On 08/03/2012 19:40, Roger Tonkin wrote:

Do you think it is just coincidence, or that the potting compost I used
(B&Q I think) was responsible?

I grew parsnips last year in conical holes, made with a spike, and
filled with B&Q compost. Most had canker attack quite badly.

The previous year the ones grown directly in the clay soil were mostly
canker free.

You may be right. There could be a B&Q compost connection.