Which pathogen is responsible for downy mildew on impatiens?
Downy mildew is caused by the water mold Plasmopara obducens. This fungus-like pathogen thrives under cool temperatures and moist or humid conditions. Disease spores (zoospores) are produced in spore-bearing structures called sporangia. The sporangia are responsible for the white, fuzzy growth on the undersides of the leaves. The sporangia are easily dislodged and can be splashed by rain or water to other leaves and carried to distant plants by air currents. When provided with a wet leaf surface (either from rain or irrigation water), zoospores released by the sporangia invade the leaf tissue and begin an infection. Impatiens downy mildew also produces a second type of spore (oospore). Oospores form inside infected leaves and stems and are released into the soil as the plant debris decays. These resting spores may survive in the soil for a period of several years.