Oak wilt is a serious and often fatal disease caused by the fungus Ceratocystis fagacearum. Central Texas is one of the worst afflicted areas in the U.S., damaging and killing numerous live oaks and red oaks. Most home owners, where these trees are present, are familiar with the danger this fungus causes, but aren’t sure on how and when to prune oak wilt susceptible trees.
Oak wilt spreads in two ways. Below ground, through the grafting of roots of different trees and above ground, carried by a vector in the shape of a sap feeding beetle called a Nitidulid.
Over 95% of oak wilt transmission occurs underground (Juzwik 1983), making it a difficult disease to prevent as it often involves trenching between properties to keep it contained.
Over ground transmission, and this is where pruning comes into play, may occur when the Nitidulid, attracted to the scent of exposed green woody tissue, whether from a pruning cut or scrape from contact with a building, deposits the fungal spores from another infected tree. To prevent this from happening the arborist needs to sanitize all cutting tools between trees and cover the wound with a thin layer of non-toxic paint immediately after the cut is made. About 24 hours after the cut is made the tree will have sealed the exposed tissue on his own, so it is important to protect our oaks for this initial period. If pruning dead wood, where the correct cut exposes only non conductive tissue, painting the wound isn’t necessary but is done anyways so as to let the public know that all precautions have been taken.
Some neighborhoods in Austin stress the importance of avoiding pruning of live oaks and red oaks in the spring months, when the beetle activity is very high. These reccomendations often lead to confusion, beetle activity doesn’t follow seasons but temperature patterns, and these patterns vary greatly from year to year. If all precautions are taken, the sanitising of the tools and immediate treatment of the wound, there is no problem pruning oaks year round.
It is worth noting that although oak wilt is a devastating disease, the damage it causes is relatively small when compared to the damage caused by the poor pruning practices performed by many unqualified “tree guys” in our city. These precautions need to always be backed up by proper care for the all round health of the tree.