Mountain laurel susceptible to many health ailments
Q: I live in the Laurel Mountains and have a significant amount of mountain laurel. Over the past few years it has slowly been dying off. The leaves get spots on them and then turn yellow and fall off. I spoke to several resources and have received answers from “they are old and just dying” to “it is a fungus.” I have used many different anti-fungus sprays with little effect. I have more than half an acre to cover so it is difficult to spray all at the same time. Any recommendations?
A: Mountain laurel (Kalmia latifolia) is a beautiful, broad-leaved evergreen shrub that is native to Pennsylvania. It also happens to be our state flower. Unfortunately, mountain laurels are prone to any number of “health issues,” and I think there may be a combination of factors affecting your plants. Let me explain.
Many broad-leaved evergreens, including laurels, rhododendrons and azaleas, are prone to winter injury (also called leaf scald or leaf burn). If winter injury is the culprit, generally only the outer leaf margins turn brown and almost look “burned.”
Winter injury will seldom kill the shrub and can be prevented with an autumn application of an anti-desiccant product such as Wilt-Pruf, or by surrounding the plants with a barricade of burlap or landscape fabric to protect them from damaging winds. Winter injury is quite common and doesn't usually effect the overall health of the shrub.
The yellowing spots you describe on the leaves are most likely the result of a fungal infection. There are a number of fungal organisms that attack mountain laurels and, while they do cause discoloration of the leaves, they seldom kill the plant. There are at least a dozen fungi that cause what's commonly called Leaf Spot on laurels, and the only way to know which one you have is to send a tissue sample to a laboratory.
However, I don't think this is a necessary step because most Leaf Spot organisms do no long term damage to the plant. Fungal issues such as these can be staved off by pruning and removing affected leaves, spacing plants properly, cleaning up any fallen leaves, and irrigating only the root zone if possible.
The branch dieback you are encountering is the real threat to your laurels. Botryosphaeria Canker is a very common disease on several dozen species of plants, including laurels. The fungal spores enter plants via pruning cuts, sites of physical damage, or certain natural openings in plant tissue. A canker forms at the site and eventually leads to the death of the entire branch.
Damage from this disease usually appears one branch at a time. The leaves will curl downward and, if you look carefully, you can often see a circular canker or area of deformation somewhere along the branch. Botryosphaeria Canker is most common in plants facing some type of environmental stress, whether it be drought, heat stress, physical damage, over-crowded conditions, or any number of other factors.
There is no one solution to canker, but the damaged branches should be pruned out and burned or thrown into the trash immediately. Do any pruning on a dry day and remove the branch 6 to 8 inches below the canker site. Since the spores of this organism can spread via your pruning equipment, be sure to clean it with a 10 percent bleach solution after moving from one plant to the next.
Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio. She is the author of several gardening books, including “Grow Organic” and “Good Bug, Bad Bug.” Her website is www.jessicawalliser.com.
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