Why are Oaks Declining? Dr. Dave Describes 3 Oak Diseases

oak wilt symptoms

Leaves showing oak wilt symptoms, Pittsburgh, PA (Photo: Dave Clement)

We’ve recently received inquiries about oak trees declining and dying. People usually want to know if this is caused by oak wilt. Importantly, oak wilt is not known to occur widely in Maryland. Two common leaf diseases of oaks in Maryland are oak anthracnose and bacterial leaf scorch of oak.

oak anthracnose

Oak anthracnose

Oak anthracnose is caused by a fungal pathogen, Apiognomonia quercina, and it typically is a cool, wet weather, springtime disease. In wet cool summers like the one we’re experiencing this year however, the symptoms can continue through the season. Most oak species will show a range of symptoms scattered through the leaf canopy. Symptoms range from small brown spots and irregular dead areas on distorted leaves to severe blight that kills twigs and causes leaf shriveling. This disease in oaks typically doesn’t cause much leaf drop.

bacterial leaf scorch on oak

Bacterial leaf scorch on oak (Photo by Mark M. Pollitt)

Bacterial leaf scorch is caused by a bacterium, Xylella fastidiosa, and it typically causes symptoms to appear later in the summer through fall. The symptoms are more severe when we have dry summers because the bacterium blocks the water conducting vessels that move water from the roots to the leaves. If this disease is present, you would see scattered yellowing and browning of the leaf margins throughout the canopy.  Leaves remain attached until autumn. So, this disease in oaks also typically doesn’t cause much leaf drop.

oak wilt leaf drop

Oak wilt in Pittsburgh, PA. Oak wilt causes extensive leaf drop by mid-summer.  (Photo: Dave Clement)

The disease that isn’t known to exist in most counties in Maryland, called oak wilt, is caused by a fungal pathogen, Ceratocystis fagacearum. This pathogen causes a vascular infection, meaning it blocks the water and nutrient “circulatory system” of the tree. It typically only shows symptoms on red, scarlet, black, and pin oaks. Oak wilt often affects closely spaced groups of trees because it is spread by root grafts. Wider spread is by sap beetle feeding. Symptoms appear in the spring and early summer beginning near the top of the tree with leaves developing an abrupt margin between green and brown dying leaf tissue. Another typical symptom is extensive leaf drop by mid-summer.

By Dr. Dave Clement, Principal Agent, University of Maryland Extension, Home & Garden Information Center. For plant disease and pest updates, follow Dr. Dave on Facebook

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