What Is Causing Oak Leaves to Turn Brown & Drop in Clusters?

Iron Chlorosis

• Oak tree leaves are likely to turn brown and drop in clusters when suffering from iron chlorosis, a condition that occurs when the tree is unable to absorb the necessary iron from the earth to maintain vigor. This is not necessarily because the soil lacks iron, but rather due to the fact that the soil contains a pH level above 7.0 and is therefore primarily alkaline. For iron to be absorbed, the soil must be largely acidic.

Chlorosis Problem

• The inability of the oak to uptake iron deprives the leaves of the ability to conduct photosynthesis, the chemical process that produces chlorophyll --- the element that adds green color to the leaves --- and offers energy to the tree and leaves. As a result, the leaves initially turn yellow while the veins maintain their green color. Eventually the oak leaves dry out, turn brown and drop in significant clusters. Often, healthy green leaves will also fall.

Oak Wilt

• Oak wilt, caused by the fungus Ceratocystis fagacearum, is an extremely dangerous disease that once contracted, typically proves lethal for the oak tree. Initial infection takes place in the root system with the first symptoms displaying in the crown as leaves begin to wilt, droop and turn brown. The fungus eventually moves all the way down the tree and, in severe cases, both brown and green leaves will drop in large clusters. There is no cure for oak wilt; infected trees must be removed and destroyed.

Bacterial Leaf Scorch

• Bacterial leaf scorch brought on by the bacterium Xylella fastidiosa first displays as a problem in the fall, usually after a summer of drought conditions. The leaves begin to redden, yellow and then turn brown, often dropping prematurely and accompanied by the die-back of twigs and branches of the oak tree. The overall vigor of the tree is affected slowly; as long as 10 years may pass before the oak must be removed.