Shot hole disease of cherry laurel

Several pathogens have been described to be involved in the development of shot hole disease symptoms on cherry laurel, including fungi (i.e., Neofusicoccum parvum) and bacteria (i.e., Micrococcus aloeverae, Xanthomonas arboricola pv. pruni , Pseudomonas syringae)

Eukaryota; Fungi; Dikarya; Ascomycota; Pezizomycotina; Dothideomycetes; Dothideomycetes incertae sedis; Botryosphaeriales; Botryosphaeriaceae; Neofusicoccum; Neofusicoccum parvum

Bacteria; Bacillati; Actinomycetota; Actinomycetes; Micrococcales; Micrococcaceae; Micrococcus; Micrococcus aloeverae

Bacteria; Pseudomonadati; Pseudomonadota; Gammaproteobacteria; Lysobacterales; Lysobacteraceae; Xanthomonas; Xanthomonas arboricola pv. pruni

Bacteria; Pseudomonadati; Pseudomonadota; Gammaproteobacteria; Pseudomonadales; Pseudomonadaceae; Pseudomonas; Pseudomonas syringae

Current name:

• Neofusicoccum parvum (Pennycook & Samuels) Crous, Slippers & A.J.L. Phillips

Basionym:

• Fusicoccum parvum Pennycook & Samuels

Taxonomic synonym(s):

• Botryosphaeria parva Pennycook & Samuels
• Neofusicoccum algeriense A. Berraf-Tebbal & A.J.L. Phillips
• Neofusicoccum italicum Dissan. & K.D. Hyde
• Neofusicoccum pandanicola Tibpromma & K.D. Hyde

Cherry laurel (Prunus laurocerasus L.) is an evergreen plant with large, glossy, dark green leaves. It is a widely cultivated ornamental plant used for hedging and screening in public greens and homes in temperate regions worldwide. It is also widely produced in ornamental nurseries.

Leaves of infected plants exhibit reddish-brown necrotic lesions often with concentric rings, surrounded by a reddish border or a light green halo. With the development of the disease, in case the disease is caused by fungi, black pycnidia forms on the lesion surface. Pycnidia contain one-celled, hyaline, fusiform to ellipsoid conidia, smooth with fine granular content, typical for Neofusicoccum parvum and sometimes also smaller one-celled, hyaline, rod shaped conidia with rounded ends called spermatia. In the final phase of disease development, necrotic lesions drop out of the leaves, leaving holes in the leaf that resemble damage from shot gun pellets (“shot hole” appearance).

Symptoms of a shot hole disease caused by Neofusicoccum parvum on ornamental Prunus laurocerasus in Serbia. (a) Leaves with a shot hole appearance, (b) brown necrotic lesion surrounded by a light green halo, (c and d) necrotic lesion with N. parvum pycnidia developed on a lesion surface, (e) close up of leaf with hole. Bars: a = 1 cm, b–d = 0.3 cm.

Since the 1980s, shot hole symptoms have been reported on cherry laurel in several countries across Europe and North America. However, the causal agents have differed between countries. Shot hole disease caused by pathogenic bacteria has been reported in British Columbia in 1980, and in Spain during the 1990, with Pseudomonas syringae identified as the causal agent. In the Netherlands (2008) and Italy (2010), Xanthomonas arboricola pv. pruni was identified as the cause. In Ireland, the disease was reported in 2017, caused by Micrococcus aloeverae. As for pathogenic fungi, the disease was found in Serbia in 2010, with Neofusicoccum parvum identified as the causal agent.

Shot hole disease has been found in Serbia, Netherlands, Ireland, Spain, Italy, British Columbia, possibly elsewhere where cherry laurel is grown.

The ecology of shot hole disease in cherry laurel remains largely unknown. However, in general, bacterial spores are disseminated by raindrops and insects, while Neofusicoccum parvum spores are spread by raindrops and wind currents.

The causal agents of the disease are currently not included in the EPPO A1 or A2 lists of regulated pests.

Although the disease primarily affects leaves and is often considered an aesthetic issue, in Serbia it significantly impacts commercial cherry laurel production in certain ornamental plant nurseries. The presence of shot holes reduces the likelihood that buyers will purchase affected plants. A similar impact may occur in other countries, particularly in ornamental nurseries where plants are not regularly treated with pesticides and are grown in proximity under mist irrigation systems, which increase humidity and favor the development and spread of the disease.

The disease is of particular concern in Serbia, as P. laurocerasus is a relict species endangered by extinction due to its limited population distribution. It occurs naturally only on the Ostrozub Mountain in southeastern Serbia, within cherry laurel–beech forests (Lauroceraso-Fagetum).

In ornamental nurseries, the best preventive measures against the disease include maintaining adequate spacing between plants to reduce humidity and avoiding the use of mist irrigation systems for the same reason. Additionally, since shot holes containing fungal pycnidia fall from the leaves, they should be collected and properly disposed of to prevent further spread of the disease.

Streptomycin can effectively control the disease when it is caused by bacteria. However, there are documented cases where streptomycin became ineffective in nurseries where it had been used repeatedly over several years. Moreover, its use on crops is now banned in many countries, including those in the EU.

Preventive treatments with copper-based or systemic fungicides may help control the disease, but there is currently no research confirming their effectiveness.

In urban areas, the best preventive approach is to avoid planting cherry laurel in locations with full sun, as it prefers partially shaded environments. It is also important to remove and destroy fallen shot hole debris to minimize disease spread.

The causal agent can be effectively identified through isolation, PCR, sequencing, and multigene phylogenetic analyses. Pathogenicity can be confirmed by fulfilling Koch’s postulates.

1.    Zlatković, M., Keča, N., Wingfield, M.J., Jami, F. and Slippers, B., 2016. Shot hole disease on Prunus laurocerasus caused by Neofusicoccum parvum in Serbia. Forest pathology, 46(6), pp.666-669.

2.    De Boer, S.H., 1980. Leaf spot of cherry laurel caused by Pseudomonas syringae. Canadian Journal of Plant Pathology, 2(4), pp.235-238.

3.    Marchi, G., Cinelli, T. and Surico, G., 2011. Bacterial leaf spot caused by the quarantine pathogen Xanthomonas arboricola pv. pruni on cherry laurel in central Italy. Plant Disease, 95(1), pp.74-74.

4.    Smith, L., Gibriel, H.A.Y., Brennan, C., del Pino de Elias, M., Twamley, A., Doohan, F., Grogan, H. and Feechan, A., 2020. First report of shot hole disease on cherry laurel (Prunus laurocerasus) caused by Micrococcus aloeverae in Ireland.

5.    Tjou-Tam-Sin, N.N.A., Van de Bilt, J.L.J., Bergsma-Vlami, M., Koenraadt, H., Naktuinbouw, J.W., Van Doorn, J., Pham, K.T.K. and Martin, W.S., 2012. First report of Xanthomonas arboricola pv. pruni in ornamental Prunus laurocerasus in the Netherlands. Plant Disease, 96(5), pp.759-759.