There are several diseases that can harm your boxwood. Most are fungi that attack the roots or leaves of the plant. It is possible to prevent an attack and, in most cases, treat the different diseases should they attack your plant. Here are the main diseases and how you can prevent or treat them.
Root Rot is a fungus that starts at the roots and works its way into the plant. The leaves where will be the first place that you notice when they start to change from normal to a darker green, to a lighter green and then yellowing from the fungi and lack of nutrients. The main treatment of root rot is actually prevention. Once it takes hold it is likely too late to transplant and save the boxwood.
To prevent root rot, take care not to overwater the boxwood. Plant in well-draining soil. If you must plant in a clay or non-porous soil, use a heavy item such as tree bark or even peat moss to loosen up the surrounding soil.
Once root rot has taken hold, do not plant another boxwood in that area—the disease is still in the soil and will take over the next plant. Either completely replace the soil or plant a root rot resistance plant in that place.
Copper fungicide or a lime sulfur treatment has been shown to help treat and prevent canker disease on boxwood. Canker disease is a fungus that attacks different stems of a plant. Leaves that are infected tend to have small rose-colored splotches of the fungus. The leaves will also change color from light green to tan and start to curl inward towards the stems.
Remove all infected stems at the base, and then spray with the first application of copper fungicide or lime sulfur mixture. Apply again in the spring before the plant starts to grow. Continue to follow this same treatment pattern until you have removed all canker fungus from your plant.
Decline is a dieases only found in the English boxwood. The American Boxwood is immune to it. It is a disease similar to fungus, caused by neglect to the soil. Mind pH levels, drought, nutrition and mulch management in order to prevent decline from harming your English boxwood. You can try to save your plant by thinning out the branches, not overwatering and making sure that the nutrient levels are correct in the soil.