How to Cut an Oak Tree to Avoid Powdery Mildew

Inspect the oak tree to determine which branches are infected with powdery mildew. They will have leaves that look as if they have been dusted with talcum powder. In extreme cases, leaves will be misshapen, yellowed and falling off. You may also notice pinhead-sized dots that are white, brown or black; these are the fruiting structures of the fungus. Powdery mildew is usually most noticeable on the upper parts of the leaves, but can also appear on undersides, stems and buds.

Choose branches that are low enough to reach from the ground and that are highly infested, dying or broken. Large branches and off the ground work must be left to professional arborists, who have the climbing skills and specialized equipment necessary to do the work safely.

Use the lopping shears to cut branches at the point of origin -- where they emerge from the trunk or larger branch -- cutting back to a strong lateral branch or to the trunk and making the cuts clean and smooth. Use pruning saws for the larger branches. According to Texas A and M University, you should make slanting cuts when removing limbs that grow upwards; this prevents water from collecting in the cuts and promotes healing. Remove branches flush to the "collar," which is the slightly swollen area where the branch joins the trunk. Do not cut them off flush at the trunk.

Disinfect the shears and saw blades after each cut to prevent spreading the powdery mildew.

Prune the oak tree lightly and judiciously, removing only diseased limbs and leaves. The City of Los Angeles' Urban Forestry Division reports that pruning living tissue increases the risk of pathogens gaining entrance into the tree. In addition, deep pruning encourages the growth of watersprouts, weak new branches that are particularly susceptible to powdery mildew.

Dispose of the leaves, twigs and branches in a closed container. Do not compost them.