How to prevent and control gray leaf spot when growing Isatis indigotica

Symptoms: On both sides of the leaves of affected Isatis indigotica, round lesions form, 2-6 cm in diameter, slightly sunken, grayish-white in the center, with brown edges. In the later stage, the lesions become thin and brittle, crack or perforate; when moist, a brown mold-like growth appears on them, which consists of the pathogen's conidiophores and conidia. Old leaves are affected first, and the disease spreads from the bottom upward. In severe cases, multiple lesions merge with one another, and the leaves wither early.

Pathogen: The pathogen is a fungus of the genus Cercospora (Cercospora sp.), belonging to the Deuteromycotina. Occurrence pattern: The pathogen overwinters with diseased plant residues and becomes the primary infection source the following year, or it spreads and expands in new cultivation areas through contaminated seeds. An average daily temperature of 23-25°C favors disease development, and the disease spreads rapidly. July to August is the peak period of incidence. Control methods: (1) Practice crop rotation and clean the field to reduce inoculum, strengthen field management, and carry out intertillage and weeding twice before the rows close. (2) After rain, open ditches for drainage to reduce field humidity. (3) In the early stage of disease, spray Bordeaux mixture (1:1:100) for protection, or spray 65% zineb 600-800 times dilution for control. During the disease period, spray 36% carbendazim wettable powder 500 times dilution or 40% chlorothalonil suspension concentrate 600 times dilution, 50% thiophanate-methyl wettable powder 600-700 times dilution, 50% carbendazim wettable powder 1500 times dilution, 65% jiaMeiLing wettable powder 100 times dilution, 50% duomeiling wettable powder 800 times dilution, spraying once every about 10 days, for 1 or 2 applications.