Managing Late‑Summer Powdery Mildew on Lawns and Ornamentals

What It Is

The white powder appearing in August is powdery mildew – the same fungus that affects roses, zinnias, phlox, and begonias. On grass it’s usually harmless but indicates cultural stress.

Lawn Care to Reduce Mildew

• Water deeply once a week (2–3 hours) rather than daily light sprinklings. Drought‑stressed grass is more susceptible.
• Fertilize on schedule: Late spring (Memorial Day), early fall (Labor Day), late fall (Halloween). Avoid early spring and summer applications.
• Late‑fall fertilization timing: Apply after top growth stops but before roots go dormant – usually between Halloween and Thanksgiving. If you mow weekly, wait until you haven’t mowed for two weeks.
• Mow high (2.5–3.5 inches) and frequently – never remove more than 1/3 of the blade.
• Limit herbicide and insecticide use – they can stress the turf.

Ornamental Plants

Phlox, zinnias, and begonias are highly prone to mildew. For these:

• Plant in full sun with good air circulation.
• Space adequately; avoid crowding.
• Water consistently; don’t let them drought‑stress.
• Prune nearby shrubs to improve airflow.
• If needed, spray with fungicides like triadimefon (Bayleton, Strike) or triforine (Funginex) every 7–14 days starting at first sign.

Roses may require a dedicated spray program for susceptible cultivars.

Bottom Line

A well‑fed, properly watered, high‑mowed lawn rarely suffers serious powdery mildew. Focus on cultural practices first; treat ornamentals individually if they’re problematic.