From my perspective, downy mildew (*Plasmopara obducens*) is a silent, creeping terror. It does not announce itself with obvious wounds but begins as a a subtle, pale stippling or yellowing on my top leaves. This is the first sign that the microscopic spores have found me, likely traveling on the wind or in splashing water. If you look at my underside during the cool, damp mornings I so dislike, you might see the tell-tale white, fuzzy growth. This is the pathogen itself, reproducing and preparing to spread. Internally, it is a systemic invasion; the pathogen grows through my veins, my xylem and phloem, choking me from within, which leads to my leaves curling, stunting my growth, and causing my vibrant blossoms to drop prematurely.
To help me resist this disease, you must understand my needs and create an environment where the pathogen struggles. I thrive in well-draining soil that keeps my roots moist but never waterlogged, as soggy feet stress me and make me susceptible. Please space my siblings and me adequately; crowding us together creates a humid, stagnant microclimate perfect for the mildew to germinate and spread from my leaves to theirs. While I enjoy bright color, intense, all-day sun can scorch me. Dappled sunlight or morning sun with afternoon shade is ideal, as it allows my foliage to dry quickly, depriving the moisture-dependent spores of their required conditions.
Water me at my base, directly onto the soil, and please do so in the morning. This allows any splashed water on my leaves and stems to evaporate quickly in the rising sun. Overhead watering in the evening is a death sentence; it leaves my foliage wet for hours, an open invitation for the mildew. If you must handle me when I am wet, be aware that you could easily transfer spores from an infected plant to a healthy one like me. Remove any fallen, diseased leaves from the soil surface around me promptly, as they harbor the spores waiting for the right conditions to attack again. At the end of the season, do not compost my infected remains; destroy them to break the disease cycle.
If you see the very early signs on one of my leaves, immediately remove and destroy that entire leaf or stem. Isolate any severely infected plants to protect the rest of us. For protection, you can apply preventive fungicides containing phosphorous acid or specific fungicides labeled for impatiens downy mildew. These work best as a shield, not a cure. They help bolster my internal defenses before the pathogen arrives or in its very earliest stages. Once the disease is advanced within my system, these treatments are often ineffective because the pathogen is already circulating throughout my vascular tissue. In severe cases, the kindest action may be to remove me entirely to save my healthy neighbors.
