Hello!

From your photo it looks like you have a scale insect. Cottony camellia scale is one possible culprit. Although the name says camellia, it can also affect hydrangeas. The spots you are seeing are egg masses underneath a protective scale. Tiny crawlers will emerge and feed on the veins in the leaves.

For control right now the egg masses should be removed with a toothbrush (or similar small brush). Sticky tape with sticky side out around twigs will capture the crawlers when the eggs hatch.

Larger areas can also be pruned off, depending on your tolerance for that.

The leaves that are infected with the scale insects may turn yellow or pale. A black sooty mold may also appear there from the honeydew that is extracted from the insects. Your hydrangea will likely be fine if there is a low level of insects, but a larger outbreak could be damaging.

Resources:

Camellia (Camellia)-Cottony camellia scale | Pacific Northwest Pest Management Handbooks (pnwhandbooks.org)

Landscape pests-Scale insect | Pacific Northwest Pest Management Handbooks (pnwhandbooks.org)

Regards,

Leo Sherry

OSU Extension Master Gardener Volunteer