Brassica black rot

What you see

V-shaped yellow wedges eating in from the leaf margin, each with the point aimed at the midrib, and blackened veins running through them. Cut the stem across and the vascular ring is black. Plants stink faintly when they collapse.

Conditions

Warm and wet - above 25C with rain or overhead watering. It moves in water and enters through the leaf edge pores and through insect and slug wounds.

Treatment

Remove and bin affected plants entirely. Copper slows spread on neighbours but will not save a plant with blackened veins. Never work in the brassica bed when wet.

Prevention

Hot-water treat seed (50C for 20-30 minutes) or buy certified clean seed - the great majority of outbreaks arrive on seed. Rotate four years and control slugs, which open the wounds it enters through.

Months: June, July, August, September, October

Plants

Cabbage, Cauliflower, Broccoli, Curly kale, Turnip, Brussels sprouts

Remedies

Bordeaux mixture (copper), Iron phosphate slug pellets

The thing people get wrong

The V from the leaf margin is unmistakable once you have seen it, and it is the single most destructive brassica disease worldwide. The uncomfortable truth is that it nearly always comes in on seed - so the cheap packet of unspecified cabbage seed is a genuine gamble, and saving your own seed from an affected crop passes the problem straight to next year.