Powdery Mildew on Coleus
Coleus (Coleus scutellarioides)
Symptoms
- white, powdery coating on leaves
- a light finger swipe removes the surface dust and shows blotchy, discolored tissue below it
- distorted new growth alongside the coating
- spreading pattern across the plant over days to weeks
Causes
Fungal infection favored by high humidity and poor air circulation
As a member of the mint family, which includes several other mildew-prone species, coleus shows a fairly typical susceptibility to powdery mildew fungi that thrive in humid conditions combined with still air, a combination that can develop when plants are grouped closely together or kept in an enclosed space.
Water sitting on leaf surfaces
Overhead watering or misting that leaves water on the leaves, especially combined with cooler nighttime temperatures, creates favorable germination conditions for mildew spores.
Dense, overcrowded growth
A coleus that hasn't been pinched or pruned regularly can become dense and overcrowded internally, trapping humid air within the plant's own foliage even in an otherwise well-ventilated room.
How to Fix It
- 1
Pinch back any dense, bushy sections first to open up the plant's own internal airflow, since coleus's naturally full growth habit can trap humid air against the stem even in a room with a fan already running.
- 2
Snip off any leaf carrying a heavy coating, prioritizing the colorful, actively growing tips where coleus concentrates its newest tissue, since that's where the fungus keeps a foothold even after visible spread elsewhere slows.
- 3
Thin out dense, overcrowded growth through pruning, which both improves airflow within the plant and encourages the bushier habit coleus benefits from anyway.
- 4
Pick up a potassium bicarbonate or neem oil fungicide labeled for mildew, and expect to keep reapplying it roughly weekly for as long as coleus keeps pushing new soft growth — a plant this fast-growing outpaces a single spray cycle before the fungus has been fully knocked back.
- 5
Switch to watering at the soil level going forward, keeping foliage dry.
Prevention
- Maximize air circulation, especially if plants are grouped closely together
- Direct the watering can right at the base of the stems rather than over the bushy top growth, since coleus's dense leaf mass holds any overhead moisture against the foliage for longer than an open-growing plant would
- Prune and pinch regularly to prevent dense, airflow-restricting growth
- Space plants with enough room for air movement between leaves
Quick Summary
| Plant | Coleus (Coleus scutellarioides) |
|---|---|
| Category | Disease |
| Likely causes | Fungal infection favored by high humidity and poor air circulation, Water sitting on leaf surfaces, Dense, overcrowded growth |
| Fix steps | 5 steps — see above |