Factsheet 04/10 Horticulture Stoneleigh Park Development Kenilworth Hardy Nursery Stock Warwickshire CV8 2TL Company T: 0247 669 2051 E: [email protected]

Bacterial shot-hole of laurel

Steve Roberts, Health Solutions Ltd

Bacterial shot-hole of cherry laurel is caused by the bacterial pathogen Pseudomonas syringae pv syringae. The disease can lead to product downgrading and losses in the case of cherry laurel and other susceptible species. This factsheet describes the symptoms and biology of the disease and summarises the available control measures.

Action points

●● Quarantine bought-in plant material ●● Dispose of visibly infected ●● Use as wide a plant spacing for at least a week and check for and plant debris. as economically possible. the appearance of symptoms. ●● Clean and disinfect production ●● Create barriers of non-hosts plant ●● Get the identity of the beds between crops. species between batches of plants. pathogen checked in new ●● Minimise the movement of outbreaks, especially on ●● Grow cherry laurels machinery and people within under protection. imported plant material. and between batches of laurels, especially when plants are wet. ●● Keep batches of plants ●● Optimise the growing medium from different sources used in terms of drainage, ●● Wash hands when moving separate if practical. conductivity and pH. between susceptible crops.

●● Focus on ensuring the health ●● Use sub or drip-irrigation or ●● Clean and disinfect status of stock plants. minimise overhead watering. and cutting tools frequently.

1 Bacterial spot symptoms on a young leaf of laurocerasus ‘Rotundifolia’ Background

Cherry laurel (Prunus laurocerasus) (HDC project HNS 71). It is one of varieties such as ‘Otto Luyken’ is one of the single most important the major causes of losses in cherry and ‘Zabeliana’, but the disease hardy nursery stock species; large laurels on commercial nurseries affects all varieties and the related numbers are produced for and widely and can result in up to 30% of stock Portuguese laurel (P. lusitanica). used in landscaping and in amenity becoming unmarketable. Losses have A 3-year MAFF-funded project on plantings. Plant propagation is via led some growers to reduce or stop the disease was completed by the cuttings and production may occur production resulting in increases in author in 1998. There has also been in containers or in the open ground. the level of imported plant material. some limited work undertaken in In an HDC-funded survey conducted The disease has been reported in other countries including France and Canada. This factsheet summarises in 1996-97, leaf spot and shot-hole of the UK, and N. America since the currently available information. cherry laurel was the most prevalent the early 1980s. It is particularly bacterial disease noted on nurseries troublesome on ground cover laurel

Disease recognition and biology

Causal organism the precise host range is unclear and Abscission layers develop around the scientific literature is confusing. the spots which then eventually drop In the UK, bacterial leaf spot and shot- This is probably due to the frequent out to give the characteristic ragged, hole of cherry laurel and related plant but incorrect assignment of unknown shot-hole appearance (Figure 2). species is caused by the bacterium strains of P. syringae to pv. syringae. Stems can also be infected to give cankers and shoot die back especially Pseudomonas syringae pv. syringae. Limited host-range studies were done in combination with frost (Figure 3). Another bacterium Xanthomonas as part of the MAFF-funded project. arboricola pv. pruni (synonymous with Pathogenic isolates from cherry laurel The shot-hole symptoms can be easily X. campestris pv. pruni) is also known were able to infect lilac and forsythia. mistaken for insect damage. On the to infect cherry laurel (and other Prunus Isolates from lilac and Prunus avium other hand, not all shot-hole symptoms species), and is often considered were able to infect cherry laurel. P. are necessarily caused by bacteria and as the primary cause of shot-hole syringae pv. morsprunorum, which they are also reported to be caused by causes bacterial canker and shot- in N. America. This is a quarantine a number of different fungi (for example hole on sweet cherry and plums, pathogen within the EU. There have the leaf spot fungi Stigmina carpophila). did not infect cherry laurel (but has been localised reports on stone Probably the most common alternative been reported to do so in Spain). cause of shot-holes in cherry laurels subjects in France and Switzerland, (especially on ) is infection and it has recently been reported by powdery mildew (Podosphaera on cherry laurel on nurseries in the Symptoms tridactyla and P. pannosa). Netherlands (2008) and Italy (2005). Another cause of confusion is marginal During the HDC-funded survey of The bacterium infects via natural leaf necrosis symptoms. This is a HNS and the MAFF funded project openings and wounds, and infections tend to be localised with little or physiological problem associated only P. syringae pv. syringae was no evidence of systemic spread with poor growing conditions (for isolated from cherry laurel, but given through the vascular system of the example drought stress, water-logging the recent reports of X. arboricola pv. plant. Precise conditions for infection or high growing media conductivity pruni from other parts of Europe, it is and disease development have levels). Typically these brown areas recommended that the causal organism not been established, however in at the edges of drop off, is confirmed by laboratory identification. artificial inoculation experiments, giving the leaf margins a notched symptoms first became apparent appearance (Figures 4 and 5). about six days after infection at Host range The presence of certain bacteria can temperatures in the range of 15-22°C. also lead to increased levels of frost The species P. syringae is divided into Disease symptoms can be quite damage in crops, as the bacteria can pathovars (pv.) on the basis of plant variable. Typically the disease is act as ice nuclei. An ice nucleus is a host range, for example P. syringae pv. seen as brown necrotic leaf spots particle which acts as a point for the berberidis infects only Berberis species around 2 to 10 mm in diameter. In the formation of an ice crystal. Many strains whilst P. syringae pv. philadelphi infects very early stages, spots may have of P. syringae (including all P. syringae only Philadelphus species. P. syringae a water-soaked margin, most easily pv. syringae isolates pathogenic on pv. syringae is considered to have a seen by holding an infected leaf cherry laurel) can act as ice nuclei broad host range that includes lilac, all up to the light. On younger leaves, and promote the development Prunus species, Phaseolus beans and spots develop a reddish border and of ice crystals within plants, many other plant species. However, a yellow chlorotic halo (Figure 1). increasing levels of frost damage. 2 Leaf spots eventually drop out to give leaves a ragged 3 Shoot dieback caused by a combination of frost and appearance Pseudomonas syringae pv. syringae

4 Shot-hole symptoms on P. laurocerasus ‘Rotundifolia’ 5 Marginal leaf necrosis can be a physiological problem caused by powdery mildew Disease sources and spread

Pathogen presence on with production stage, site and P. syringae is generally considered commercial nurseries sampling date. In general, the to survive poorly in soil and site with the most severe disease growing media and so these are As part of the MAFF-funded problems also had the highest unlikely to be significant sources project, studies were undertaken levels of contamination. Plants in of inoculum. Crop debris and at two commercial nurseries. protected environments (cuttings residues from previously infected The nurseries were located in and liners) had lower levels of batches may provide an inoculum different parts of the country contamination compared with source over the short-term, (South coast and North West) plants in the open (stock plants especially from crop to crop within and followed different production and marketable plants). a growing season, and particularly practices. Samples of apparently if the rate of debris breakdown is healthy leaves were collected at limited by dry or cold conditions. Inoculum sources each stage of production (from stock plants, through to rooted Stock plants, imported plants, Spread cuttings, liners and marketable previous years’ production and plants) at each nursery and alternate host species may all In common with many other tested for the presence of serve as sources of inoculum. bacterial diseases, spread from P. syringae pv. syringae. plant to plant occurs by water- Detection of the pathogen splash (rain or irrigation), wind Pathogenic isolates of P. on apparently healthy leaves driven rain and via the movement syringae were detected on means that the absence of of people, animals, insects apparently healthy leaves at all symptoms does not guarantee and machinery within crops. stages of production on both that the material is free from nurseries. Contamination varied the pathogen (Figure 6).

6 The potential sources of inoculum within cherry laurel production. Control strategy

Due to the potential for cross- drained, have a low conductivity The bio-pesticide Serenade infection with other susceptible and a pH of around 6.5. ASO (active ingredient Bacillus species it is important that subtilis) has a Specific Off-Label Nursery hygiene and management of this disease Approval (SOLA 0246/2009) is approached on a whole use of disinfectants for use on ornamentals, and is nursery basis. There is little Dispose of infected plants and known to have activity against value in putting effort into sweep up beds between crops bacteria, but its efficacy as a eliminating the disease in cherry to minimise the amount of crop foliar spray for control of bacterial laurels if there is regular re- debris remaining between leafspot and shot-hole of cherry infection from other species. batches of plants. Production laurel has not been examined. areas and pruning tools should be Recommendations from other Cultural methods of control disinfected to minimise pathogen parts of the world are often spread. P. syringae pv. syringae is vague and generally indicate Bought-in plants and sensitive to most general purpose the use of copper compounds or stock plants disinfectants, in HDC project HNS the antibiotic Streptomycin (not 91, both Jet 5 (peroxyacetic acid) approved in the UK). However The symptomless contamination and Menno Florades (benzoic there have been no definitive of stock plants with the pathogen acid) were effective at reducing studies and results reported for suggests that a strategy based on the pathogen to undetectable copper compounds are variable. the production and maintenance levels in laboratory tests. In an HDC-funded trial on bacterial of pathogen-free stock plants diseases of hardy nursery stock Wash hands when moving should be an important part of (HDC Project HNS 91) none of the between susceptible crops any management strategy for this compounds examined (including and try to avoid the movement disease. Bought-in plants should Bordeaux mixture, fosetyl- of machinery and staff within be quarantined in a separate area aluminium and peroxyacetic acid) susceptible batches of plants from susceptible varieties until the gave satisfactory control despite health status of the plant material especially when the plants are wet. being applied on 26 occasions can be ascertained (by laboratory over a period of 37 weeks. examination if required). If Foliar treatments practical, keep batches of plants In areas where copper and/or from different sources separate. Streptomycin have been used Currently the only approved extensively on ornamentals, Production system fungicides with known activity P. syringae has developed against bacterial plant resistance. Nevertheless, The most effective way to pathogens are those based on copper resistance was never manage bacterial shot-hole is copper compounds or the bio- detected in isolates obtained to grow cherry laurels under pesticide Serenade ASO. during the MAFF study on plant protection with sub or drip- material obtained in the UK. irrigation to prevent the spread of The copper based products the pathogen from plant to plant Cuprokylt and Cuprokylt FL If the decision is made to apply by water splash. Plants should (copper oxychloride) have activity sprays to control the disease, it be grown at as wide a spacing against bacterial diseases. Neither is important that consideration is as economically possible. If of these products has a label also given to treating stock plants, the nursery production system recommendation for use in hardy young plants and other potential permits, create barriers of non- ornamental plant production or hosts on the nursery. Both copper host plant species between a Specific Off-Label Approval based fungicides and Serenade batches of plants. A suitable (SOLA). Use is therefore currently ASO work as protectants; growing medium should be via the Long Term Arrangements therefore applications made used to grow the plants in, for Extension Use (LTAEU), after symptoms are widespread the medium should be well until a SOLA can be obtained. are likely to have little impact.

Disease diagnosis

Shot-hole symptoms present a lesions have dropped out. Samples Clinic, Sand Hutton, York YO41 problem for successful diagnosis with a range of symptoms (packed 1LZ (www.fera.defra.gov.uk) or as the infected tissues are often in absorbent paper in plastic bags Plant Health Solutions Ltd, Ryton missing. It is therefore important within a box for protection) should , Wolston Lane, Coventry to send samples for laboratory be sent to a laboratory specialising CV8 3LG (www.planthealth.co.uk). diagnosis at the earliest stages in the diagnosis of bacterial of infection and certainly before diseases; for example Fera, Plant Further information

A summary of the MAFF-funded nursery stock: Examination of www.hdc.org.uk. A further HDC project (HH1731SHN ‘Aetiology the distribution and importance funded project HNS 179 ‘Management and control of bacterial leaf spot of bacterial diseases’ and HNS 91 of bacterial canker in Prunus of cherry laurel’) can be found at ‘Bacterial diseases of HNS: species’ has recently commenced. www.planthealth.co.uk/publications/. Chemical control’) are available Reports on the HDC funded projects from the HDC by either phoning referred to (HNS 71 ‘Ornamental 0247 669 2051 or visiting

Acknowledgements

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