Illinois Extension

We've seen quite a few boxwood samples at the Plant Clinic already this year. Boxwoods can get a number of pest and disease issues in Illinois; the most common are winter damage, a few fungal diseases, and a few insect pests. We haven't seen much winter injury this year (which isn't a surprise, given the mild winter). The two most common problems seen so far have been Volutella blight and boxwood leaf miner.

We have seen a lot of Volutella, a common fungal pathogen that attacks boxwood (we also found Volutella on a pachysandra sample, a relative of boxwood). Volutella produces salmon-pink spore masses on the undersides of affected leaves, and can also cause stem lesions. Management for Volutella includes avoiding overcrowding when installing plants, pruning to improve air circulation around the plants, and fungicide applications that start when the plant is dormant and continue at labeled intervals through the spring or summer.

Macrophoma leaf spot is another fungal foliar disease of boxwoods. This disease is much less common in Illinois and rarely causes enough injury by itself to pose a threat to healthy boxwoods. Leaves turn straw-colored or white, and small, black fruiting bodies are produced on the affected leaves. Usually cultural controls aimed at increasing air circulation and increasing plant vigor are enough to manage this disease.

Boxwood leafminer is a common insect pest of boxwoods. The insects lay their eggs inside the leaves in early summer. Larva hatch and devour the inner tissue of the leaf, creating pockets or blisters that turn yellow, then brown as the season progresses. The larva overwinter in the leaves; in spring, they produce cocoons on the underside of the leaf. They pupate in the cocoons, then emerge as adults. Emergence usually occurs in central Illinois as weigela begins to bloom (mid-May). Contact and systemic insecticides are available for use in controlling boxwood leafminer.

Boxwood psyllid is another common insect pest of boxwoods. These tiny winged insects cause cupping of the leaves at the tips of branches. Scouting for this damage is very easy, as the cupped leaves should be readily apparent. The insects are also large enough to see, and can often be found flying around boxwood plants in late May and early June. Both contact and systemic insecticides are labeled for use against this boxwood psyllid.