Detailed Explanation of Control Methods for Licorice Leaf Spot Disease

① The pathogen is a type of imperfect fungus (Cercospora astragali), belonging to the subdivision Deuteromycotina → Moniliales → Cercospora fungus. It damages leaves; affected leaves produce circular or irregular lesions, with the center of the lesion grayish-brown and the edges brown, and grayish-black mold-like growth on both the front and back of the lesions. Fruiting bodies grow on both sides of the leaf, but mainly on the upper surface; the stroma consists of only a few brown cells; conidiophores are born in bundles of 6 to 12, pale brown, with the apex lighter in color and narrower, unbranched, having 0 to 5 geniculate nodes, with the apex nearly truncate, spore scars prominent, and 1 to 7 septa; conidia are whip-shaped to obclavate, colorless and transparent, straight to curved, with the base truncate to nearly truncate, the apex slightly blunt, and 3 to 10 septa.

② Symptoms: Lesions on the leaves of infected plants are circular to irregular in shape, 1 to 3 mm in diameter, with the center grayish-brown and the edges brown, and grayish-black mold-like growth (the fruiting bodies of the pathogen) on both sides.

③ Disease occurrence pattern: The pathogen overwinters as mycelium on the remnants of diseased plants. When conditions are suitable the following spring, conidia spread via wind and rain to cause primary infection. The conidia produced on the lesions then continue to multiply and cause secondary infection. It generally occurs in July to August.

④ Control measures: In autumn, clean the garden and centrally dispose of the remnants of diseased plants to reduce the source of the pathogen; spray a 200-fold dilution of non-toxic high-lipid film for protection; at the early stage of the disease, spray a 1:1:(100 to 160) Bordeaux mixture or a 1500- to 2000-fold dilution of 70% thiophanate-methyl wettable powder; during the disease period, spray a 500-fold dilution of 65% mancozeb 1 to 2 times.