American boxwood (Buxus sempevirens) and littleleaf boxwood (Buxus microphylla) are very common shrubs in Fairfield County and Westchester County landscapes. Landscape architects, landscape designers, and homeowners all love them for their rich evergreen color, dense growth habit, and resistance to deer browsing. Unfortunately these two shrubs also have a number of damaging insect and disease problems.
Boxwood insect problems include boxwood leafminer (Monarthropalpusi flavus), boxwood spider mite (Eurytetranychus buxi), and boxwood psyllid (Cacopsylla busi). The most common and most damaging of these three insect problems is boxwood leafminer whose symptoms include lumpy (especially on the bottom) and yellowed leaves. Boxwood mite symptoms include leaves that slowly turn grayish yellow as tiny leaf puncture wounds from these very small insects become more numerous and coalesce. Boxwood psyllid symptoms are cupped leaves. This leaf cupping actually shields the insect as it feeds on leaves. Boxwood leafminer and boxwood mite can cause significant damage but are easy to control, even organically. Boxwood psyllid is more difficult to control but doesn’t do any appreciable damage.
Boxwood diseases are much more difficult to identify and control. Most boxwood diseases are fungal pathogens, generally referred to by their scientific names: Volutella, Paecilomyces, Macrophoma, Phytophthera, and Thielaviopsis. These fungal diseases can have very similar symptoms including discoloration of leaves, wilting, dieback, and general decline. Symptomatic boxwoods often have more than one fungal pathogen. Positive diagnosis of these diseases is only possible with laboratory analysis. There are no effective organic or chemical control options for these diseases so management efforts are focused on prevention and sanitation.
Prevention takes two forms: planting disease free specimens, and maintaining proper cultural practices. Planting disease free plants is critically important. When new boxwoods are purchased, roots and soil from root balls should be tested before new boxwoods are transplanted into the landscape. This is especially important to prevent the introduction of Thielaviopsis as well as plant parasitic nematodes into your landscape. These two problems typically come from wholesale nurseries and once they are established in your landscape cannot be controlled or eliminated. Of recent boxwoods we have tested, roughly 25% had Thielaviopsis or parasitic nematodes, or both. Infested plants should be returned to the nursery where they were purchased.
Another means of preventing fungal disease problems in the landscape is to maintain good cultural practices. Boxwoods thrive in well drained soils. Often soils in suburban landscapes are compacted and don’t drain well. Additionally irrigation systems are often overused. This combination of compacted soils and over irrigation leads to water logged soils. Water logged soils are the perfect environment for Phytophthera and Thielaviopsis. Loosening soil and ensuring adequate drainage at time of planting, and proper irrigation including monitoring of soil moisture levels are critical to maintaining boxwood health.
Additional causes of disease in common and littleleaf boxwoods are several species of plant parasitic nematodes. Nematodes are non-segmented microscopic worms. There are thousands of species world-wide which fill many ecological niches. The few species that attack boxwoods can be very damaging and virtually impossible to control. Symptoms can be very similar to symptoms of the above fungal pathogens. Positive identification of parasitic nematodes requires the expertise of a nematologist. The only management option available to control parasitic nematodes is soil sterilization, which is not practical in suburban landscapes. These must be control by sterilization at wholesale boxwood growing facilities. All boxwoods purchased for landscape installation should be tested to be sure you are not introducing infested and disease boxwoods into your landscape.
Green Cross ISA Certified Arborists can assist you with identification and management of all of these boxwood problems.