Severe Powdery Mildew on Crimson Sentry Maple: Fungicide Rotation

Your three Crimson Sentry maples (Acer platanoides 'Crimson Sentry') have aggressive powdery mildew unresponsive to monthly sprays. The issue is likely fungicide resistance or insufficient spray frequency for high disease pressure.

Why Monthly Sprays Fail

• Mildew spores reproduce every 5–7 days in favorable conditions.
• Monthly intervals leave huge protection gaps.
• Same fungicide class repeatedly → resistance.

Effective Program

1. Spray every 7–14 days from bud break through summer (shorter interval when humid/wet).
2. Rotate fungicide modes of action (FRAC groups):
   • Group 3 (DMI): Myclobutanil (Immunox), Propiconazole (Infuse), Tebuconazole
   • Group 11 (QoI): Azoxystrobin (Heritage), Trifloxystrobin (Compass)
   • Group 7 (SDHI): Fluxapyroxad (Broadform)
   • Group M2 (Sulfur): Safer Garden Fungicide (do not use >85°F or near oil)
3. Cover all leaf surfaces—use spreader-sticker.
4. Start at bud break—preventive is far better than curative.

Cultural Helpers

• Full sun, wide spacing, avoid overhead irrigation.
• Rake/destroy all fallen leaves.
• Thin canopy for air flow.

Professional Options

Commercial applicators have access to stronger rotations (Banner MAXX, Bayleton, Eagle, Strike, Cygnus). If DIY fails, hire a licensed arborist with a proper spray program.

With consistent rotation and frequency, you can break the mildew cycle.