The usual sign of cabbage root fly are sickly looking plants with out an obvious cause. The only way to tell for sure is to pull the plant up and carefully split the stem, if you find bored holes and little white maggots then the plant had cabbage root fly. These can be prevented either by being in the middle of a number of plots away from hedges and other places where the fly overwinter, or by putting collars of cardboard, roofing felt, or similar around the plant - cut out a 6" diameter circle. Make a cut from the edge to the middle, then cut out a 1/2" diamter hole in the middle for the stem and place around the plant, when planting. You don't need to heel in! Pressing in firmly with the hand, and planting the seedlings deep works perfectly well on all bar the lightest soils.
The other likely cause would be club root, but again you need to lift the plants to find out, and it takes several weeks for the roots to swell to make a confirmed diagnosis. The wisdom from most books is there is no cure, but anecdotal evidence from a large number of sources confirm that treatment of the ground with Jeyes Fluid or Armatillox works against it (I can confirm the latter). It can also be combatted by not planting out until the plants are a good 6-7" tall and have a good root ball in a 6" pot, adding lime and crushed egg shells when repotting, and adding a lime, crushed eggshell, fish/blood/bone, and compost mix to the planting hole. If you want to line the agro-chemical company's pockets you can also buy one of the F1 clubroot resistant strains from the Kilaxy family. I don't, so despite my father-in-laws plot having clubroot, both he and I successfully grow a wide variety of open pollinated varieties by a mixture of these methods.
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