Bacterial shot-hole of cherry laurels: prevention and control

Find information on the prevention and control of bacterial shot-hole of cherry laurel.

This information was last updated in 2023.

Bacterial diseases are 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) propagation/planting material and good nursery hygiene. It is important to remember that the pathogen may be present in the absence of disease symptoms. 

Clean start

The apparent absence of infection or contamination with the pathogen in some stock plants (see CP 191) suggests that a strategy based on the production and maintenance of pathogen-free stock plants may be feasible.

Stock plants used to provide propagation material should be kept separate from production material and other susceptible hosts. Ideally they should be grown under protection and watered via a sub-irrigation system.

They should be tested for the presence of Pss and any testing positive or showing symptoms should be discarded. When propagating, wash and disinfect hands and secateurs/knives regularly, ideally between stock plants.

As part of AHDB CP 191, we established cv. Otto Luyken in micropropagation, as a first step to production of high-health stock plants. These plants were then successfully weaned, potted-on and remained pathogen-free until the end of the project.

Many production nurseries buy in liners from specialist nurseries, this means they have little control over the health status of the material. Due to the high intensity of production, there is much greater potential for rapid spread both within and between batches of plants from different origins. Disease symptoms may also not be apparent at the time of despatch to the production nursery. Freedom from symptoms does not mean that young plants are free from the pathogen.

Irrespective of the above, all bought-in plant material should be quarantined and carefully checked over several weeks for the presence of symptoms; the entire batch should be rejected if any symptoms are found.

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. Similarly, increasing the spacing between individual plants will also reduce the rate of spread.

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.

The most effective way to manage bacterial shot-hole is to grow cherry laurels under protection with sub or drip-irrigation to prevent the spread of the pathogen from plant to plant by water splash. Plants should be grown at as wide a spacing as economically possible. If the nursery production system permits, create barriers of non-host plant species between batches of plants.

It also makes sense to optimise the growing medium which ideally should be well drained, have low conductivity and a pH of 6.8-7.

Nursery hygiene

Bacteria can be passively spread by anything that comes into contact with an infected plant or contaminated surface. It is therefore important to develop a culture of good hygiene amongst all workers across the nursery.

Staff

Staff should wash/disinfect hands when moving between crops/batches. One way to encourage this is to issue all workers with their own disinfectant hand gel, and/or place dispensers at entrances/exits to glasshouses. It is particularly important to try to avoid the movement of machinery and staff within and between susceptible batches of plants when the plants are wet.

Cuttings, pruning, trimming

Taking cuttings, pruning and trimming are potentially highly effective ways of spreading bacterial diseases; not only are the bacteria spread on cutting blades, but they are simultaneously inoculated into a fresh wound. Outdoors, such operations should be done during periods of dry weather, not just dry on the day of the operation, but forecast to be dry for the next few days until wounds have become more resistant to infection. Pruning and cutting tools/knives/secateurs should be disinfected as often as possible during operations, and certainly when moving between different batches.

Clean-up

Dispose of infected plants and sweep up beds between crops to minimise the amount of crop debris remaining between batches of plants. Production areas should be disinfected to minimise pathogen spread.

Many bacterial pathogens can survive for long periods in dry crop debris, or in dried-on slime or ooze from infected plants. It is therefore important to remove crop debris and thoroughly clean down and disinfect surfaces and structures between crops/batches and particularly after infected crops.

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.

When working at a bench it is better to have at least two implements in use, so that one can be left to soak in disinfectant solution for as long as possible. Out on the nursery, this is not practical, but the results from HDC HNS179 [add link] suggest that the use of iso-propanol impregnated disinfectant wipes are a good compromise.

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.

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

Infection and disease development

Pss can live on plant surfaces without causing symptoms. It enters plant tissues through natural openings, such as stomata and lenticels and through wounds. Thus, any events causing damage to plants, such as hail, frost, pruning, or other mechanical damage, may provide opportunities for infection.

Once inside the plant, bacteria multiply and symptoms develop as a plant response to invasion. Infections tend to be localised with little evidence of systemic spread within the the vascular system. Precise conditions for infection and disease development have not been established, but in artificial inoculation experiments, symptoms first became apparent about six days after infection at temperatures in the range of 15-22°C.