Laurel hedge: Fungicides can control powdery mildew
Question: My laurel hedge is eight feet high and three feet wide and 80 feet length, the problem is it is sick. It is old and is being attacked by some horrible pest. Can you advise me on a treatment for this (leaves enclosed) as I would hate to have to remove it. (Mrs V Thornington, Oulton Broad)
The problem is not a pest but that really is where the good news ends. 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 making affected leaves look very unsightly, just like the ones you have sent.
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.
The powdery mildew pathogen P.pannosa also attacks roses and a distinct variety attacks peaches. P.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.
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'. Severe attacks result in a ragged appearance to the leaves.
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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. Where the attack is very unsightly, consider clipping 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 (Plant Rescue Fungus Control), myclobutanil (Bayer Garden Systhane and other formulations), tebuconazole (Multirose 2) and triticonazole (Scotts Fungus Clear Ultra) 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.