Sudden Oak Death

 

 

Sudden oak death is a disease caused by the water mold Phytophthora ramorum. Currently it is limited to California and Oregon and parts of Europe. This pathogen can infect a wide variety of plants, but oaks are most severely affected. Non-oak hosts may serve as important reservoirs of the disease and transport of infected ornamentals, such as rhododendron, may be important in moving this disease out of its current range.

Oaks infected with Phytophthora ramorum. Photo credit: Joseph O'Brien, USDA Forest Service, Bugwood.org.

 

Weeping canker on infected oak. Photo credit: Bruce Moltzan, USDA Forest Service, Bugwood.org.

Symptoms include the wilting of new shoots, chlorosis of older foliage, and dead brown foliage that remains on the tree. Bleeding cankers exuding dark sap may appear on the lower trunk and the bark may fissure. As the tree dies back, suckers will appear the following spring only to wilt and die.

Oak leaves with symptoms of infection with Phytophthora ramorum. Photo credit: Joseph O'Brien, USDA Forest Service, Bugwood.org.

 

Canker on oak crown caused by Phytophthora ramorum. Photo credit: Joseph O'Brien, USDA Forest Service, Bugwood.org.

 

The weakened trees attract insects and allow the growth of other pathogens/saprophytes. Ambrosia beetles may attack dying trees, producing fine white sawdust at their entrance holes.

Tube of sawdust excavated by an ambrosia beetle. Photo credit: David Jenkins, South Carolina Forestry Commission.

 

Later, bark beetles will attack, producing red sawdust. Severe infestations of bark beetles can girdle trees.

Small black domes may appear on the bark. These are the fruiting bodies of Hypoxylon fungus, an opportunistic invader of weakened trees.

Fruiting bodies of Hypoxylon fungus. This is an opportunistic pathogen that only invades stressed or diseased trees. Photo credit: Joseph O'Brien, USDA Forest Service, Bugwood.org.

 

Phytophthora ramorum produces resistant spores that are able to survive winter. It also produces flagellated spores that move in water. The spores can spread through splashing rainwater and in streams, where it can be moved greater distances. Wounds facilitate infection but are not required for this pathogen to gain entrance to its hosts.

Like many diseases, early detection is key in controlling sudden oak death. There is no cure for infected trees. Isolating infected trees and practicing good sanitation (removing foliage and debris from shoes when leaving an infected area) help reduce the spread.

If you see symptoms consistent with sudden oak death, please contact the South Carolina Forestry Commission Insect and Disease Staff (David Jenkins ((803) 896-8838) office; (803) 667-1002 cell; or at djenkins@scfc.gov) or your local Forestry Commission office.