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.
What are laurel leaf diseases?
Leaves of 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.
Laurels suffering from leaf diseases may show the following symptoms;
- Both powdery mildew species initially grow over the leaf surface, visible as a white powdery coating
- 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
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'
- Little can be done by cultural means to prevent infections when conditions are suitable. Plants usually grow through the problem, with new leaves being unaffected when growing conditions change
- 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
Fungicides containing difenoconazole (Westland Plant Rescue Fungus Control), myclobutanil (Doff Systemic Fungus Control and other formulations), tebuconazole (Bayer Garden Multirose concentrate2) and triticonazole (Scotts Fungus Clear Ultra and other products) are approved for use against powdery mildews on ornamental garden plants and would probably give useful incidental control of shothole fungi as well, although this is not claimed by the manufacturers.
Plant and fish oil blends (Vitax Organic 2 in 1) and sulphur with fatty acids (Scotts Natural Fungus and Bug Killer) can be used on all plants against powdery mildews but are unlikley to have any incidental effect on leaf spot fungi or bacterial shothole.
Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)
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.