Occurrence Patterns and Control Methods of Black Spot Disease in Impatiens

Black spot disease of Impatiens is an important disease in the production and cultivation of Impatiens. It occurs commonly in our country and causes serious damage, so it should be taken seriously.

Symptoms: The disease damages leaves, especially the older leaves in the middle and lower parts, which are susceptible to infection. Affected leaves develop circular or oval spots that are dark brown, 2-10 mm in diameter, with inconspicuous concentric rings and darker edges. When the disease is severe, the spots can expand and merge into patches, and on the spots appear pale green or olive-black mold-like growth, which is the pathogen's conidiophores and conidia. This causes the diseased leaves to shrivel and die.

Pathogen: Alternaria tenuis Nees., belonging to the Deuteromycotina, genus Alternaria, a fungus. The conidiophores are erect, brown, with flexuous bends, and the apex often enlarges and bears conidial scars, measuring 5-125 um x 3-6 um. They have beaks, dark brown, measuring 1-58.5 um x 1.5-7.5 um, with 1-9 transverse septa and 0-6 longitudinal septa.

Occurrence patterns: The pathogen overwinters as mycelium on diseased plant residues. In the following spring, when conditions are suitable, it produces conidia for primary infection and re-infection. It is generally spread by airflow and prefers high-humidity environments. Rainy weather and heavy dew favor disease occurrence and epidemics. In Northeast China, the peak period of disease occurrence is generally July-August, and it gradually decreases in early to mid-September.

Control methods

(1) Agricultural control: Strengthen cultivation management, apply fertilizer and irrigate reasonably, maintain appropriate ventilation and air permeability, and improve plant disease resistance. Drain promptly after rain, remove diseased residues, and pick off lower old leaves and diseased leaves. Bury them deeply in a centralized place or burn them to reduce the amount of overwintering inoculum and improve the soil.

(2) Implement a crop rotation system.

(3) Chemical control: Finally, spray 1% Bordeaux mixture once before disease onset for protection. After disease onset, spray 50% carbendazim or 50% thiophanate-methyl wettable powder at 800-1000 times dilution, or 50% Tuzet at 800 times dilution.

Commonly used agents: carbendazim, thiophanate-methyl wettable powder, Tuzet.