Bacterial blight of coriander: prevention and control

This page provides information on the prevention and control of bacterial blight of coriander caused by Pseudomonas syringae pv. coriandricola

This information was last updated in 2023.

Coriander bacterial blight is very difficult to control once disease has become established in a crop and symptoms are widespread. The most effective way to manage the disease is to use ‘clean’ seed which has been tested and shown to meet minimum seed health standards.

Seed testing

It is important that seed is tested, as visual inspection of seed crops is not a reliable indicator of the health of the harvested seed.

Growers should also be aware that there are no official health standards and no statutory requirement for coriander seed to be tested for bacterial blight.

It is recommended that seed health tests should be designed to achieve a tolerance standard of 0.03% (i.e. less than 1 infested seed in 3,000) and an analytical sensitivity of 900 CFU (colony forming units, a measure of bacterial numbers) with 95% probability. This means testing at least 9,000 seeds.

This standard was developed from disease transmission and spread data obtained in Research Project FV 318. The data were incorporated into mathematical models to examine the risks of sowing seed with different infection levels in relation to the probability of detecting them.

Seed treatment

Hot water treatment has considerable potential to reduce or even eliminate seed-borne Pseudomonas syringae pv. coriandricola. Infestation was reduced to undetectable levels in five out of six seed lots, and with a 20-fold reduction achieved in the remaining seed lot, by treatment at 53°C for 30 minutes.

This temperature-time regime is at the borderline of safety for germination, and so there was a slight reduction in germination compared to untreated seed for some seed lots (improvement in others). In these seed lots reducing the temperature by 1°C achieved similar levels of control without loss in germination.

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 there should be a 2 year break to ensure residues have disappeared.

Hygiene

Bacteria can be passively spread by anything that comes into contact with an infected plant or contaminated surface. It is important to remember that significant numbers of the pathogen may be present before symptoms appear.

Good hygiene is essential, to prevent moving the bacteria within crops and cross-contamination between crops. This means minimising the movement of machinery and people within and between crops and  cleaning and disinfecting equipment, clothing and footwear.

Bacterial pathogens can survive for long periods in dry crop debris, such as the dust/residue from infested seed lots. It is therefore important to clean and disinfect drills between seed lots.

Disinfectants

The efficacy of disinfectants is dependent on having sufficient contact time. Most standard disinfectants are likely to be effective against bacterial plant pathogens when used at recommended concentrations in long soaks.

On plants grown for seed, stem lesions may result in collapse; on infected flowers, petals may become brown and fall prematurely. Water-soaked lesions can develop on the green unripe fruit; these can later become dark and shrivelled.

The disease can be confused with physiological disorders such as ‘oedema’, ‘blue spot’ or ‘tip-burn’, so it is important to obtain an accurate diagnosis. A characteristic feature of both ‘blue spot’ (Figure 4) and ‘oedema’ (Figure 5) is that unlike bacterial blight, the lesions are only apparent when viewed from the upper leaf surface.

Pesticide sprays

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

Research in Australia on seed/spice crops indicated that the use of copper sprays may give a reduction in disease in some circumstances, when applied at the early stages of crop development and before disease symptoms are seen, but results were variable and unlikely to be economic. In any case there are currently no approvals for the use of copper compounds on coriander.

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 on other crops has shown the reductions achieved may not justify the costs.

Author

Dr Steven J. Roberts, Plant Health Solutions Ltd., Warwick, UK.

Please note this information was last updated in 2023. Biocidal and plant protection products must only be used in accordance with the authorised conditions of use. Regular changes occur in the authorisation status of biocides and plant protection products. For the most up to date information, please check with your professional supplier, BASIS registered adviser or the Chemical Regulation Division (CRD) of HSE (https://www.hse.gov.uk/crd/) before use.

While the Agriculture and Horticulture Development Board seeks to ensure that the information outlined on this page is accurate at the time of publishing, no warranty is given in respect thereof and, to the maximum extent permitted by law the Agriculture and Horticulture Development Board accepts no liability for loss, damage or injury howsoever caused (including that caused by negligence) or suffered directly or indirectly in relation to information and opinions contained in or omitted from this information.