The carrots on the neighbouring allotment to mine are systematically
being wiped out by something or other, first the foliage gradually
goes yellow with a hint of reddy-brown around the edges on some
plants, then the whole lot wilts and collapses. There are a few very
small aphid type bugs on the foliage, but the roots don't show any
signs of anything untoward,any ideas?

Here's a less than brilliant photo if it helps. Half of his row of
carrots hardly got going after germination and are now well gone, but
the previously very healthy other half of the row is about to go the
same way.

http://img.photobucket.com/albums/v5...A/DSC02038.jpg