Black rot: take care to prevent this destructive disease

Xanthomonas campestris can destroy a cabbage plant. It can also occur on dead organic matter under the proper conditions, warns Bill Kerr

- Bill Kerr

Black rot (Xanthomonas campestris) is the most well-known disease of cabbages and other brassicas. Occurring around the world, it can render a cabbage crop totally unmarketable.

Black rot is a bacterial disease, and many weeds host the pathogen. The bacteria are airborne. The disease typically starts on the edges of the leaf, with conspicuous V-shaped lesions appearing where the veins reach the margin. This is because, when the soil is moist, droplets of sap exude from the water-secreting pores (hydathodes) at night. The droplets can be seen in the early morning before the dew evaporates.

The bacteria in the air then alight on these exposed droplets, and are sucked back into the leaf when the sun dries the dew. This is the main entry point for the bacteria.