Q What happened to the Brussels sprouts on my allotment?

Mandy Beagent, Berkshire A

PIPPA SAYS Your photo shows raised, knobbly growths on the stem, which caused it to split. This would have significantly weakened your plants and the splits would let in other pathogens too. The cause is an aerial form of crown gall, Agrobacterium tumifasciens. This can cause a range of effects and levels of damage.

Unfortunately, there is no easy fix once you have the organism in your soil. I would advise removing all plants showing these symptoms and, in future, keep a close watch so that you can catch it earlier.