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I have half a dozen heucheras, several years old, in pots on a basement patio, in Brighton. They are superb this year, growing well and flowering but the lime green one has suddenly developed small rust coloured spots on the top of the leaves with same colour blobs under the leaves. My gardening book says ‘leafy gall’ but gives no instructions and Google doesn’t know what ‘leafy gall’ is! I don’t know if it is infectious, if the plant should be destroyed or cut back. I have never re-potted or divided them.
Heuchera rust is what it is.
Cut off ALL foliage scoop soil from around the plant and throw away (don’t put any in compost bin )
Then spray for a couple of months, as indicated in bottle with Rose Clear for black spot and rust.
Keep an eye on all others as it can spread!
Any leaves that come back with spots whilst it’s growing back pinch off immediately.
Don’t touch other leaves when you handled the rust foliage as it can spread that way
Hope this helps
Vicky and Richard Fox