From our perspective, rooted in place, the first sign of trouble is often a subtle change in our light intake. A fine, white, powdery film begins to manifest on our upper leaves and flower buds. This is not dust; it is the mycelium of a fungal pathogen, most likely *Podosphaera xanthii*. This substance interferes with our photosynthesis, the very process that sustains us. We cannot flee from this invasion, so we must rely on our internal defenses and the aid of our caretaker to halt its progress before it covers our entire surface, stunting our growth and deforming our beautiful blooms.
We petunias thrive in conditions that are warm with plenty of direct sunlight and good air circulation around our leaves. The powdery mildew fungus, however, exploits a very specific weakness. It adores the combination of warm days and cool, humid nights. It does not require free water on our foliage like many other fungi; high ambient humidity is sufficient for its spores to germinate and invade our tissues. Dense plantings where our leaves touch, or sheltered locations with stagnant air, create the perfect microclimate for this pathogen to flourish at our expense.
Our first line of defense is always prevention, and we communicate our needs through our appearance. Please provide us with ample space when planting so our leaves can breathe and dry quickly. Water us at the soil level in the morning, avoiding splashing moisture onto our foliage, which only aids the enemy. If you see our leaves becoming overly dense, judicious pruning to improve air flow feels like a relief, allowing sunlight to penetrate and strengthen our inner parts. Removing severely infected leaves, though a wound, is a necessary sacrifice to prevent the spread to our healthier parts.
When the fungal pressure is too great for our natural resilience, we welcome gentle, organic reinforcements. A weekly spray of a solution containing potassium bicarbonate is highly effective. It creates an alkaline surface on our leaves that is inhospitable to the fungus, effectively halting its progress. Another ancient remedy we respond well to is a milk solution (one part milk to two parts water). The compounds in milk act as a natural antiseptic and seem to stimulate our own defense systems. For severe, persistent cases, a biological fungicide containing *Bacillus subtilis* is beneficial. This bacterium colonizes our surface, outcompeting the pathogenic fungus for space and resources, acting as a protective shield.
