Ask Mr. Smarty Plants

Monterrey (or Mexican white) oaks (Quercus polymorpha) are native to West Texas, Monterrey Oak is resistant to oak wilt and is a hardy evergreen oak tree spreading to 60’ feet wide as it matures and a height of 80’. It is a relatively fast growing oak, and practically evergreen in Austin. It is more resistant to oak wilt and other diseases and pests than other oaks. But it is sometimes afflicted with powdery mildew (Microsphaera alni or Phyllactinia guttata)

The Texas A&M AgriLife Extension website for the Texas Plant Disease Handbook says that powdery mildew occurs on all groups of oaks. Infected leaves have a faint indistinct spot on the upper leaf surface and a white to off-white powdery growth on the lower surface. The fungus will most often be found along the veins and midribs of the leaf. In severe cases, infected leaves will be slightly disfigured.

And from the Clemson Cooperative Extension website: Humidity is an important factor related to the onset and spread of powdery mildew. Unlike most fungi, these do not require free water to germinate; only high levels of relative humidity. High relative humidity favors spore formation, and low relative humidity favors spore dispersal, which explains why powdery mildew tends to be a problem when the days are cool and the nights are humid. Temperature is also a factor. Although powdery mildew can occur all season long, it is less common during the heat of the summer.

Fertilize to optimize plant health, but avoid overfertilization with nitrogen as it stimulates young, succulent growth which is more susceptible to infection.

Contact an arborist if there is a severe case of powdery mildew on your Monterrey oak that needs treatment. Pale, curled leaves with considerable white powder on the upper side of the leaves, stem elongation, reduced vigor, and partial defoliation are the signs. A mild case of powdery mildew will look distasteful but will probably be tolerated by the tree. The age of the tree is also a consideration regarding whether treatment is necessary. Young trees or trees under stress should have proactive treatment sooner.