Black Rot of Crucifers

Classic symptoms are yellow, V-shaped lesions that begin at the leaf margins and point inward toward the midrib, following the pattern of water flow into the leaf. The veins within and beyond the lesions blacken as the bacteria move through the vascular system, and cutting the stem may reveal darkened vascular tissue. Severely affected plants can be stunted, drop leaves, and become predisposed to secondary soft rots.

Xanthomonas campestris pv. campestris survives on and in infected seed, in crop residue, and on cruciferous weeds and volunteers. The bacteria enter leaves mainly through the natural water-releasing pores at the leaf margins (hydathodes) and through wounds, then colonise the vascular system. They spread within crops by splashing rain and irrigation water, on tools and equipment, and by movement of infected transplants.

• Infected seed and transplants are primary sources of the bacterium
• Bacteria enter through leaf-margin pores and wounds
• Colonisation of the vascular system drives the V-shaped lesions
• Splashing water, tools, and residue spread the disease within crops

A wide range of brassica crops is affected, including cabbage, broccoli, cauliflower, kale, and related mustards and radishes, along with cruciferous weeds that act as reservoirs. Warm, wet, humid conditions with rain, dew, and overhead irrigation strongly favour infection and spread, and dense plantings that stay wet increase risk.

• Warm, wet, humid weather with rain, dew, or overhead irrigation
• Use of infected seed or transplants
• Cruciferous weeds and volunteers that reservoir the bacterium
• Wounding and handling of wet plants that spreads bacteria

Because the bacterium is seed-borne and spread in water, management begins with clean seed and transplants and relies on sanitation, rotation, and water management to limit spread. There is no in-crop cure once plants are systemically infected, so prevention is key.

• Use certified, tested, or treated seed and healthy transplants
• Rotate away from brassicas and control cruciferous weeds and volunteers
• Avoid working in and handling plants when foliage is wet
• Reduce overhead moisture and improve drying where feasible
• Sanitise tools and equipment and remove infected residue