Black rot affects cabbage and related crops (brassicas, mustard & radish) worldwide and is caused by the bacterium Xanthomonas campestris pv. campestris. Black rot is one of the most serious cabbage / kale diseases in warm climates. Diseased plants may rot quickly before or after harvest because of secondary infection from bacterial soft-rot.
Soft-rot bacteria may invade heads of black-rot-infected plants, causing tissue to become slimy and foul-smelling. The black rot bacterium can over-season on infected cabbage seeds, in weeds belonging to the Brassica family (including: black mustard, field mustard, wild turnip, wild radish, shepherd's purse, and pepper weed); or in infected plant material in the soil. The bacterium can persist in plant residue for 1-2 years or as long as the plant debris remains intact.
In Kenya, black rot is endemic and the cause of much damage (Onsando, 1988, 1992). The disease is considered of intermediate economic importance in Mozambique (Plumb-Dhindsa and Mondjane, 1984). Black rot is widespread in Zimbabwe where it is considered the most important disease of brassicas (Mguni, 1987, 1995).
Black rot is a pathogen of most cultivated cruciferous plants and weeds. Cauliflower and cabbage are the most readily affected hosts in the crucifers, although kale is almost equally susceptible. Broccoli and Brussels sprouts have intermediate resistance and radish is quite resistant, but not to all strains. Kohlrabi, Chinese cabbage, rutabaga, turnip, collard, rape, jointed charlock (Raphanus raphanistrum) and mustard are also susceptible hosts.
Symptoms on cabbage
The plant can be infected at any time during its life cycle. On young seedlings a yellowing appears along the margin of the cotyledons, which later shrivels and drops off. On the margins of mature leaves, similar yellowing appears. Initially, a small V-shaped area develops, but as the diseased area enlarges, the veins become distinctly black. In contrast to Fusarium yellows the veins are brownish in colour. The affected stem, when cut crosswise, shows a characteristic black ring. In later stages the entire head may turn black and soft due to secondary infection by soft rot bacteria (Erwinia carotovora var.carotovora).
|Bacterial black rot. Note blackening of water-conducting tissues of the stem
|© A. M. Varela, icipe
|Bacterial soft rot. Note slimy rot (whitish) of the centre of the cabbage head
|© A. M. Varela, icipe
Affected plant stages
Seedling stage, vegetative growing stage and heading stage (cabbages).
Affected plant parts
Leaves, seeds, stems, vegetative organs and whole plant.
Leaves: 'V' shaped lesions
Seeds: discolourations; lesions.
Stems: Internal discoloration (black in colour) .
Vegetative organs: internal discolouration (black in colour); dry rot.
Whole plant: plant death