Deciding which season is Diabolo ninebark's (Physocarpus opulfolius 'Monlo') time to shine can be difficult. In summer, pinkish-white flowers glow against its purple-black leaves. In winter, strips of old bark peel from its naked branches, revealing underbark layers of reddish- or golden-brown.
Deciding when Diabolo looks less than its best is much easier: It's when the Podosphaera aphanis var. physocarpi fungus, or powdery mildew, strikes. The disease commonly infects several ninebark cultivars throughout their growing range in U.S. Department of Agriculture plant hardiness zones 3 through 7. Eliminating powdery mildew requires patience.
How Powdery Mildew Spreads
Wind spreads powdery mildew spores from plant to plant, but they only germinate if the leaves they land on are dry. They're most damaging to ninebarks growing in excessive shade, and most active during humid weather with temperatures between 60 and 80 degrees Fahrenheit. Bright, direct sun and temperatures above 95 degrees F often kill the disease.
Treating an Infected Plant
Before breaking out a chemical fungicide, consider spraying an infected Diabolo with organic, plant-based, ready-to-use neem oil.
On a day when the temperature will remain below 90 degrees F, water the Diabolo well.
Put on the protective clothing and gear; it guards against skin, eye and respiratory irritation from the drifting spray.
Spray until the neem oil drips from all Diabolo's surfaces, including the backs of the leaves.
Repeat weekly until the humid weather and favorable temperatures subside.
Stopping Future Infections
Once you've rid Diabolo of powdery mildew, keep the disease away with a few preventive measures:
- Trim any overhead branches shading the plant.
- Wet the leaves when you water to wash off spores already on them and keep landing ones from germinating. Do this in mid-morning, so the sun dries the leaves quickly. Otherwise, other fungi that germinate in water may move in.
- Apply fertilizer only at the manufacturer's recommended rate. Overfertilizing stimulates lots of new, infection-susceptible growth.