How to control leaf spot disease in hydrangea (Impatiens)
[Alias] Impatiens brown spot disease. Hydrangea leaf spot is one of the common diseases on hydrangea.
[Distribution] Sichuan, Henan, Guangdong, Taiwan, China, Jilin, Shanghai, Zhejiang, Hunan and other places.
[Symptoms] The disease mainly occurs on leaves. Leaf spots initially appear as light yellowish-brown water-soaked small dots, then expand into circular or nearly circular shapes, 2-5 mm in diameter, with a maximum of about 15 mm. Later the center becomes light brown, the edge dark brown, slightly raised, with inconspicuous concentric rings, and black small dots, which are the fungus's pycnidia, are produced on the diseased area. On severely infected leaves, the spots merge together, causing the leaves to turn brown and withered yellow, until the plant dies. It occurs from August to September.
[Pathogen] The pathogen is a fungus, Cercospora fukushiana, belonging to the Deuteromycotina, genus Cercospora, class Coelomycetes, order Sphaeropsidales. The stroma is not well developed; conidiophores are clustered, light brown, rarely septate or branched, straight or wavy, or with 1-3 geniculate bends, measuring 10-100 um x 4-6 um. Conidia are narrowly club-shaped, straight or curved, tapering toward the apex, with a truncate base, with many septa but inconspicuous, the basal end flatly cut, the apex pointed, 30-140 um x 3-4.5 um.
[Disease pattern] The pathogen overwinters on diseased leaves and plant debris in the soil. The following year, when environmental conditions are suitable, conidia are produced from the pycnidia, and the conidia spread by wind and rain. A large number of conidia are produced on new lesions and spread by airflow, causing multiple reinfections. High humidity and rainy conditions are favorable for the occurrence and spread of the disease. Generally, the disease occurs from August to September and gradually decreases in October. Old leaves are susceptible to the disease.
[Control methods]
(1) Horticultural control: Impatiens prefers fertile sandy loam and is not tolerant of waterlogging. Therefore, planting in sandy loam is suitable to facilitate drainage; for potted impatiens, the pot should be tipped promptly after rain. In late autumn, diseased leaves and diseased plants should be collected and destroyed to reduce the source of infection the following year.
(2) Collect seeds from disease-free, healthy plants, and disinfect the seeds with potassium permanganate or zineb before sowing.
(3) Chemical control: Before the disease occurs, spray 0.5% Bordeaux mixture for prevention. Remove diseased leaves promptly when found early. Spray for control at the early stage of the disease or when the disease is severe. You may choose 50% carbendazim 500 times dilution, or 75% chlorothalonil 700 times dilution, or 25% carbendazim wettable powder 300-600 times dilution, or 50% thiophanate-methyl 1000 times dilution, or 50% zineb 800-1000 times dilution, or 50% Wanmeiling 600 times dilution. Alternatively, spray 40% carbendazim-sulfur suspension 500-600 times dilution, 50% mixed sulfur suspension 500 times dilution, 1:1:200 Bordeaux mixture, 77% Keshake wettable powder 500 times dilution, 50% copper fatty acid amide (DT) wettable powder 400-500 times dilution, once every about 10 days, for 2-3 applications.