Rex begonia spots on leaves: causes and what to do
8 results in the diagnosis tree begin with spots on the leaves, 5 of them marked likely when the answers fit. They are ordered here for Rex begonia (Begonia rex), using what its sources say about light, soil moisture, humidity and pests and diseases and 2 problems recorded for it.
Sources last read 14 September 2026.
Begonia rex, the king begonia, fancy-leaf begonia, or painted-leaf begonia is a species of flowering plant in the family Begoniaceae. It is a rhizomatous begonia first brought to England from Assam, and documented in 1857. Wikipedia, CC BY-SA 4.0
Native range: S. Bhutan to N. Myanmar (subtropical). Also sold as fan plant and painted-leaf begonia.
What Rex begonia’s sources say that bears on this
- Light
- Partial shadeRHS · read 13 September 2026
- Soil moisture
- Well-drainedRHS · read 13 September 2026
- Humidity
- ASHRAE recommends indoor relative humidity between 30% and 60%Penn State Extension · read 13 September 2026
- Pests and diseases
- Can be troubled by aphids, vine weevil, thrips, tarsonemid mite, mealybugs and caterpillars. Diseases: rhizome rot, stem rot, powdery mildews and grey moulds.RHS · read 13 September 2026
How this shows up on a Rex begonia
Pale powdery spots that merge into a white film
White patches like a dusting of flour appear on the upper surfaces of the leaves of the rex begonia, spread and merge; affected leaves may yellow and curl.
RHS lists powdery mildew among the diseases it may get. It favors crowded plants, poor air movement and roots kept too dry.
Telling it apart: Where the white is. Mildew coats the tops of leaves; mealybugs hide in joints and under leaves.
RHS · read 13 September 2026Soft patches that grow a gray fur: botrytis
Damaged or dying tissue on the rex begonia is covered in gray, fuzzy growth, and the rot spreads into healthy parts.
RHS lists gray mold among the diseases it may get. Gray mold (botrytis) enters through damaged or dying tissue and spreads in cool, humid, still air.
Telling it apart: The fur. Gray mold carries a gray, dusty coat; a plain leaf spot does not.
RHS · read 13 September 2026Possible causes, likeliest first for a Rex begonia
Most likely powdery mildew
Ranked higher because Rex begonia's recorded problems point this way.
- Pale powdery spots that merge into a white film RHS
Why: A white coating that wipes off is the surface growth of a fungus.
What to try:
- Remove the worst-affected leaves.
- Improve air movement around the rex begonia and space it away from others.
What would settle it: Wipe a patch. Powdery mildew comes away and leaves a yellowed patch beneath. White that does not wipe off is part of the leaf, or a mineral deposit from hard water.
Most likely cold
Ranked higher for Rex begonia because of what its sources say.
Its RHS hardiness rating puts this plant's cold floor at 10°C or above, so a cold windowsill at night is a strong explanation.
- H1B — 10–15°C RHS · read 13 September 2026
Why: Soft, dark patches rather than crisp ones are damaged tissue rather than dried tissue.
What to try:
- Move it away from the glass at night, or keep the curtain on the room side of the rex begonia.
- Leave damaged leaves until new growth appears, then remove the worst of them.
What would settle it: Where the rex begonia sits at night, and what the glass feels like at that hour. Damage that appeared after one cold night, on the side facing the window, is the pattern.
Most likely spider mites
Why: Spider mites feed on individual leaf cells, and each emptied cell shows as a pale speck.
What to try:
- Move the rex begonia away from other plants.
- Wash the leaves, top and underside, with a strong spray of water in a shower or sink.
What would settle it: Hold a sheet of white paper under a leaf and tap the leaf. Tiny specks that move across the paper are mites.
Less likely for a Rex begonia, still possible
Questions
- What usually causes spots on the leaves on a Rex begonia?
- For Rex begonia: powdery mildew, cold and spider mites.
- How can I tell which one it is on my Rex begonia?
- What do the marks look like? Pale, dry, bleached patches on the side facing the window; Brown or black spots, many with a yellow ring around them; Tiny pale speckles all over, sometimes with fine webbing underneath; A white powdery coating that wipes off; Silvery streaks with tiny black dots; Dark, soft, water-soaked patches.
- What do Rex begonia's sources say about its light?
- RHS: Partial shade.
- What does this look like on a Rex begonia?
- White patches like a dusting of flour appear on the upper surfaces of the leaves of the rex begonia, spread and merge; affected leaves may yellow and curl. Telling it apart: Where the white is. Mildew coats the tops of leaves; mealybugs hide in joints and under leaves.
- Is Rex begonia safe to handle while I check it?
- Its sources list it as toxic to cats and dogs. Wash your hands after handling the roots or sap, and keep trimmings away from pets.
Other plants that show spots on the leaves
Polka dot begoniaFuzzy gray growth over spreading patches on leaves and stems
Aloe veraWashed-out patches that do not spread to the shaded side
Areca palmWilting with white, waxy patches on the root ball: a root pest
Arrowhead plantBrown or black spots on the leaf blades that grow and merge
Chinese money plantWhite, powdery patches on the leaves
Chusan palmBleached patches on the side facing the glass
Something else wrong with the Rex begonia?
What this does not cover
This page orders a decision tree's results using a plant's sourced care. It has not seen your plant, its pot or its window, and two problems at once are common: that is the usual reason a confident answer turns out wrong. The questions in the diagnosis tool narrow it further than a list can.