English Boxwood Decline

• The leaves of the English boxwood are affected by decline, which is a fungal disease that attacks when the growing conditions of the boxwood are inadequate. Excess water, improper pruning and a lack or excess of fertilizer all contribute to growing conditions that contribute to English boxwood decline. This disease causes the leaves to turn yellow and die, in addition to premature defoliation. Treat your English boxwood with a fungicide and keep the growing conditions optimal to avoid decline.
  
  

Spider Mites

• Spider mites are small pests related to ticks that feed on numerous trees and shrubs, including boxwoods. Two-spotted spider mites attack boxwoods, according to the North Carolina State University Cooperative Extension Service. These mites feed on the leaves of the boxwood, sucking sap from them and causing the leaves to die early in the growing season. Apply a targeted pesticide or miticide to control the presence of two-spotted spider mites and stop them from damaging the shrub.
  
  

Weather Damage

• Severely cold winters and springs have a tendency to damage boxwood leaves, especially if the shrubs are already stressed or damaged by poor growing conditions from the previous season, such as drought or a lack of fertilizer. Winter damage causes leaves to turn yellow, brown or orange. Protect boxwoods from weather damage by providing them with covers and planting them away from low-lying areas where they are more likely to be subjected to very low temperatures during the winter.
  
  

Leafminers

• Leafminers cause primarily cosmetic injuries to boxwoods; the damage they do is not often severe enough to warrant chemical treatment although severe infestations with large numbers of curled, yellowed leaves do require pesticide sprays to stop the boxwood from being damaged further. Leafminers tunnel into leaves and cause them to become yellow and curled, weakening the boxwood and preventing the leaves from photosynthesizing. A few leaves that turn yellow and die, after curling up, are probably infested with leafminers.