Impatiens Downy Mildew: Soil Persistence, Solarization, and Rotation

A New England gardener's impatiens died (likely downy mildew). She bagged them and asks if the disease persists in soil, if winter kills it, and what to plant next year.

Downy Mildew (Plasmopara obducens) Facts

• Soilborne oospores survive 2–3 years in soil.
• Winter cold does not kill it.
• Only proven eradication: solarization — copper fungicide drench + black plastic cover for 3+ months (full growing season).

Action Plan

1. Spray entire bed with copper-based fungicide.
2. Cover with black plastic (weighted down).
3. Leave until next May (Memorial Day).
4. Then plant non-host annuals: petunias, marigolds, zinnias, salvia, celosia.
5. Do not plant impatiens for a full season.

Rotation Is Key

Downy mildew is host-specific to Impatiens walleriana. Most other annuals are immune. Rotate beds yearly anyway — it breaks cycles for many pathogens.

Follow this, and next year's shade beds will be clean.