What is oak wilt?

Oak wilt is one of the most destructive tree diseases in the United States. Oak wilt kills lots of trees in Central Texas. Some say that it’s prevalent in epidemic proportions here. Well, what is oak wilt? Oak wilt is an infectious disease caused by the fungus Ceratocystis fagacearum. This fungus invades and infects the water-conducting system in susceptible trees. All oaks are susceptible to this disease to some degree. Some species are more affected than others. In fact, infections have been found in 16 native oak species.

Oak wilt affects two main types of oak species specifically – the white oak and the red oak. White oak trees have rounded leaf edges and the pores are clogged with tyloses while red oaks have pointed leaf edges. Their pores are large and open. Because of this difference, red oak trees tend to die faster from oak wilt than white oaks.

So, how does oak wilt disease spread?

The oak wilt fungus spreads in two basic ways. First, there is a transfer of spores from diseased to healthy trees by insect vectors and movement of the fungus from a diseased tree to a healthy tree through root grafts. If conditions are right after the oak has wilted and died, spores are produced on fungal mats that form under the bark of the oak. The mats produce asexual spores called endoconidia. Endoconidia are barrel-shaped spores that are produced in chains. If compatible mating types are present, these mats will produce sexual spores (ascospores) in fruiting structures. Certain species of sap beetles are attracted to these mats because they smell fruity.

The beetles will visit the mats to feed and breed. This is where they pick up the disease. These same beetles are also attracted to the bleeding sap of wounded oak trees and can then deposit the spores picked up from the fungal mats. This method of transmission is important for introducing the fungus into a new area which it could not have reached by transmission through root grafts.

Of course, transmission via root graft is the most common means of spreading the disease. Trees within as much as 50 feet of a diseased tree can be infected. Oak wilt usually moves from diseased trees to healthy trees through roots that have become interconnected. In this case, spores that have been produced inside the tree travel through the vascular tissue of a tree. Most root grafts form between oaks of the same species. Red oak develops root grafts more commonly than white oaks and grafts between red and white oaks are very rare.

Next week, we’ll focus on treating oak wilt. If you have questions about this disease, please feel free to call us at Austin Tree Service. Our phone number is 512-341-8888.