Black rot (Xanthomonas campestris)
Black rot disease is often seen as a light brown to yellow ‘V’ shaped lesion on the leaf, typically starting at the leaf margins. When the leaf veins are cut in half, the veins will be black. Black rot is caused by the bacteria entering the plant through natural leaf openings, or from damage caused by insects, other pathogens or mechanical damage.
The black rot bacteria can spread in water splash from rain or irrigation and under warm moist conditions can spread very rapidly throughout a crop. It can also be seed-borne. Black rot can persist for many seasons on common alternative hosts such as the brassica weeds, wild radish and wild turnip.
Bacterial soft rot (Erwinia and Pseudomonas species)
Soft rot disease is common on many vegetables, not just brassicas. It causes a soft mushy breakdown on leaf stalks, heads and storage roots. The decay is often foul-smelling but there is no mould associated.
Infection is through damaged areas often resulting from fertiliser burn or hail injury in the ﬁeld, but can be associated with harvest damage.
The soft rot bacteria may be carried on cutting knives or on residue in produce bins. Postharvest rot is common where the temperature of the harvested produce is allowed to rise and the cool chain is not maintained.