How to identify Oak Wilt before it's too late
What Is Oak Wilt?
Oak wilt is a vascular wilt disease caused by a fungus that clogs the water-conducting (xylem) vessels of trees in the Beech family (predominately oak species but also chestnuts and several others).
The severity and rate of mortality are also due in large part to the self-preservation response of the tree, in which the tree shuts down and kills most of its colonized tissue in order to compartmentalize or trap the fungus and limit its full movement within itself.
Oak wilt is the worst tree pathogen in Texas. It is in the top 5 tree pathogens in the USA. The national oak wilt map is eye-opening to the Texan as it clearly shows that our severe problem is actually only a small percentage of the total expanse of this disease. It is sobering to see how seriously our entire national tree cover is being affected by Oak Wilt.
What Does Oak Wilt Look Like?
I manage thousands of acres of oak wilt in over a dozen counties within central Texas. This amount of experience with the disease day in and day out throughout the year, with different cold- hardiness zones, elevations, soil types, annual rain fall variations, etc., has in a way provided me a state of the art “laboratory” by which I can form observations and continually improve the management plans I provide my many clients.
I’ve included a number of photo and video collections below of just some of my observations regarding this disease. One thing I want to stress here – many of these photos call into question – further, require significant changes in current education and protocol regarding this disease.
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Oak Wilt Symptoms
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Live Oak Symptoms
The native Edwards Plateau Escarpment Live Oak is obviously the most common focus of concern and management. This is because it is both the most prevalent type of oak and for that matter of any tree species in the regions, as well as its genetic proclivity to more aggressively expand the mot by expansive root sprouting and grafting.
The most typical symptom expressed is that of veinal necrosis – seen in the slightly aged brown leaf above. There are many examples of veinal necrosis below as well. There are numerous reasons why this is the classic leaf symptom BUT it is not the only leaf symptom. Regularly I observe that the classic symptom does not represent even 50% of the symptomatic leaves on the ground or in the tree. Most importantly, there are often occasions when the classic leaf symptoms are few or even non-existent at the sites I visit. It is for this reason I have chosen to take photos of all the variations of symptoms I see and consequently look for during diagnosis, in order to better assist those hoping to have better information to help them learn and investigate possible
oak wilt situation.
List of foliar symptoms (roughly in order of frequency):
***Most All the below symptoms include leaf drop save for tufting, flashing, canopy yellowing, and
blitzkrieg ghosting, which may or may not exemplify full or partial marcescence.
- Veinal necrosis (with lateral veins or without)
- Leaf Tip Burn (often with tip shriveling)
- Leaf Margin Burn (often with leaf cupping)
- Bronze Wash
- Veinal Chlorosis
- Canopy yellowing
- Veinal Banding
- Blitzkrieg Ghosting
“Blitzkrieg Ghosting” Live Oak Wilt Symptom
Spanish Red Oak Symptoms
Many of the native Red Oaks in the hill country are a mostly-pure Spanish Red Oak strain. I sometimes see some traces of Texana in them and more curiously – I often see Black Jack cross-pollination (or vice versa) when these two in close proximity to each other. The Spanish Red Oak is without a doubt the second most prevalent native oak species. It is very unfortunate that more effort is not being made to help train property owners to find and destroy red oaks with wilt. Beetles and paint is a moot point if sporulation of the red oaks isn’t allowed to occur due to a pro-active response. I believe that significant effort in this one specific task would have one of the most powerful impacts on disease management and must become a major focus of efforts from here on out. In many cases there are not many fungal mats at any given time on any given 1000 acres, rather more likely 10-20, which is a manageable task to monitor and remedy. Further, studies done in other states give us a reasonable belief that either the use of fungicide or herbicide can prevent sporulation without labor-intensive tree removal and destruction (burn, mulch). Red Oak routinely pass the fungus to trees within grafting distance. They are sometimes grafted with other oaks.
Bigelow Oak Symptoms
Quercus sinuata var. breviloba
There is way too much confusion with nomenclature with this tree species. Most call it a (“bastard”) shin oak. There are four other oak species each with their own unique latin etymology. Generic names like shin or scrub actually make identification more of a challenge rather than easier – the purpose for common names to begin with. This tree is also called a Limestone Durand Oak as it is genetically similar to the Durand Oak, but most recent progress in DNA I believe warrants the giving this tree its own name, therefore I have for over a decade called this tree Bigelow Oak. Note the first picture – for those who know this tree by its very unique papershell, exfoliating bark one would likely conclude it is undoubtedly a Bigelow. Surprise surprise, the branching structure, leaf shape and color is Live Oak. These two trees cross-pollinate. These two trees regularly graft. If one or the other gets oak wilt, one should anticipate the odds are quite high that most of the trees in neighboring or converged mots are connected. This white oak is not resistant to oak wilt!
Mexican White Oak Symptoms
Often called the Monterrey Oak, I strongly dislike this term as it has a connotation of uniqueness of genetics and reclusive location. This tree will cross-pollinate and graft with dang near every oak – hence polymorpha (i.e. many forms). It is not resistant to oak wilt! It readily grafts with Live Oaks, etc. that have oak wilt. There will be massive losses of this over-planted species and a secondary epidemic outbreak will occur in those locations where this tree was planted in heavy numbers – almost to the point of replacing the monoculture of Live Oaks with a monoculture of this species.
Chinkapin Oak Symptoms
This species is becoming a more prevalent selection as an “oak wilt resistant” shade tree. I can confirm that, the native Chinkapin, which is quite reclusive and found mostly in riparian areas in the more mountainous areas in central Texas, does contract oak wilt from grafts both from Live Oak and Lacey Oak and Bigelow Oak, and it spreads it readily to other Chinkapins within root-graft distance. When it contracts this disease it will often die within the first two years. By contrast, the Chinkapin Oak grown/sold by most nurseries in region is not a pure genetic specimen from our area. The genetics are from out of state. It is highly probable that these trees will not fare any better than their local region counterpart. For example over in Kansas, experts in that area of the country are saying, "Oak wilt can attack all species of oak, but the red oak group is most susceptible. This group includes northern red, shingle, shumard, blackjack, and pin oak. The white oak group is more resistant to the oak wilt fungus bur, chinquapin, post, and white oaks, may survive several years of infection. In Kansas, oaks are usually found woodlots or in urban subdivisions that were built in areas where native oak stands were present."
I will be watching these planted trees closely and suspect I will have a solid position on this tree quite soon. What I am certain of is that I regularly see in these Chinkapins from the nursery, anatomical features of the Mexican White Oak. As mentioned above, the Mexican White Oak will continue to cross with other oaks and make the likelihood of mortality only worsen for their crossbred progeny. In a later section, we will cover the serious implications for tree nurseries and landscape tree design and install.
Black Jack Oak Symptoms
The good tree with the cute duck feet-shaped leaves. The bad tree that possibly forms pressure pad fungal mats more consistently that Red Oaks. The ugly in that they are rare and many people are unaware how to identify them and therefore unable to proactively monitor them for possible contagion.
Lacey Oak Symptoms
Also known as the Blue Oak. Beautiful tree. Yes, this tree spreads oak wilt at the same speed as well as has as high of a mortality rate as Live Oak. This white oak species is not resistant to oak wilt! One of the videos I’ve taken recently is of a transference of oak wilt from a brand new Live Oak mortality center to a nearby Bigelow Oak mot and then into a Lacey Oak mot. Within another recent video, I have captured a large Lacey Oak mortality center with a transference into a Spanish Red Oak mot.
Post Oak Symptoms
Would it be a bit interesting if you happened to hear a Texas Forest Service member say if they had numerous Post Oaks and one got oak wilt – that forester would proceed with injections – even up three times? Maybe confusing?
Often when I see Post Oak mortality centers – I see ample evidence of canker scaring on the trunks. All too frequently the dismissive, inexperienced, and possibly lazy investigator concludes immediately that drought caused hypoxolon to overtake and kill all those trees. I am here to tell you emphatically that I regularly see active hypoxolon sporulation on live oaks at the later end of dying from oak wilt. Years later, the only evidence that may be left will likely by the hypoxolon canker scars – does that mean there was never any oak wilt present?!?
It is possible that Post Oaks, while they don’t die immediately from oak wilt (like the Bur Oaks in the upper Midwest – see below), very well may succumb to hypoxolon as a result of the weakened health. I am not equating all or even half of hypoxolon scaring to oak wilt – just that it is very likely and probable that some of those cases are a result of a weakened tree due to the virulent oak wilt pathogen. Very often the leaf symptoms are hard to differentiate from other causes – not nearly as easy as Live Oaks.
Shumard Red Oak Symptoms
Bur Oak Symptoms
Bur Oaks (along with the White Oak – Quercus alba) are regularly treated for oak wilt in order to prevent mortality in the Upper Midwest. A Minnesota DNR forester during a Sustainable Forests Webinar, distinguishes between “susceptibility” of the red and white oaks in this way – they both can and will acquire to disease by way of overland vector or by root system (assuming both wounded and visited by a beetle or grafted to a neighboring infected tree), therefore both are equally susceptible. The “resistance” between the two he explains in this fashion – Red oaks always die and do so much faster, while the native white oaks will take one or more years to progressively die unless treated with fungicide. Personally, I would only accept the use of the term “resistant” for a tree who acquires oak wilt and can on its own record fight it off and or live with it in such a way that the tree’s value not only doesn’t diminish but it continues to increase as it continues to grow. Doesn’t that sound like a more accurate and appropriate use of the term…?
Also, quite pertinent here – Dr. William MacDonald during a presentation at a Symposium in Pittsburg, makes a very strong remark about the surprise and great concern that occurred by expert here and especially those in England when two of the greatest European oaks – Quercus robur (English Oak) and Quercus patraea – Sessile Oak were inoculated with oak wilt spores here in the U.S. and the mortality response was very similar to Red Oaks! Without intensive research showing routine capacity of any oak to resist the fungus effectively on its own – no positive claim of resistance should be made! This must become the new protocol adopted by those who wish to responsibly educate and fight this disease with an eye to the future.
Joseph OBrien, USDA Forest Service, Bugwood.org
White Oak Symptoms
Images Courtesy of Rainbow Treecare Scientific Advancements
Fred Baker, Utah State University, Bugwood.org
Northern Red Oak Symptoms