Identification and Control Methods for Geranium Alternaria Leaf Spot

Geranium Alternaria leaf spot is a common leaf disease of geranium. The pathogen has a wide host range and can attack many woody and herbaceous ornamental flowers, seriously affecting their ornamental value.

Symptoms: In the early stage of the disease, chlorotic spots appear on infected leaves, and the lesions gradually expand to form larger circular spots. The spots are dark brown with concentric rings. In the late stage of the disease, a dark green mold layer appears on the spots, which consists of the conidia of the pathogen.

Pathogen: The pathogen is Alternaria tenuis Nees, belonging to the Deuteromycotina, Hyphomycetes, Hyphomycetales, genus Alternaria.

Disease cycle: The pathogen overwinters as mycelium in diseased tissue. When environmental conditions are suitable, it produces conidia, which spread by wind and rain to cause infection. During the growing season, the conidia can reinfect plants repeatedly.

Control: (1) Reduce sources of infection: promptly prune diseased leaves when found and destroy them together with the diseased leaves. (2) Chemical control: In the early stage of the disease, spray 1:1:200 Bordeaux mixture, or 50% Tuzet wettable powder, or 65% zineb wettable powder at 600-800 times dilution, once every 10-15 days, spraying 2-3 times.