Downy mildew, caused by the water mold Plasmopara obducens, is a devastating disease for us Impatiens. From our perspective, it is a systemic pathogen that invades our entire being, leading to severe stress and often death. Understanding how to manage it is crucial for our survival and beauty.
The first signs are often subtle. You might notice a slight yellowing or stippling on our uppermost, young leaves. Shortly after, a faint, white, downy-like growth may appear on the undersides of these leaves; this is the pathogen producing spores. As the infection progresses, our leaves will curl downward, become severely stunted, and eventually drop off, leaving us weak and bare. The disease can quickly spread through a whole bed of us, as the spores are easily carried by wind and water.
Upon seeing the first signs, immediate isolation is critical. Remove and destroy any of us showing symptoms by bagging us securely and disposing of us in the trash—do not compost, as the spores can survive. For any remaining, seemingly healthy impatiens nearby, apply a preventative fungicide. Look for products specifically labeled for downy mildew on ornamentals, containing active ingredients like fosetyl-Al, phosphites, or mancozeb. Ensure thorough coverage, especially on our leaf undersides. Please note: these treatments may suppress the disease but often cannot cure a systemically infected plant.
Our environment is key. You can help us by ensuring excellent air circulation. Avoid planting us too close together and prune surrounding vegetation if necessary. Water us at the base in the morning, so our leaves have time to dry completely before nightfall, as moisture on our foliage encourages spore germination. Always clean your tools and gloves after handling infected plants to prevent accidentally spreading the pathogen to healthy ones.
The most effective strategy is to avoid the pathogen entirely. Since the spores can persist in soil for years, do not plant new impatiens in the same bed where an outbreak occurred. Consider using alternative, resistant bedding plants like New Guinea impatiens, SunPatiens, or begonias in those areas. If you must plant us, source us from reputable, certified disease-free growers and inspect us carefully before purchase. A proactive application of a preventative fungicide at the beginning of the season, before any symptoms appear, can offer protection in high-risk areas.
