A rex begonia is easy to buy for the leaf pattern alone, that swirl of silver and deep maroon that doesn’t look real in photos. A faint white dusting on a few leaves, right before they start yellowing and dropping, is easy to mistake for a pest, and it’s tempting to spend a weekend checking for bugs that aren’t there. A closer look at the white coating itself — not spots, not webbing, an actual dusty film — points to something else entirely.
Yellowing begonia leaves come down to four fairly distinct causes: overwatering or root rot, powdery mildew, low humidity, and normal lower-leaf aging. Begonias are particularly prone to powdery mildew compared to a lot of other houseplants, which makes it worth checking for specifically rather than assuming yellowing always means a watering problem.
| Symptom | Likely cause |
|---|---|
| Yellowing paired with a white, dusty or powdery coating on the leaf surface | Powdery mildew |
| Yellowing paired with soft, mushy stems or consistently wet soil | Overwatering or root rot |
| Yellowing at the edges, dry and crispy, room on the dry side | Low humidity |
| Yellowing limited to the oldest, lowest leaves; rest of the plant full and healthy | Normal aging |
Powdery mildew
Begonias, especially rex and rhizomatous types with their fuzzy or textured leaf surfaces, are notably susceptible to powdery mildew, a fungal infection whose most obvious symptom is a white powdery coating on the surfaces of infected leaves and stems, often followed by yellowing and eventual leaf drop in the affected areas. It tends to develop in conditions with poor air circulation, high humidity combined with still air, or when leaves stay damp for extended periods.
The distinguishing feature here is the coating itself, a dusty white film that’s genuinely different from pest damage or nutrient issues, and it usually wipes off somewhat if you touch it, though it returns without treatment. Improve air circulation around the plant, avoid getting water directly on the leaves when watering, and remove and dispose of heavily affected leaves. A diluted fungicide, including some homemade options like a mild baking soda solution, can help control an active outbreak.
Overwatering or root rot
Begonias are fairly sensitive to consistently soggy soil, and overwatering that progresses to root rot commonly shows up as yellowing leaves paired with soft, mushy stems, sometimes with a sour smell from the soil. Unlike powdery mildew, there’s no white coating involved, just discoloration and a general wilted, unwell appearance despite the soil being wet.
Check the soil moisture and the base of the stems. If everything feels consistently damp and the stems near the soil line are soft rather than firm, let the soil dry out significantly, and consider repotting into fresh, well-draining soil if rot has clearly set in.
Low humidity
Rex and rhizomatous begonias in particular tend to want more humidity than a lot of common houseplants, and persistently dry air can cause yellowing that starts at the leaf edges and moves inward, typically with a dry, crispy texture rather than the softness associated with overwatering.
If the room runs dry, especially during winter with heating running, grouping the plant with others, using a humidifier, or setting it on a pebble tray can help. This is worth ruling in before assuming a watering problem if the soil moisture actually looks fine.
Normal lower-leaf aging
Like most houseplants, begonias naturally shed their oldest, lowest leaves over time as the plant puts energy into newer growth. This shows up as isolated yellowing on just the bottom-most leaves while the rest of the plant continues looking healthy and full.
If it’s just a leaf or two at the base, and new growth at the top of the plant looks healthy, this is likely just normal aging. You can trim the yellowing leaves for appearance, but no other intervention is needed.
A few things people ask
Is powdery mildew contagious to my other plants? It can spread to other susceptible plants nearby, particularly ones with similarly textured leaves, so isolating an affected plant while treating it is a reasonable precaution.
Should I mist my begonia to raise humidity? Generally not recommended — Purdue Extension advises against misting plants or wetting the leaves when watering — since getting water directly on begonia leaves, especially fuzzy-leaved varieties, can actually encourage powdery mildew and other fungal issues. A humidifier or pebble tray is a safer way to raise ambient humidity.
Can a begonia recover from a bad powdery mildew outbreak? Yes, in most cases, once air circulation and watering habits are corrected and the fungicide treatment has had time to work. Removing heavily affected leaves speeds recovery since it reduces the amount of active fungal material on the plant.
Do all begonia types get powdery mildew equally? Rex and rhizomatous begonias with fuzzy or heavily textured leaves tend to be more susceptible than smoother-leaved types like wax begonias, though any begonia can develop it under the right conditions.
For the full rundown on keeping begonia happy long-term, light, watering, soil, and propagation, see the Begonia Care Guide. Next reads: Why Are My Anthurium’s Leaves Turning Yellow?, Why Is My Cyclamen Turning Yellow? and Why Are My Orchid’s Leaves Wrinkled?.