4 septa. The conidia are borne at the tips of conidiophores, whip-shaped, with pointed tips, slightly curved, and mostly septate after maturity.

3. Occurrence and development pattern

The pathogen overwinters as mycelial masses on diseased leaves or plant residues. When the temperature is suitable the following year, conidia are produced and spread by rain or wind. The spores fall on leaves and, upon contact with dew or water droplets, produce germ tubes that invade the leaf tissue. It generally occurs more severely from April to July.

4. Control methods

(1) Strengthen nursery management: Remove diseased leaves and clear away fallen leaves on the ground or in pots, and burn them in a centralized manner. Maintain ventilation and light transmittance in summer, and avoid excessive humidity.

(2) Chemical control: During the disease period, spray 50% carbendazim wettable powder at 500-800 times dilution, or 50% thiophanate at 800-1000 times dilution, or spray with 1:1:100 Bordeaux mixture.