Laurel: leaf diseases

Cherry laurel (Prunus laurocerasus) is generally considered a tough evergreen but its leaves can be affected by diseases such as powdery mildew, leaf spot fungi and bacterial shothole. Affected plants or hedges look unsightly.

Laurel: leaf diseases

Laurel: leaf diseases

Quick facts

Common name Powdery mildew, leaf spot fungi and bacterial shothole
Scientific name Podosphaera pannosa, Podosphaera tridactyla, Stigmina carpophila, Eupropolella britannica and Pseudomonas syringae
Plants affected Cherry laurel (Prunus laurocerasus); most of the diseases also on Portuguese laurel (Prunus lusitanica)
Main causes Fungi and bacterium
Timing Spring to autumn

What are laurel leaf diseases?

Leaves of cherry laurel (Prunus laurocerasus) are often affected by powdery mildew (Podosphaera tridactyla and Podosphaera pannosa), by leaf spot fungi (Stigmina carpophila and Eupropolella britannica) and bacterial shothole (Pseudomonas syringae), all of which can cause holes, tattering and distortion in the leaves. Most of the diseases (apart from Eupropolella) are also found on Portuguese laurel (Prunus lusitanica).

Symptoms

Laurels suffering from leaf diseases may show the following symptoms;

Powdery mildew:

  • Both powdery mildew species initially grow over the leaf surface, visible as a white powdery coating, particularly on the underside of the leaf
  • Later, underlying tissues go brown and die
  • Unusually for powdery mildew infections, the brown tissue then drops out, often leaving irregular holes in the leaves, and tattered edges which look more like insect damage than disease. The margins of the holes often have a corky, brown appearance

Leaf spot fungi and Bacterial shothole:

  • The leaf spot pathogens Stigmina and Eupropolella cause brown spots on the leaves. The centres of the spots may eventually fall out, leaving irregular holes in the leaves that resemble damage from shotgun pellets – hence 'shot-hole'
  • The bacterium Pseudomonas syringae causes water-soaked lesions that enlarge and turn tan with chlorotic (yellowing) halos. After leaf defences halt the enlargement of a lesion, the dead part eventually falls out – again, also referred to as 'shot-hole'

Control

Non-chemical control:

  • Little can be done by cultural means to prevent infections when conditions are suitable. Plants may sometimes 'grow through' the problem, with new leaves being unaffected when growing conditions change so that they are less conducive for the diseases
  • If the attack is very unsightly, consider trimming to remove affected leaves and encourage new growth. However, avoid heavy pruning as this will stress the plants and may aggravate the problem
  • Feeding may be helpful, although laurel is usually robust enough not to require it

Chemical control:

The fungicides tebuconazole (Provanto Fungus Fighter Concentrate), tebuconazole with trifloxystrobin (Provanto Fungus Fighter Plus, Toprose Fungus Control & Protect), and triticonazole (Scotts Fungus Clear Ultra and Scotts Fungus Clear Ultra Gun) are approved for the control of powdery mildews on ornamental plants. Tebuconazole with trifloxystrobin also carries a recommendation for control of 'leaf spots' of ornamentals, and may have some activity against the fungal leaf spot diseases of laurels. It is unlikely to have activity against bacterial leaf spot.

The following products contain a combination of both insecticide and fungicide, enabling the control of both insect pests and disease: myclobutanil containing cypermethrin (Westland Resolva Rose 3 in 1, Doff Rose Shield Bug & Fungus Killer, Scotts Roseclear Ultra Gun 2, Vitax Rosegarde) and triticonazole containing acetamiprid (Scotts Roseclear Ultra and Scotts Roseclear Ultra Gun).When a proprietary product contains an insecticide as well as a fungicide it would be preferable to use an alternative product if pests are not a problem on the plants treated.

SB Plant Invigorator, Resolva Natural Power Bug and Mildew Control, RHS Bug and Mildew Control, Ecofective Bug & Mildew Control and the Ecofective ‘Defender’ range contain a blend of surfactants and nutrients and can be used on any edible or ornamental plants, with no harvest interval. They have a physical mode of action and may be used against powdery mildews, as well as a range of pests such as whiteflies, aphids, spider mites, mealybugs, scale insects and psyllids.

Inclusion of a product does not indicate a recommendation or endorsement by RHS Gardening Advice. It is a list of products currently available to the home gardener.

Download

Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)

Links

Chemicals: using a sprayer
Chemicals: using safely and effectively
Chemicals: storing and disposing safely

Biology

The powdery mildew pathogen Podosphaera pannosa also attacks roses and a distinct variety attacks peaches. Podosphaera tridactyla is found on numerous host species of the genus Prunus. As with other powdery mildews, these species grow initially over the leaf surface, feeding from the tissues but not killing them, and producing white, airborne spores which spread infection. Later the tissues die and, unusually for powdery mildew infection, drop out leaving holes and tattered edges to the leaves. An overwintering stage is of minor importance, most survival through the winter is as mycelium on the evergreen leaves. 

The shothole fungi survive the dormant season in lesions on twigs, buds or leaves and produce spores that are mainly dispersed by rain. Infections occur in young leaves, but as they expand the infection stabilises and the healthy leaf tissue pulls away from the lesion which drops out, leaving a hole. Severe attacks result in a ragged appearance to the leaves. Stigmina carpophila attacks other Prunus species but Eupropolella britannica has only been recorded on Prunus laurocerasus.

The bacterium Pseudomonas syringae is readily spread in wind-driven rain and penetrates host tissues through natural openings or wounds. The bacteria survive on plant tissues when the conditions are unfavourable to multiplication and infection.

The bacterium Xanthomonas arboricola pv. pruni also affects Prunus species (including laurel) and although not thought to be established in the UK, there have been cases confirmed by Defra. The disease is established in several European countries, where it has also been found affecting commercial production of Prunus laurocerasus. Symptoms include shothole, fruit and leaf spotting, stem cankers and severe defoliation.
Any suspected cases should be reported to the relevant plant health authority, whose contact details can be found on the UK Plant Health Information Portal


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