Cauliflowers Wilting Problem?

I planted 12 cauliflower plants in my allotment 2 months ago, I used landscape fabric to keep weeds down & spread slug pellets to keep them at bay. About 3 of these plants have now suffered a strange problem whereby the outer leaves are wilting and just a few inner leaves are OK. On inspection, the stem seems to go from about 4mm thick where it enters the ground to about 1mm thick in the ground, clearly causing some water supply problems. What is causing this? Is it slugs eating the stalk or some other diesease? All info welcome.