Bigleaf hydrangea 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 Bigleaf hydrangea (Hydrangea macrophylla), 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.
Hydrangea macrophylla is a species of flowering plant in the family Hydrangeaceae, native and endemic to Japan. 5 m (8 ft) or more broad with large heads of pink or blue flowers in summer and autumn. Wikipedia, CC BY-SA 4.0
Native range: SC. Japan (to Izu Ōshima), Kazan Rettō (temperate). Also sold as French Hydrangea, Hortensia, Lacecap Hydrangea, Mop-head Hydrangea, mophead hydrangea and penny mac.
What Bigleaf hydrangea’s sources say that bears on this
- Light
- Full sun, Partial shadeRHS · read 13 September 2026
- Soil moisture
- Moist but 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 vine weevil, hydrangea scale, capsid bug and aphids. Diseases: honey fungus, powdery mildews, grey moulds and a leaf spot can occur.RHS · read 13 September 2026
How this shows up on a Bigleaf hydrangea
Brown or black spots on the leaves that grow and merge
Spots on the bigleaf hydrangea start small and water-soaked, then darken and merge; badly marked leaves may fall.
RHS lists leaf spot among the diseases it may get. The pathogen enters through the leaf surface while it is wet and kills the tissue around it.
Telling it apart: Where the spots are. Disease scatters spots anywhere, often where water sits; cold damage and scorch mark one side.
RHS · read 13 September 2026Fuzzy gray growth over spreading patches on leaves and stems
Damaged or dying tissue on the bigleaf hydrangea is covered in gray, fuzzy growth, and the rot spreads into healthy parts.
RHS lists gray mold among the diseases it may get. Crowding and damp air around the plant let spores settle and grow.
Telling it apart: Where it starts. Botrytis begins on damaged or dying tissue; mildew starts on healthy leaves.
RHS · read 13 September 2026Possible causes, likeliest first for a Bigleaf hydrangea
Most likely a leaf-spot infection
Ranked higher because Bigleaf hydrangea's recorded problems point this way.
- Brown or black spots on the leaves that grow and merge RHS
Why: Spots with a yellow ring around them are the mark of a fungus or bacterium spreading through the leaf, the ring being tissue that is dying ahead of it.
What to try:
- Remove the affected leaves and put them in the bin rather than the potting mix.
- Water the soil rather than the leaves, and stop misting.
What would settle it: Whether the spots grow. Mark the edge of one with a pencil dot and look again in a week; an infection advances, damage from a single event does not.
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 bigleaf hydrangea 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.
Most likely powdery mildew
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 bigleaf hydrangea 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.
Less likely for a Bigleaf hydrangea, still possible
Questions
- What usually causes spots on the leaves on a Bigleaf hydrangea?
- For Bigleaf hydrangea: a leaf-spot infection, spider mites and powdery mildew.
- How can I tell which one it is on my Bigleaf hydrangea?
- 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 Bigleaf hydrangea's sources say about its light?
- RHS: Full sun, Partial shade.
- What does this look like on a Bigleaf hydrangea?
- Spots on the bigleaf hydrangea start small and water-soaked, then darken and merge; badly marked leaves may fall. Telling it apart: Where the spots are. Disease scatters spots anywhere, often where water sits; cold damage and scorch mark one side.
- Is Bigleaf hydrangea safe to handle while I check it?
- Its sources list it as toxic to people. Wash your hands after handling the roots or sap, and keep trimmings away from pets.
Other plants that show spots on the leaves
African violetFuzzy gray growth over spreading patches on leaves and stems
Barberton daisyDark spots with a yellow halo spreading across the leaves
Bearded irisFuzzy gray growth over spreading patches on leaves and stems
Blue passion flowerBleached patches on the side facing the glass
Busy LizzieAngular pale patches and a downy coating on the undersides
Calla lilySilvery patches dotted with tiny black spots: thrips
Something else wrong with the Bigleaf hydrangea?
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.