Q: The stems of cherry laurel are turning white. It looks like mold or some sort of infection. Some leaves look OK while others have turned yellow or brown. Can I treat it?
This actually isn’t fungal, and instead is a population of a pest insect called white prunicola scale. Scale insects suck plant juices, and they live either on foliage for part of the year or, in this case, year-round on bark. The white material is wax produced by the insects to shield their bodies from the elements and predators.
Both sexes of mature prunicola scale have white parts to their shell-like body covering. The juvenile males tend to congregate on the underside of branches in clusters. This makes the infestation much easier to detect, even though it’s a bit late to be considering management tactics at that point, since the scale have been present and breeding for one or more years.
Favored host plants include cherry laurel and their flowering cherry tree relatives, but they can also feed on some unrelated shrubs, such as lilac, holly, boxwood, and privet (which is invasive). When Extension receives diagnostic questions about this scale, the insects are most often on cherry laurel that are pruned into a hedge or to fit within a restricted space by a home foundation.
Managing high populations of scale often involves the use of insecticides. Replace infested plants if you’re not willing to (or if it’s not feasible to) spray. Scale insects are at their most vulnerable to low-toxicity insecticides like horticultural oil when they are newly-hatched and mobile, a life stage appropriately called a “crawler.” If you want to try a pesticide that does not have a long residual, which reduces the risk of exposure to beneficial insects, you need to determine when crawlers are active so you can time the spray appropriately.
White prunicola scale has three generations per year in Maryland. While other insecticides provide longer-lasting control with fewer applications, these ingredients may harm other organisms, such as pollinators visiting the plant’s flowers. Maryland law requires certain products be applied by a certified pesticide applicator to minimize pollinator exposure.
Keep in mind that dead scale, or the wax that covered them, do not necessarily fall off the plant right away, so an infestation may look heavier than it really is, or it may appear that a pesticide application or natural predation isn’t working. Consider hiring an experienced professional to help scout for crawlers and to decide what pesticides may be warranted. Crawlers are barely visible to the naked eye, and distinguishing live from dead scale requires magnification.
Q: This was my first year having a vegetable garden. Any steps I should be taking now?
It’s a good time to remove any plant debris for crops that have finished for the year. For example, leafy greens like kale which you may shelter through the winter under row cover can stay, but tomatoes, potatoes, peppers, beans, cucumbers, squash, pumpkins – any warm-season crop that finishes ripening fruits or dies due to an overnight frost, whichever comes first – should be taken out.
Dispose of stems, fallen leaves, and fruits; any residue left behind can harbor pests and diseases over the winter that can increase the chances of a recurring problem next year. (
Although it’s too late now to plant a cover crop for winter, you can use mulch to protect any exposed soil from erosion and weed encroachment: arborist wood chips, straw, pine needles, or fallen leaves, all of which improve soil health as they decompose.
University of Maryland Extension’s Home and Garden Information Center offers free gardening and pest information at extension.umd.edu/hgic. Click “Ask Extension” to send questions and photos.
