Question: I planted several mountain laurels last year, and I noticed that over the winter they have developed black spots with a white center on all of the leaves. They are located on the edge of my property in a wooded area in partial to full shade. Can you tell me what this is? What is the best way to control the problem? Are the plants dying?
Answer: Mountain laurel (Kalmia latifolia) is a commonly used native ornamental shrub revered not only for its beautiful flowers but also for its attractive evergreen foliage, which adds color to the landscape year-round. These shrubs are shade tolerant and often used in wooded areas. They are a close relative of azaleas and rhododendron.
The foliage of mountain-laurel can be infected by at least 14 different species of fungi and suffers from an assortment of environmentally caused leaf disorders. Two of the most common are leaf spot and leaf blight.
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Leaf spot, which is what appears in the photo, is caused by the fungus Mycosphaerella colorata (Phyllosticta kalmicola), and is characterized by round to irregular, greyish white to silvery spots, ⅛ to ½ inch in diameter, with reddish to dark purple borders that may even appear almost black. Black specks, the fruiting bodies (pycnidia) of the fungus, form in the centers of older spots. Leaf spot is most severe under crowded, shaded and excessively moist conditions. As the disease progresses, the leaves will start to turn brown and may curl, eventually falling to the ground.
Leaf blight is caused by the fungus Diaporthe kalmiae (Phomopsis kalmiae) and can be distinguished from leaf spot by the larger, circular, brown lesions that often develop a zonate pattern. The lesions frequently first appear near the margin or tip of the leaf, gradually enlarge, merge with other lesions, and result in the death of the entire leaf blade. The fungus may also spread from the leaf through the petiole into the twig, causing twig blight.
The first step to controlling these diseases is to keep the plants healthy by providing them with acidic soil that is high in organic matter and is well-drained, but moist. Watering during dry periods in summer and fall and applying organic mulch will help prevent drought and winter injury.
Dead or diseased leaves should be removed from the bushes and soil surfaces and destroyed. Avoid placing any diseased materials in your compost bin, as this increases the risk of spreading the disease. If only a light infection of leaf spot or leaf blight is detected, removal and destruction of diseased leaves is often sufficient. While these diseases will not typically kill a mature mountain laurel, young seedlings or shrubs that lose all of their leaves are likely to decline and die from the stress.
Do you have a gardening related question you would like answered here? Please forward your questions to Belinda Chester, Master Gardener Program Coordinator, Rutgers Cooperative Extension Office, 6260 Old Harding Highway, Mays Landing, NJ 08330. You can also submit questions at Rutgers-atlantic.org/garden or email them to currents@catamaran.com; please include “garden question” in the subject line.
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