Decline (Twig Blight, Dieback): Boxwood decline is a poorly understood complex involving the fungi Volutella, Paecilomyces, Macrophoma and Phytophthora, as well as cold injury and nematodes (microscopic round worms). This phenomenon is also closely related to cultural problems associated with boxwoods, such as improper pH and nutritional status, drought, poor drainage, and improper mulch management. Volutella can cause a dieback on all types of boxwood. Macrophoma can cause leaf blight, but it usually acts as a weak pathogen. Paecilomyces buxi has been consistently associated with roots of English boxwood exhibiting the syndrome of boxwood decline.

Symptoms consist of weak and spindly plants. Dead or dying branches occur randomly in the bush. The older leaves drop prematurely and the remaining foliage develops a yellow color. Leaves often have pink eruptions of spores on black fruiting bodies. Dead areas or cankers develop along branches or near the crown. Various species of nematodes (microscopic worms that feed on the roots) also appear to be involved.

Prevention & Treatment: A thorough diagnosis of the associated factors is important before corrective action is taken. This should include a nematode analysis, soil analysis, and evaluations of drainage in the area and the degree of rooting in surface duff (litter).
Crowded growth and dead leaves in the branch crotches tend to maintain high levels of humidity in the canopy, making conditions conducive to die-back diseases. Prune dead stems back to healthy tissue. Disinfect pruning shears frequently in household bleach diluted 1:9 with water. Removal of dead branches and leaves from crotches of the plant, as well as yearly renewal of mulch material, will also aid in control.

Proper cultural practices, such as providing water when necessary, avoiding over-watering or excessive fertilizing, and thinning shrubs to allow better air circulation are of utmost importance in maintaining a vigorous condition. To prevent winter injury, make sure sufficient soil moisture is available during the fall. Plants in highly exposed situations may require wind protection. This is especially important in upstate areas where the soil can freeze and remain frozen on sunny days. When this happens, the foliage continues to transpire but the roots cannot replace the lost moisture from the frozen soil.