July 1, 2021: Many oak trees in Maryland are showing a symptom called flagging -- brown branch tips in the canopy, some of which hang downward and eventually break off. This is due to the egg-laying of the periodical cicadas. They use 200+ species of plants for egg-laying and oaks are a prefered species. While the damage can appear to be severe, this is not considered detrimental to mature established trees. Further information on flagging is provided on our cicadas page.
We have received numerous inquiries about oak trees declining and dying in recent years. Residents often 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 symptoms. Photo: Joseph OBrien, USDA Forest Service, Bugwood.org
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 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 isn’t known to exist in most counties in Maryland. It 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.