Got a problem in your plot? Let our Garden Guru Martyn Cox help you out

Q I have two pieris in pots with stems that are covered in a black soot-like substance. I was wondering what it is and how to get rid of it. M. Hughes

A This is almost certainly sooty mould, a fungi that grows on honeydew, a tacky substance secreted by aphids. The mould itself is unsightly but not really detrimental to growth. Wipe the stems with a cloth to remove the mould and then control the aphids to prevent it returning.

Q I’d like to plant some herbs and wondered whether there is a difference between oregano and marjoram. Kate Edwards

A Well, they both taste and look similar and are closely related. Common oregano (Origanum vulgare) and marjoram (Origanum majorana) are both perennial, although oregano tends to be better suited to our climate. Perhaps the most noticeable difference is in the taste; oregano is the classic pizza topping, while the flavour of marjoram is sweeter and less pungent.