Symptoms and Control Methods of Brown Spot Disease in the Medicinal Plant Licorice

Licorice, scientific name Glycyrrhiza uralensis Fisch., also known as meicao and micao, is a perennial herbaceous plant in the Fabaceae family. Its roots and rhizomes are used as medicine. It has the effects of harmonizing the middle and relieving urgency, moistening the lungs and detoxifying, and harmonizing various medicines. It is produced in Inner Mongolia, Gansu, Shanxi, Shaanxi, Liaoning, Jilin, Heilongjiang, Hebei and other provinces.

The symptoms mainly damage the leaves. The lesions on the leaves are round or irregular, 1-3 mm in size, with a grayish-brown center and brown edges, and grayish-black mold-like substances, namely the pathogen's conidiophores and conidia, grow on both sides.

The pathogen Cercospora astragalis Woronichin Fisch. is called Astragalus Cercospora and belongs to the Deuteromycotina fungi. The fruiting bodies grow on both sides of the leaves, mostly on the leaf surface. The stroma is composed of a few brown cells. The conidiophores are borne in bundles of 6-12, light brown, unbranched, with 0-5 geniculate nodes, nearly truncate at the apex, with obvious spore scars, and 1-7 septa, measuring 24-71 x 4-5.5 (μm); the conidia are whip-shaped, colorless, with 3-10 septa, measuring 32-80 x 3-4.5 (μm). This fungus also infects Astragalus and purple vetch.

Transmission routes and disease conditions: The pathogen overwinters as mycelium on diseased residues. When conditions are suitable the following spring, it produces conidia, which spread through wind and rain to cause primary infection. Later, the diseased parts produce spores again to cause secondary infection. It occurs in Inner Mongolia, Liaoning, Heilongjiang and other provinces from July to August, and the disease is severe when there is heavy autumn rain and heavy dew.

Control methods: (1) Clean the garden in autumn and promptly dispose of diseased residues to reduce the source of primary infection. (2) Spray 70% mancozeb wettable powder at 500 times dilution in the early stage of the disease.