Well, Spring is getting closer and closer everyday here in the Hill Country. The Redbuds are blooming their beautiful splash of color that is so eye catching in front of a backdrop of green junipers. The fragrant scent of agaritas in bloom wafts in the air as you walk about the ranch. And it's so fun to try and spot the Sandhill Cranes overhead as they fly in V-shaped formations day after day sounding their trumpetlike song heading North to cooler climes. Daytime temperatures reached into the eighties this past week, yet evenings are still cool enough for a jacket. Perfect weather! And if it ever rains again (I feel another brutal summer approaching) we might have some wildflowers to admire.
As the days heat up, not only do the birds and plants become more active, but a particular little beetle comes out of a winter's dormancy. The Nitidulid beetle is rather infamous around here as it is the little critter that carries the deadly spores of the Oak wilt fungus on its body. For those of you not familiar with Oak wilt, it is a deadly disease that has been responsible for the loss of over 10,000 trees in the Austin area over the past 15 years. Oak wilt is found in over 25 states in the U.S. and over 65 counties in Texas alone. Oak wilt is one of the most destructive tree disease in the U.S and is killing oak trees in Central Texas at epidemic proportions. This fungus, known as Ceratocystis Fagacearum, invades and colonizes in the water conducting tissues of healthy oaks. The reaction of the tree to this invasion results in the blockage of these water conducting tissues, ultimately resulting in 'die back.'. Infection and death of a tree can occur very quickly and the disease is able to spread rapidly within an area. Oak wilt has created some very devastating results. In Austin, property values have been considerably reduced in some places due to the loss of old growth Oaks. Also, increased utility bills is something to consider in urban areas when a giant tree has been lost that used to shade the house from the brutal sun.
I understand Oak wilt is probably not what you want to think about when you think of Spring at the ranch. However, I got to wondering that many of you might not be aware of the damage that this fungus is able to produce. And since Rockmoor is home to many beautiful Oaks, I believe a bit of knowledge and education on prevention is beneficial. The more folks visiting Rockmoor that are aware of this disease the better chance we have of preventing it.
There are three types of Oaks, the Reds (Spanish, Blackjack,Shumard, ect.) the Whites(Post, Chinkapin, Lacey, Shin) and the Live Oaks (mainly the Escarpment; what the 'big tree' is down at the creek) Rockmoor has all three types. The Red Oaks are particularly susceptible to oak wilt and when infected generally die within the course of a single summer. The White Oak group is somewhat less susceptible and when infected can live several years after infection, losing branches over time from the top down. The Live Oak susceptibility is somewhat variable, depending on certain conditions, but wilt and death usually occur within 6 months of infection. Oaks with oak wilt stand out with their dead crowns compared to a green canopy in the summer. So much,that infection can be see spotted from the air.
Oak wilt can spread two ways. One is from the transfer of spores from diseased trees to healthy trees by insect 'vectors'(our little beetle friend). The other way is from the movement of the fungus from a diseased tree to a healthy one via root grafts.
The 'overland' transmission begins with a Red Oak. After a tree has died, spores on a fungal mat are formed under the bark. As this mat grows it expands. Eventually this expansion of the mat cracks the bark open. This fungal mat has a distinct odor. The Nitidulid beetle is attracted to this fruity smell. The beetle thus enters the mat to feed. And he gets the sticky spores all over him and then he travels to another tree. These beetles are drawn to the fresh 'wounds' of trees emitting sap. The oak wilt spores then infect another tree. (kinda' sci-fi creepy, huh?) Experiments have shown that these little beetles are able to travel over a mile and can appear on a fresh wound of a tree in 15 minutes. Or less! Because these beetles don't have 'chewing mouth parts' they need some other creature or physical damage to create an open wound. Thus, and this is VERY IMPORTANT, NEVER, NEVER prune an Oak in the Spring to early Summer months (let's say February to June in Texas) unless you plan on immediately painting the wound with a pruning sealant. (black paint works just fine) If a tree becomes damaged from a storm or construction, as will happen, that wound needs to be painted as soon as possible. In extreme cold or extreme heat the beetle is not active, thus these are the best time for pruning Oaks.
Live Oaks tend to be infected by oak wilt via the root system, which is very difficult to control. Trees within 50 feet of one another can be infected. Trees of the same family tend to interconnect their root systems, called root grafts. And the oak wilt will travel through these roots. Spores that have been produced inside the tree travel through the vascular tissue. The leaves on a diseased Live Oak often develop yellow (chlorotic) veins that eventually turn brown (necrotic). This symptom is identified as 'veinal necrosis'. And this a good diagnostic tool to identify oak wilt. The picture at the top of this entry is a classic veinal necrosis image.
Foilar symptoms on Red Oaks is less distinct, they are more readily identified by the fungal mats. These mats on Red Oaks can be found by looking for incongruous narrow cracks in the bark of a dying tree that leads to hollow areas between the bark and wood. It often has the distinct odor of fermenting fruit, that the beetle is so fond of.
So how can you treat Oak Wilt if it's discovered? The most common technique is trenching with a big piece of equipment (trencher, rock saw,ect.) The point of trenching is to sever the root systems from one tree to the next in order to save the healthy ones that have yet to be infected. The area is typically trenched within 100 feet on all sides of the infected area. As far as utilizing a fungicide there is only one that has proven effective for Live Oaks, it's called Propiconazole. Also known as AlamoTM. It is essentially injected into the tree. I've seen both of these methods utilized and it's quite a bit of work. Not to mention very expensive.
Of course the best way to treat oak wilt is prevention. Painting and sealing all cuts on a tree is absolutely essential. This cannot be stressed enough. And if you do suspect an infected tree, quick removal is vital. If everyone keeps an eye out for sick trees and practices common sense when pruning then we should enjoy our healthy Oaks around Rockmoor for generations to come.
Now if we could just find some disease to help us thin out these invasive Ashe Junipers....
Till next time,
Sally Anne
*photo- Oak Wilt Specialist of Texas
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