Holly is a relatively hardy bush that gardeners appreciate for its low-maintenance needs. However, holly is sometimes subject to a few problems. Powdery mildew is the most common of these and afflicted bushes quickly become covered in unsightly white "powder." The best way to tackle powdery mildew is to prune your holly bush as soon as possible. Then wait to see if the mildew persists before spraying with a fungicide. If your holly repeatedly develops powdery mildew, it may be a sign that it is being grown in unfavorable conditions.
Prune all of the leaves that have been affected by the mildew. Also clear any affected leaves that are on the ground around the bush.
Prune any of the holly bush's branches that are dead or diseased. Compromised branches are quite attractive to mildew. Trim the branch all the way back to the main branch. If any pruning wounds are larger than 1 inch, cover them with pruning paint.
Improve the holly bush's air circulation. Prune any branches that are crossed or rubbing against each other. The less crowded the interior of your holly bush, the less likely it will be to develop powdery mildew. Prune any other branches as you see fit. Holly bushes can stand a fair amount of pruning. Continue to prune your holly bush annually in December to keep air circulating in its center.