Oak Wilt

Disease Overview

Oaks are Blaine's most plentiful and treasured neighborhood shade trees. Unfortunately, hundreds of oaks are lost every year to oak wilt, a vascular disease caused by the fungus Ceratocystis fagacearum.

Red Oaks are more susceptible to oak wilt and once infected can die within weeks. White oaks are not as susceptible and in some cases can take years before they die.

Disease Spread

The oak wilt fungus spreads in two ways: via root graft and sap beetles.

Root Graft Transmission

A transmission via root graft is the most common source of infection, as trees within as much as 15 meters (50 feet) of an infected tree can be infected. 

Sap Beetle Transmission

The second method of infection is via sap beetles (also known as picnic beetles). These beetles are attracted to the bleeding sap of the oak tree, as well as the fungus in an infected or dead tree.

New infection centers are started when the beetle lands on a fresh oak wound generally during the months of April, May, or June. The fungus may then be inadvertently transferred to healthy but injured trees. This is less common as trees are rarely infected this way unless injured, but it is the only way to jump barriers (rivers, for example) and infect trees in new areas.

Infected trees are inspected in the fall to determine which trees could potentially sporulate. Sporulation is the reproductive phase of the fungus. Any tree deemed by the city to have the potential to sporulate the following spring is required to be removed before March 1. 

All trees required for removal must be brought to a city approved disposal site, utilized, or covered in four millimeter plastic for one year.