Prunus Lusitanica is the popular Portugal laurel. It has thick, glossy pointed leaves and grows as a large bush or small tree. The Portugal laurel is slower growing than cherry laurel and has small white flowers followed by red or purple fruit. Laurel plants do not often have problems with pests or disease, however, shot hole disease is a common fungal ailment in the Prunus genus. Bacterial canker is another problem that the genus encounters.
Bacterial canker is a common affliction on Prunus, on both the fruiting and ornamental varieties. It is caused by two different bacteria that affect the leaves, stems and trunk of an infected plant. The genus Prunus includes apricots, cherries, plums and other fruiting trees, as well as the non-fruiting varieties like Portugal laurel. The leaves get brown, necrotic spotting, and the woody parts of the plant weep and become gummy and gluey with affected sap. Cankers appear as wounds on the wood and the stems and branches can become girdled by the bacteria and die.
Control of Bacterial Canker
Canker begins in winter when the bacteria ooze out of the sores in the wood. Cool, wet weather hastens the spread of the disease as the bacteria are carried on water to unaffected parts of the laurel. The disease can also spread systemically inside the plant, which makes control difficult. Copper fungicides help, if applied before the problem is too severe. Pruning out affected parts, cleaning up the infected dead wood and leaves below the plant, and sanitising any cutting tools between uses seems to provide even better control than chemicals.
Shot Hole Disease
Shot hole is another possible reason for brown leaf spots. The initial presentation is a reddish-brown sore in the leaf surface, which will have a yellow halo around it. As the disease progresses, the centre of the spot dies and falls out, leaving what looks like a bullet hole. The disease is caused by several bacteria, and it was recently discovered that there is a fungal association as well. The disease is the most pronounced in mid- to late summer when overhead irrigation is used.
Control of Shot Hole Disease
Copper fungicides were once the method of choice but provided little control of the disease. More recently, new fungicides have been tested that limit the infection. Chlorothalonil, thiophanate methyl and mancozeb were successful in trials of the sprays. Laurel trees are being bred for resistance to the disease, and a full sun exposure seems to prevent the infection. Mechanical cleanup is always a useful when used in conjunction to chemical preventatives. The infected dead material needs to be properly destroyed, as the bacteria may still be present after composting.
- The Center for Applied Nursery Research: Evaluation of Shot Hole Disease Development on Laurels and Efficacy of Control; Dr. Jean L. Williams-Woodward
- Washington State University Extension: Bacterial Canker of Prunus; Ralph S. Byther
- Royal Horticultural Society: Bacterial Canker
- Clemson Cooperative Extension: Laurel; Marjan Kluepfel, Bob Polomski; May, 2009
- Washington State University: Gardening in Western Washington: Evergreen Shrubs for the Northwest; Mary Robson; March 19, 2000