Oak wilt is an fungal parasite that affects the vascular system of an oak tree. On June 18-19, 2014, Dr. David Appel and Jim Houser of the Texas A&M Forest Department updated ISA Certified Arborists, including Shane Kelly of Canopy Tree Service on the latest treatment techniques.
Arborists care about treating oak wilt because this parasite can wipe out the majority of oak trees in an area if allowed to become established. Oak wilt lives in and feeds off the portion of the tree known as the xylem. The xylem are layers of sieve tubes that move nutrients around the tree by negative pressure caused by water evapo-transpiring through tiny leaf openings known as stomatas. The presence of the oak wilt prevents the tree from receiving needed water and so it “wilts.” Oak wilt does not always kill the oak tree directly or instantly. The tree, especially red oaks, don’t recognize or respond to the presence of oak wilt right away. They wait until the disease has spread and then initiate a late stage protective response that kills itself. How could the oak tree kill itself? It seals off compartments of the tree and attempts to regrow around the diseased areas. In sealing off those compartments it deprives healthy tree tissue of needed nutrients. The sieve tubes are a crucial part of the process of transpiration. The more sieve tubes that are blocked off due to the late recognition and response to the oak wilt the lower the tree’s chance to survive. White oaks fair better than red oaks because unlike the red oaks, the white oaks actually recognize and respond to the presence of oak wilt while it is a small problem. The fewer sieve tubes that are shut down in an early response to oak wilt the more likely the tree will survive.
Oak Wilt Aerial Spread with Red Oaks
Oak wilt spreads in two main ways: aerial and root grafting. Arial spread of oak wilt occurs mainly in red oaks. The oak wilt creates a fungal mat. Fungal mats are how oak wilt spores are released to affect other trees where spores are released and carried by insects. Fungal mats are only dangerous when a Red Oak dies during the cooler months. One insect that is particularly troublesome is the nitidulid. The nitidulid is a hairy beetle that is attracted to fungal spores, fresh tree wounds or any other source of vegetation. The hairy beetle picks up spores from the oak wilt mat and carries them to the next tree they travel to. Arial oak wilt transfer is highly affected by temperature. Oak wilt spores does not survive above 95 degrees or below 45 degrees. Painting fresh cuts blocks the scent of a recently cut oak tree and prevents the nitidulid from coming to it’s next meal.
Oak Wilt Grafting Spread with Live Oaks
Clusters of trees together have intertwined root systems. Oak wilt spreads about 75 feet per year through the roots of live oaks. To prevent the spread of oak wilt in live oaks on ranch land or in rural areas, it is optimal to dig a four foot trench around the tree about 100 feet out from the edge of the affected tree area. In rural areas with ranch land, trenching can be an effective solution, especially if all the neighbors cooperate. Inside the trenched area, the trees are eliminated by pushing them down to kill them quickly. Once the trees are pushed down, another parasite that is a more powerful saprophyte than the oak wilt “Hypoxylon Canker” and which is more efficient at consuming dead trees will also consume the oak wilt.
In a town or on small lots with many utilities, trenching is not practical. Tree injections performed by a skilled arborist can address the oak wilt. A fungicide called Propiconazole can be injected into the tree by one of two methods: One method is macroinjection and the other is micro injection. Success rates are fairly equal, but sometimes one can succeed while the other fails.
Oak wilt is a powerful parasite that can attack and kill the most vigorous oak trees. Interestingly, an oak that is already stressed and in defense mode may actually fare better when challenged by oak wilt.
Oak Wilt Spread Myths
Fire wood is often associated with the spread of oak wilt. In order for fire wood to spread it , it would have to be a red oak with fungal mats, or an infected tree immediately transferred to a susceptible root zone. Although oak wilt is a powerful parasite, it is no match for saprophytes such as Hypoxylon Canker that is found in all trees. Also fire is usually hotter than 95 degrees and kills the spores.
Tools are another method of transport blamed for spreading oak wilt. Chainsaws are unlikely to spread oak wilt because they are covered with gasoline and oil and produce heat way in excess of the temperature that spores can tolerate. Also, bare metal is not a surface that oak wilt can establish itself on. Tools that are more likely to spread oak wilt are digging tools such as shovels, picks and back hoes. They work underground in the root zones and are protected from heat. They can possibly have chunks of mud stuck to them with bits of infected root along for the ride.
Best Time to Identify Oak Wilt
The best months to identify oak wilt are May and June in San Antonio. An arborist will take samples of the tree and put it into a zip lock bag with blue ice and deliver it to Texas A&M lab for analysis.
Oak Wilt has affected trees in 74 counties in Texas and the list is growing. For information on its spread, check out the article with the Texas A&M Forest Service If you suspect you have oak wilt, call Canopy Tree Service for an evaluation.