Oak wilt is caused by a fungus called Ceratocystis fagacearum.  Once inside a healthy tree, the fungus quickly travels through the water conducting tissues (xylem) located just beneath the bark.  The tree's internal defense system begins to fight off the fungus by producing gums and resins (tyloses) which plug the water conducting vessels.  Unable to contain the fast moving fungus, the tree eventually cuts off its own internal water supply.  This causes the tree to wilt and die.

All species of oaks are susceptible to this disease.  Northern red and northern pin oaks (Red Oak Group) are highly susceptible to oak wilt and do not recover once infected.  The initial symptoms of oak wilt are easily identified in red oaks by the rapid wilting of the leaves in the upper crown of the tree.  Wilting usually begins in the uppermost part of the crown and spreads downward.  Individual leaves generally wilt from the tips to the bases with the edges turning pale green and then light tan or brown.  Fallen leaves are often still green at the base (Figure 1).  Once the initial symptoms appear, red oaks generally wilt completely within several weeks.  Also, when oak wilt symptoms are first noticed in red oaks, the fungus has already spread throughout the tree and into the root system.

Figure 1.  Leaves from oak wilt-infected red oaks.
 

Bur and white oaks (White Oak Group) are somewhat resistant to oak wilt.  If white oaks become infected with the disease, they generally wilt very slowly, dying one branch at a time.  Leaf discoloration of infected white oaks can resemble autumn colors.  White oaks can live with the disease from 1 to 20 years, or in some cases, they may actually recover.

How Does Oak Wilt Spread?

Overland Infection
The oak wilt fungus is spread two ways.  Initially, healthy oaks can be infected above ground by several species of sap beetles of the family Nitidulidae.  These beetles are attracted to the fruity-smelling spore mats produced by the fungus between the bark and wood of red oaks that died from the disease.

These spore mats generally develop between April and July on red oaks that wilted the previous summer.  They produce pressure pads that crack open the bark, providing the beetles access to the spore mats (see figure below).  The sticky fungus spores adhere to the beetles when they visit the mats.  Several species of sap beetles are also attracted to fresh wounds on healthy oaks during the spring and summer.  Sap beetles infect healthy oaks with the disease as they inadvertently deposit the fungus spores while they feed on fresh open tree wounds.  Although this activity by sap beetles accounts for less than 10 percent of the spread of oak wilt, this is the only way new infection centers get started.  Once inside the tree, the fungus quickly travels through the xylem tissues of the tree and into the root system. Oak wilt-killed red oak with a sporulating mat (gray area) and black pressure pads. This red oak wilted in early June 1999; photo was taken November 30, 1999.

Not all oak wilt-killed red oaks will produce spore mats the following spring.  If an infected red oak wilts early in the growing season, it may produce the spore mats in autumn of the same year (Figure 3), or it may dry too fast to produce them at all.  Consequently, these trees are not considered infectious for the overland spread of oak wilt the following spring.  Actually, only about one-third of infected red oaks will be infectious for the overland spread of the disease.  Infectious red oaks are referred to as Potential Spore Producers (PSPs).  All oak wilt-killed red oaks are re-inspected in the fall by Forestry Division personnel to determine which trees are Potential Spore Producers and thus, infectious for the overland spread of the disease the following spring.

Root Graft Infection
The other way the oak wilt fungus is spread is underground through the root system.  When oak trees of the same species are growing close together (within 50-60 feet), they generally will form a common root system as roots entangle and graft together.  The fungus travels rapidly through these grafted roots, infecting healthy oaks up to sixty feet away.  Fungus transmission through root grafts is responsible for approximately 90 percent of the spread of oak wilt.    

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