Oak Diseases

Strong as an Oak.

There is perhaps no tree as tough, stately and impressive as an Oak.  The wood of Oak is hard, heavy, and can withstand the fiercest of its enemies.

Here in Sarnia Lambton, we have a special fondness for Oaks.  Now that Ash trees have all but disappeared, Oaks are our dominant tree species.  Our beloved Canatara Park is filled with Oaks and woodlots throughout SW Ontario are undoubtedly anchored by Oaks.

If Oaks are so strong and resilient, you would think they rarely suffer from insects and diseases.  Not so.  Oaks are home to more pests than you can shake a branch at.

In the book “Diseases and Pests of Ornamental Plants” by P.P. Pirone, often considered to be the bible of tree pests, there are no less than 19 insects listed that commonly call Oaks their home, as well as 11 fungal diseases.

Anthracnose of Oak is having a heyday this year because we’ve had more frequent spring rains than normal.  White Oaks (rounded leaves) are more susceptible than Red Oaks (pointed leaves).

Infestation begins early in spring as soon as leaves uncurl.  Irregularly shaped spots appear along the margins of the leaf, turning brown and causing distortion.  In short order, depending on the severity of the fungus, spots run together forming blotches of light brown tissue.

Heaviest infestation occurs on the lower limbs where humidity and moisture is greatest.  Freestanding Oaks are less susceptible than Oaks in a crowded forest.

Anthracnose may cause leaves to become unsightly but will not cause damage to healthy trees.  There is no pesticide or fungicide available to combat the disease.

The best control for Anthracnose is keeping the tree in good health.  Providing water during extended dry spells, cleaning up the infested leaves in fall, and putting down a layer of mulch to retain soil moisture are good cultural practices.

Other common diseases of Oak include Leaf Spot, Powdery Mildew, Leaf Scorch, Crown Gall and Canker.  Control measures are rarely warranted.   Oak Wilt is an aggressive fungus blocking the flow of water and food from roots to leaves and crown, potentially killing the tree.  Unfortunately, there is no control for Oak Wilt.

Insects that feed on Oaks include tent caterpillars, leaf rollers, borers, Gypsy Moth and spider mites.  A common issue for Oaks are galls, a round bubble that forms as a reaction from a mite.  Like diseases, Oaks will generally withstand any number of insects that feed on them.