Trenching plays a key roll in the oak wilt suppression program, it is designed to circumvent the disease center disrupting the interconnected tree roots. Approximately 70 percent of all the oak wilt trenches installed statewide have held the disease centers in check, but it is important to understand that trenching will never stop or slow the spread of oak wilt in the live oak trees. However, keep in mind, the beetle is not looking for stop signs or trenches, the next meal is the only priority. Oak wilt is becoming more wide spread throughout Texas, in the past 12 plus years Austin has increased from 70 to over 250 oak wilt centers. That represents a 350 per cent increase in the disease out breaks. The number of counties have also increased from 55 to 68 in the recent years.These outbreaks are primarily in high density populated areas.
Root Transmission: Regardless, once a tree has the disease root transmission occurs and it is spread from tree to tree through its “community” root system. Live oaks grow in Motts of an inter-connected roots system, they are basically one tree, living on a common root system. This makes it difficult to stop or control the disease. Removing the bad apple does not suffice as the entire roots system of the Mott is totally infected and it is necessary to remove and destroy the entire Mott and surrounding trees within the 100 foot proximity.
The Beetle, overland transmission: If beetles are found at cracks or wounds in the bark on the Texas Red Oak it could be a sign that Oak Wilt is not far away. The fungus, Ceratocystis fagacearum, creates large areas of mycelium(Spore Mats) under the bark that invite beetles of the Nitidulidae family to feed on the bark. This fungus, which is in the same genus as Dutch Elm Disease (Ceratocystis ulmi) distributes itself similarly. The Nitidulid beetles carry the fungus on them and then transport the disease to other healthy trees. Therefore, healthy trees that have been wounded or have open cracks are targets for beetles to spread Oak Wilt.
Nitidulid Beetle (Photo Courtesy of USDA Forest Service)
Severing the roots system is one way to stop or slow further transmission. It is necessary to create a barrier surrounding the infected trees. Once all of the infected trees are located it is important to mark the secondary trench at least 100’ from the last infected tree in order to make sure the trench fully contains the infected roots. A primary trench, another 100 feet beyond the secondary trench and a third buffer trench 50-100 feet beyond primary trench, is the best strategy to create a safe zone that should be disease free. (See Map below) Once the trench perimeters are established a trench depth of at least 48” needs to be cut through the roots and in certain cases 6 feet is recommended.
Click on map to enlarge
Removal of the dead or severely infected trees is of the highest priority in controlling oak wilt, as the pathogen can remain in the root system for 3-4 years. Once the water content in the roots drops below 38% the pathogen is destroyed. Whenever and wherever it is possible, the best strategy is to remove the infected or dead trees as soon as possible, using a bull dozer to uproot the entire Mott. As for disposal, its best to just burn the uprooted trees as fire will destroy the oak wilt fungus or pathogen.
In some cases it is not possible to install trenches. It becomes necessary to inject high dollar value trees to create a barrier in-order protect other trees and stop the progression of the disease. A therapeutic dosage rate or 20 mils per dia. inch is recommended for any suspect trees or the large trees in close proximity of the disease center. Annual re-treatment of these trees is also recommended, as it takes about three years for a tree to recuperate under perfect conditions. Some trees are just beyond saving at the time of treatment. It is far better to inject trees on a preventive basis rather than wait until your trees become infected and retreat on a bi-annual basis to protect the new growth wood. However, some of the treated trees in the yard will survive. You can also lose some of the otherwise survivable, treated trees, if they aren’t watered during hot and dry summers. It is important to deep water during these periods of drought to help survival.
The application of a herbicide to the tree stumps as a safety barrier: The intention is to kill the root system of the dying or infected tree root system and stop the spread of oak wilt, which has proved to be a more dangerous practice than beneficial. The herbicide applied to the stump is absorbed into the root system and continues dispersal through the transportation across root grafts as a result of the movement of water to transpiring healthy trees from the non-transpiring stump. In parts of these root zones, all the oaks are grafted together. Under these conditions the herbicide dispersal can enlarge rapidly, and become as deadly as the oak wilt disease itself. This is a practice by unscrupulous, and uninformed tree services with the intentions for financial gain only, are in need of very serious scientific council.
A personal Note: From the powers to be, our oak trees are not considered a highly valuable commodity in Texas. Perhaps it would make much more sense and a more feasible project to consider an attack on the beetle, to ultimately control and suppress oak wilt once and for all. Remember the fruit fly epidemic in California, scientist came to the call and in a very short period almost totally eradicate the fruit fly. Why? Because the financial losses to the fruit industry and other support related industries would have devastated the entire California economy.
If you can eat it, don't fix it.