Black rot of brassicas: prevention and control

This page provides information on prevention and control of black rot of brassicas caused by Xanthomonas campestris pv. campestris.

This information was last updated in 2023.

Black rot is very difficult to control once disease has become established in a crop and symptoms are widespread. The best means of control is prevention through the use of clean (pathogen-free) seed and transplants. It is important to remember that the pathogen may be present in the absence of disease symptoms.

Seed testing

The most effective means of control is to use healthy seed: i.e. seed which has been tested to confirm that it meets the required standards.

It is important for growers to be aware that there are no official health standards and there is no statutory requirement for brassica seed to be tested for Xanthomonas.

Modelling of different risk scenarios has indicated that effective control (overall risk of disease less than 10%) in transplanted brassicas can be achieved by testing 6 x 10,000 seeds by a method that incorporates a centrifugation step.

Not all commercial seed suppliers apply this standard. Some may routinely test a total of 30,000 seeds; this standard is not adequate for transplanted crops, therefore it is vital to check with seed suppliers how many seeds have been tested or what standard has been applied.

Seed treatment

Hot water and similar physical treatments may be considered if valuable seed stocks are known to be infected, but it is vital that post-treatment testing is performed to demonstrate the treatment has been effective.

In research project CP 191 we showed that hot water treatment could give significant reductions in disease in the field.

Plant raising

Most vegetable brassica crops are grown from module transplants raised by specialist nurseries.

Plant-raisers should ensure that seed has been tested to the highest standards, and good hygiene practices [link to Hygiene below] should be followed at all times .

Xanthomonas can spread very rapidly during plant raising with typical overhead gantry irrigation systems. Minimising the amount of overhead irrigation will reduce the rate of spread. A system for producing high transplants using a sub-irrigation system has been shown to be feasible (see research project CP 191) and gave a 30% yield increase compared to conventionally produced transplants.

Cultural practices

Bacterial pathogens are easily spread by water splash. It is therefore important to minimise overhead irrigation and use sub-, drip- or trickle-irrigation systems wherever possible.

Soft, lush growth leads to increased susceptibility to infection. It is therefore important to avoid over nutrition, and another reason to keep watering to a minimum, i.e. grow plants as hard as possible.

Infected crop debris should be chopped as much as possible to encourage rapid breakdown and ideally there should be a 2-3 year break from brassicas to ensure residues have completely disappeared.

Hygiene

Bacteria can be passively spread by anything that comes into contact with an infected plant or contaminated surface. Bacterial pathogens can also survive for long periods in dry crop debris, or in dried-on slime or ooze from infected plants.

Good hygiene is essential, to prevent cross-contamination between different batches of transplants on the nursery and to prevent spread from affected to nearby healthy crops in the field:

• Module trays should be cleaned and disinfected before re-use.
• Remove crop debris and clean and disinfect glasshouse bays batches of transplants and particularly after infected crops.
• Don't go into wet crops.
• Spray infected crops last.
• Machinery, equipment, clothing and footwear should be washed down and disinfected after visiting infected crops.

Disinfectants

Most general purpose disinfectants are likely to be effective against Xanthomonas when used at recommended concentrations, but it is important to ensure to ensure contact time is as long as possible for maximum efficacy.

Pesticides

Attempts at control with pesticide sprays in the field are unlikely to be cost-effective, as significant spread has most likely already occurred when symptoms are seen.

Currently (2022), there are a limited number of approved biological products (Serenade ASO, Amylo-X WG) that may have activity against bacterial plant pathogens, but previous work has shown the reductions achieved may not justify the costs.

Previously, trials have shown that frequent sprays with copper oxychloride (Cuprokylt) can give significant reductions in the rate of spread in transplants. However, the use of copper sprays is no longer approved.

Resistance

Breeding resistant varieties has long been recognized as an important target for disease control, and was the focus for a lot of work in the late 1990s and early 2000s. It should also be noted that some varieties are already resistant to some races of the pathogen and some varieties have field tolerance. Useful sources of resistance have been identified and their inheritance studied, particularly in non-vegetable brassica species (i.e. not B. olearacea), but seems not to have been a priority for commercial breeders in recent years.