This is how I can tell. Here is a healthy Broccoli plant - foliage bright and turgid (i.e. it is "perky" and stands up firmly):
Whereas this plant's foliage is dull lifeless and flaccid (i.e. it is floppy):
This means that its roots have been attacked and it is no longer able to get moisture up into its leaves. It will most likely die.
And here are the culprits - a cluster of white maggots at the base of a PSB plant - the larvae of the Cabbage Root Fly.
So, the brassicas collars that are supposed to provide a physical barrier to stop the fly laying its eggs near the plant stem didn't work. Nor did the application of a double dose of Nemasys Grow Your Own nematodes, administered exactly in accordance with the supplier's instructions. I am, as they say, "gutted".
Worst of all perhaps is the fact that the photo above, showing the maggots, is taken of the base of one of my reserve plants, grown specially to replace any casualties in the main batch of plants! So far, two of my six precious PSB plants are showing signs of attack.
My immediate action (after the initial shock) has been to put a squirt of the only bug-spray I have in my shed around the base of each of my plants. It is not one that mentions being able to kill Cabbage Root Fly larvae - but then which one is??? Maybe it will work, maybe not. All I can do is wait and hope. And keep the plants well watered so that they are not put under undue stress.
On second thoughts, I think maybe I'll also sow another batch of PSB seeds. I know it's late, but they might just come on in time...