Xanthomonas Black Rot
Xanthomonas campestris
Pathogen
Bacterium
Hosts
Field brassicas
Symptoms
Symptoms are initially expressed as `v` shaped lesions of the leaf margins and a blackening of the veins and the vascular system in the leaves, stems and roots (hence the name black rot). Eventually whole leaves can yellow, wilt and die back.
The seed can be infected by the bacteria moving through the vascular system into the flowering parts.
Development
This is principally a seed-borne bacterial disease which is the main source of infection and it passes from the seed into the crop seedlings as they emerge. It can survive both `on` and `in` the seed but this can be controlled using hot water treatment of seed.
It can also be passed from plant to plant by rain splash or wind blown water droplets , gaining entry through the stomata or via wounds/abrasions on the plant surface. Another common method of entry into the plant is via the leaf hydathodes (water pores) which exude plant sap in early morning.
Thought to have a short persistence in the soil, up to 6 weeks, but can survive for up to 2 years in crop debris.
Favourable Factors
Warm, wet and humid conditions.
Importance
An economically very important worldwide disease affecting all species of cultivated brassicas. A particular problem in countries where clean seed is less readily available and where brassica crops are grown in short rotations.
Control
Clean seed
Tolerant varieties
Removal of cruciferous host weeds and crop debris
Rotation
Some fungicides have been shown to have activity in helping plants to combat this disease
Field shot from artificially inoculated trial (Photo D. Pop)
Note the characteristic “v” shaped lesion on the leaf edge (Photo D. Pop)
Leaf symptoms from an artificially inoculated trial (Photo D. Pop)
