A Detailed Guide to the Prevention and Control Methods and Occurrence Patterns of Chrysanthemum Black Spot Disease

Chrysanthemum black spot disease is one of the most common leaf diseases in chrysanthemum cultivation. It is caused by a fungal infection and usually begins on the older leaves at the lower part of the plant. In the early stage of the disease, small pale yellow or purplish-brown spots appear on the leaf surface, then gradually expand into round or irregular dark brown lesions, and later small black granules also form on the lesions. When the disease is severe, the lesions merge into large patches, and the leaves turn yellow, wither, and drop off, clearly affecting the ornamental effect and the plant's growth.

The pathogen mainly overwinters latent in diseased plant debris or in the soil, and the following year it spreads through wind, rain, and splashing water, invading through leaf stomata or wounds. Disease tends to be more severe in plots with high temperature and high humidity, continuous rainy seasons, overly dense planting, poor ventilation, or excessive nitrogen fertilizer, so during the rainy season pay special attention to changes in the lower leaves and deal with diseased leaves promptly when found.

Routine control should focus mainly on cultivation management. Remove diseased leaves promptly and burn or bury them deeply in a centralized way, avoiding leaving debris in the soil where it can become a source of infection; water at the root zone as much as possible and splash the foliage less, and pay attention to drainage after rain; plant at a reasonable density and maintain ventilation and light penetration between plants; balance nitrogen, phosphorus, and potassium in fertilization to prevent excessive nitrogen fertilizer from making the branches and leaves tender and reducing disease resistance.

Chemical control should seize the early stage of the disease. Broad-spectrum fungicides such as carbendazim, mancozeb, and chlorothalonil can be selected for alternating spraying, generally once every seven to ten days, and two or three consecutive applications can basically bring the disease under control. Disinfecting the soil before potting cuttings and regularly spraying for protection during the growing period can significantly reduce the incidence of the disease.