Compact (or dwarf) English laurels (Prunus laurocerasus "Nana") are smaller versions of English laurels (Prunus laurocerasus). While English laurels can reach 20 feet or more in height if untrimmed, the compact English laurel only reaches 8 to 10 feet tall. Compact English laurels are slightly less hardy than their larger cousins, growing well in U.S. Department of Agriculture plant hardiness zones 7 through 9, while the large version is hardy down to USDA zone 6. Compact English laurels are most often used as hedge plants, and bear attractive, glossy, evergreen foliage.
1. Bacterial Diseases
A number of bacterial diseases may strike the leaves of compact English laurels. Most infections are caused by the bacteria Pseudomonas or Xanthomonas, and cause water-soaked, brown spots on the leaves. Often, these spots are surrounded by a reddish border with a yellow halo. These bacteria are wind-driven to the shrub and penetrate the tissues though natural openings, like the stomata, or through wounds, like those caused by breakage, insects, or pruning. There is no cure once the plant has been infected, but it's seldom fatal.
2. Fungal Disease
Fungal diseases on the leaves usually create a "shot-hole" effect, leaving small holes in the leaves. Several fungi create the same effect, including Cercospora, Blumariella, Stigmina, and Eupropolella, among others. These fungi leave brown to tan spots on the leaves, which often dry and fall out, creating the hole. Preventative fungicides may only be effective if fungi are actually causing the holes, because this symptom may also be indicative of other diseases or conditions, making control problematic if not correctly diagnosed. Seldom are these fungal diseases fatal.
3. Molds and Mildews
Molds and mildews, while technically fungi, are often far easier to treat than other fungal infections. In fact, many gardeners may consider them cosmetic issues and not treat the infection at all. Powdery mildew causes a talcum powder-like coating over the top of the leaf. Spraying a baking soda solution of 1 teaspoon baking soda per 1 quart water often stops the infection from spreading. Sooty mold is a black, powdery substance on the leaf surface, but it grows in the honeydew left behind by insects rather than feeding on the leaf. Control the problem by cleaning leaves and controlling insects, such as aphids, which create honeydew.
4. Conditions that Mimic Disease
Some English laurels show sensitivity to copper, producing symptoms similar to fungal or bacterial infections. Ironically, copper-based fungicides are often recommended for fungal and bacterial infections, but tests involving other Prunus laurocerasus varieties indicate "Nana" may also be susceptible. Shot-hole-like symptoms may be caused by cold weather or excess boron in irrigation water, making diagnosis difficult.