Symptoms and Control Methods of Downy Mildew on Impatiens
The scientific name of downy mildew on Impatiens is Plasmopara obducens (Schrtiter) Schroter, and it causes damage worldwide. It mainly affects the leaves and reduces their ornamental value.
Symptoms: On affected leaves, chlorotic patches initially appear, irregular in shape, later turning into yellow-brown or brown necrotic spots; on the underside of the leaves there is a white mold-like growth, relatively thick and dense, and in severe cases it can cover the entire leaf, causing the leaves to scorch and affecting their ornamental value.
The pathogen is called Impatiens axis downy mildew, belonging to the oomycetes. 2-5 sporangiophores emerge from the stomata, with the upper part branching monopodially 4-7 times, the terminal branches straight, conical, with a bluntly flat tip; sporangia measure 10-27 x 10-18 (um), nearly round.
Disease characteristics: This disease cycles year-round with no obvious overwintering period. In the north, the pathogen overwinters as oospores in the soil with diseased plant residues. The following year, under suitable environmental conditions, the oospores spread via water flow or rain splash. In the south, the pathogen uses sporangia for primary infection and reinfection; after the sporangia germinate, primary infection occurs, and the sporangia produced on diseased parts spread via airflow to cause reinfection, allowing the disease to spread. The disease is prone to occur at temperatures of 15-17°C, with high humidity or large day-night temperature differences, heavy fog and dew, heavy clay soil, and low-lying terrain; in summer and autumn, continuous rainy weather, many rainy days, and high rainfall cause severe disease or major epidemics.
Control methods
(1) Strengthen cultivation management, apply fertilizer and water reasonably, regulate ventilation and light penetration, and improve plant resistance. Choose high-lying plots for planting, drain water promptly after rain, maintain appropriate temperature and humidity, clean up diseased residues, and reduce sources of infection.
(2) Select disease-resistant varieties.
(3) Chemical control: At the early stage of disease onset, promptly spray 70% phosethyl-Al and mancozeb wettable powder at 500 times dilution, or 64% oxadixyl and mancozeb wettable powder at 400-500 times dilution, or 72% cymoxanil wettable powder at 600 times dilution, once every about 10 days, for 2-3 applications.
Commonly used agents: phosethyl-Al and mancozeb wettable powder, oxadixyl and mancozeb wettable powder, cymoxanil wettable powder.