What to Do When Impatiens Gets Powdery Mildew? A Detailed Guide to Prevention and Treatment

[Distribution] It occurs in all regions around the world.

[Symptoms] The disease mainly occurs on leaves and tender shoots, and in severe cases harms flower buds and capsules. In Beijing, the disease begins after June, and after July the leaf surface gradually becomes covered with a white powdery layer, gradually expanding until the entire leaf is covered with a white powdery layer; in the later stage, yellow small spots form on the powdery layer, and the color gradually deepens, turning dark brown; these are the cleistothecia of the pathogen, which appear spherical under a magnifying glass. The dark brown small spots mainly appear on the upper surface of the leaves, while spots on the underside are sparse. In severe cases, the leaves wither and drop early. In Shanghai, the disease is generally more severe after flowering. Affected plants are stunted, tender shoots are curved, leaves are uneven, and flowers are deformed.

[Pathogen] The pathogen is a fungus, Sphaerotheca balsaminae (powdery mildew of impatiens). The cleistothecia are spherical to nearly spherical, with mycelium-like appendages. Each cleistothecium contains 1 ascus; the ascus is short-elliptical or oval and contains 8 ascospores; the ascospores are elliptical, colorless, and single-celled.

[Disease pattern] The pathogen overwinters as cleistothecia on diseased plant residues, and also as ascospores and conidia. It spreads by wind. In Shanghai, the disease is more severe after flowering, and leaf drop occurs in October. Records from Beijing show that seedlings can be infected when the temperature is above 20°C, and after July the leaves gradually become covered with a white powdery layer. Powdery mildew is prone to occur under high temperature, low light, poor ventilation, and high air humidity.

[Prevention and treatment methods]

(1) Horticultural control: Do not plant too densely, provide appropriate ventilation, and strengthen light exposure; to improve disease resistance, increase the application of phosphorus fertilizer. After the flowering period ends, pull out the remaining plants, clear away fallen leaves, and destroy them promptly. Promptly prune out diseased parts or remove and destroy the entire plant.

(2) Chemical control: During the disease period, spray with 20% triadimefon 4000-fold solution, or 70% thiophanate-methyl 1000-1500-fold solution. Avoid spraying at high temperatures above 32°C to prevent phytotoxicity.