Can anyone tell me what this is growing on my outside wall please? And how to treat it ? Thank you Kate
Looks like a member of Pezizales order, probably of genus Peziza, maybe even specifically Peziza domiciliana. Maybe a proper roof or some kind of other barrier, that keeps the wall dry, would help to avoid this species to spread? Treatment with a gas flame would be more radical method. Of course, chemical treatment with fungicides would help too, if using a poison is an option for you.
If the brickwork mortar has enough organic matter to support fungal growth and fruiting, the bricks probably need to be repointed.
