Oak Wilt Resistant?

"Resistant" Monterrey Oak
Nine months into symptomology

“Susceptible” / “Resistant” Quandary

A. Merriam Webster

  1. Susceptible: open, subject, or unresistant to some stimulus, influence or agency
  2. Resistant: giving, capable of, or exhibiting resistance; i.e. the inherent ability of an organism to resist harmful influences (such as disease,…etc.)
Mexican White Oak With Oak Wilt
Mexican White Oak
with immediate flash mortality

B. Northern State Experts

  1. General consensus is that “all oaks are susceptible” to oak wilt. By the statements made below by various state experts, it would appear that by "susceptible" these experts recognize that all oaks species are capable of becoming infected and affected to the point of mortality is a likely consequence. The position that most susceptible oak species will eventually succumb significantly or completely to the disease is a significant divergence from the Texas Forest Service (TFS) position.
  2. General consensus is that white oak species are “more resistant” than Red Oak species. Here is where the significant difference between the Northern experts appears to differ from the TFS perspective below. The spectrum of resistance is presented as variations in duration of time and progressive decline until mortality occurs. Cases of survival and recovery are the exception not the norm. A second aspect of resistance they convey with the use of “resistant” is the probability or likelihood of contraction by those in the white oak group is diminished due to the fact that white oak species populations are smaller than the red oaks, there is less frequency of grafting of white oaks to red oaks as well as white oak group intra-species and interspecies grafting.


Mortality durations (if untreated):

Red Oak = less than 1 year;
European white oaks = less than 1 year;
Bur or Chinkapin Oak: 1-5 years;
White Oak: 4-8 years

A very interesting insight by one northern expert is that he suggested not to use the term “resistant” on account of the connotation it can convey to the public a 100% capacity to avoid infection and or impact of the disease. Instead, he prefers the use of the term “tolerance,” which has a more accurate connotation of assumed infection of and impact by the disease, which it can for a time successfully tolerate for brief or in some instances longer duration of time.


National Resources Conservation Service (NRCS) View

“Post oak may be the most resistant while Lacey oak and bur oak are the least.”


“Post oak tends to be the most resistant of the group while bur oak is the most sensitive…Whereas in the red oak group the disease may kill the tree in a few weeks, in the white oak group it may take several years. However in bur oak, symptoms are essentially the same as in the red oaks, and the tree may die within one growing season.”


Great Plains View

Once infected…many white oak species die from oak wilt. Although an infected, highly resistant white oak may not die from oak wilt, the death of major limbs may render the tree undesirable as a landscape or specimen tree.”


National View

“Other white oak species (e.g., Q. macrocarpa, Q. Fusiformis, Q. virginiana) may exhibit moderate resistance, with tree mortality occurring several years following infection.”


“The white oak group is more resistant to the oak wilt fungus: Bur, Chinquapin, Post and White oaks, may survive several years of infection.”



“Trees in the white oak group typically develop symptoms more slowly (than red oaks). For example, Bur Oaks typically die after 1-7 years [if untreated], showing progressive dieback during the process. White oaks may take up to 20 years to die, and some white oaks survive the disease [if untreated].”



“White oak species, on the other hand, are more resistant and may live for several years after infection.”



“Oaks in the white oak group (white, swamp white, bur, and others with rounded edges) are less susceptible. Infected trees in this group will drop their leaves on one or more branches for several years in a row. In other words, trees in the white oak group take longer to die and show more chronic symptoms.”



“White Oaks may also recover from (oak wilt) infection – there are exceptions.”



“Affected branches of Bur Oaks are scattered through the crown. Progressive development of the disease may occur year to year with tree death occurring between 2 and 5 years or longer…”



“The white oak group contains species that are much more resistant to oak wilt, so oak wilt symptoms appear more gradually. Oaks in this group may have a single limb or scattered limbs with disease symptoms, and the disease will progress down the tree only a short distance in one growing season. 

Premature leaf drop is generally not pronounced and an infected tree may survive several years before it dies. However, infection will result in dead limbs throughout the crown of the tree and will become more noticeable each year as the infection spreads. It is easy to mistake the gradual dieback for other oak problems.”



"Oaks in the white group (Bur, Chinquapin, Post, Swamp White, and White Oak) are more tolerant of the disease and may survive infection for one or more years while displaying decline symptoms."



“Oak wilt moves much slower in members of the white oak group, usually killing a branch or two a year, and it can take years for infected white oak group trees to succumb.”



“White oak group – no extensive leaf drop, trees die within a few years”


New York
“White oaks can take years to die…”



“The [white oaks] may die in one year, but usually die slowly over a period of several years or more. After two or more years of progressive die-back, infected white oaks have sparse crowns and eventually die from oak wilt or secondary causes. Bur oaks are intermediate in susceptibility and may be killed as quickly as red and black oaks or as slowly as [the white oak].”



C. Texas Forest Service [1]

Screen Shot 2019 11 11 At 2.34.49 Pm
Fall 2003 
Tfs Pre Change Image
Early November 2019 Prior Message


Tfs Changed Content Image
November 9th Updated Message


After an aggressive solo campaign that consisted of: building my website treatise on oak wilt, many emails and phone calls – both with peers, AgriLife agents, online journalists, and numerous Texas Forest Service personnel, coincidentally, various adjustments to the symptomology white oaks species and oak wilt as well as a shift favoring the more technically accurate term “tolerance/tolerant” occurred in various places within www.texasoakwilt.org. I am not claiming credit for the impetus of the change but merely just conveying accurately a part of the whole historic picture.

Let's look at the most obvious shift in the TFS position on white oak tolerance (see image above of pre-update and update side by side). What is without a doubt 100% crystal clear is there was a full-scale abandonment and deletion of “white oaks rarely die from oak wilt.” This move to completely abandon that untenable position, which has remain unaltered on www.texasoakwilt.org from 2003 – November of 2019, is a great move forward, but it does raise questions. So now we can be sure that some or most or all of the white oaks don’t rarely die, but many questions arise, such as: How often do they die? How tolerant is each species? Upon what basis is that tolerance substantiated? What is clear is that they are not yet saying most will die nor are they saying it would appear that even most will experience significant canopy loss, both of which is the general experience of Northern experts as well as my experience with white oak species (only exception – the Post Oak) in Texas. What the TFS does say is that only “some” canopy loss will be exhibited. One wonders is “some” defined as 49% canopy loss – or maybe even more? If so, I’d venture to presume most consumers would respond that, that amount of canopy loss would be better described as "unacceptable" rather than “some” – at least enough to likely remove and replant.

Next, looking at the adjustment to the Lacey, White Shin, and Chinkapin, we see that the TFS allows that these particular species sometimes have interconnected root systems like Live Oaks and when they do that it may cause “higher infection and mortality rates than other white oak species.” Let's break this down. Why “sometimes” interconnected roots? Those of us who spend considerable time around these three species in their native habitat know that they are frequently in mott groupings. Similar to Live Oaks – it is in their DNA. There are many videos on my YouTube page that display this fact quite succinctly. In regards to higher mortality rates – what does it matter if it is one infected Lacey Oak or 100 Lacey Oaks? If the tolerance level of Lacey Oaks is low and full mortality is say 80% in a year to three years – like the Live Oak, then is it not just a question of mathematics to project from a singular tree to the 100 trees? If not, I’d like to understand what exactly the TFS is thinking along with a thorough exposition for that basis. Lastly, and most importantly, - why is the seminal TFS oak wilt update I’ve quoted and provided a link to below not yet uploaded to www.texasoakwilt.org website where one would reasonably expect it to be published? Why not just copy and paste the updated terminology - including the wording that they “may die in large numbers from oak wilt”? Though a definite move in the right direction this new website update is not nearly far enough to reflect consistent, repetitive field observations and it creates many new questions.

The consistent position by the Texas Forest Service is that “all oaks are susceptible to infection from the fungus.” Further, this susceptibility is tiered in the following way:

Screen Shot 2019 11 11 At 2.33.29 Pm
Inception of TFS Website - Fall 2003 

“Red oaks are extremely intolerant of the fungus”, “Live Oaks are intermediate in their tolerance”, and “white oaks show some tolerance.”
(Each quote above reflects new Nov. 2019 updates favoring new tolerance theme)

  1. Regarding resistance, historically the Texas Forest Service has repeatedly grouped the entire white oak family in the “resistant” category (see the screenshot w/link from inception of website up until 2019 update, TOWQ course presentation image w/link below, prior brochures still being handed out, joint educational activities with AgriLife, etc,.) Even given the oak wilt update document as well as the Nov. 2019 update – the TFS message still falls considerably short and a divergence from the Northern states “may survive” to the TFS “may die” remains regarding white oaks. An in-depth exposition on the tolerance level of each white oak species is needed from the TFS.

Here is the quote from the TFS oak wilt update document that clearly shows the intra-department divergence of position, and which much more conforms with the northern states experts regarding white oak group mortality:

“However, those white oaks such as Shin Oak, Chinkapin oak, and Lacey oak that grow in stands with interconnected root systems also may die in large numbers from oak wilt.”

“Also may die in large numbers” sounds a lot like – no exactly like, an oak wilt mortality center. Oak wilt mortality center language was previously restricted to what happens with red oaks and Live Oaks but now “also” these three white oak species!

Albeit, this statement appears to be guarded and tentative – one wonders if the phenomenon of a white oak mortality center is only a theoretical possibility, or has it been observed only once, or observed numerous times, or is the fungus evolving (becoming more virulent), or has the phenomenon of Texas white oaks dying in large numbers previously gone somehow unnoticed somehow, or…? What I especially wonder is (and very likely the reader of this page), what about the other White Oaks?! We all are likely on the same page more or less on the Post Oak, but with the Lacey, Shin and Chinkapin oaks are all on the possible oak wilt mortality center list – that only leaves the Bur Oak and Mexican White Oak in limbo if you will. Though maybe not much field observation on Bur Oak exists here as of yet – we know they regularly treat them up north with fungicide to preserve them from dying and that they produce fungal mats – a fact scientifically documented decades ago. As for the Mexican White oak – on what scientific basis is this white oak not included in that same list of potential white oaks capable of mortality at the singular or group level.


Further clarification on this by the Texas Forest Service (and many other issues) is still needed.

Dr. David Appel is one of the most prolific contributors to the cadre of oak wilt research for over the last two decades. In 1994 he did a great service to the Californian community by measuring the impact of oak wilt by means of an assay of sprouts upon 11 of the most common oaks that inhabit California proper for just under 3 months of observation. What in my assessment is the most succinct concluding remark found within his study?

“None of the California Quercus spp. tested in this project exhibited any unusual immunity or resistance to the oak wilt pathogen. It is likely that an introduction of C. fagacearum into certain California woodlands would lead to significant losses, especially in areas where deciduous red oaks are intermixed with [Cali] live oaks; examples of such stands are common (9, 16).”

Texas Chapter International Society of Arboriculture 2019 Training
Texas Oak Wilt Qualification Training Manual 2019

Given our focus here on the white oak species, I’d be amiss if I didn’t point out that 6 of the 11 California oaks tested were white oaks. There were two red oak and two “intermediate” oaks (which he proffered may well produce fungal mats). The intermediate and red are of course quite a concern as one would typically expect, but very notably 3 of the white oaks (Q. dumosa, Q. durata, and Q. suber) had very high (higher than our Southern Live Oak and close to the rating of our Escarpment Live Oak) ratings of symptoms and dieback in less than three months! In no way shape or form could they be included in the resistant category of the Q. alba or even the Q. stellata). One wonders:

  • What would have been the results for those three species and especially for the other 3 white oaks at 6, 12, 18, 24, and 60 months?! (Note: There was  definite decline from week 7 to week 11 in at least study I – study II & III were just a singular 10 week study it appears)
  • Regarding the levels of classification wherein Dr. Appel appears to be defining “resistant” – would the death of 49% of above-ground tree parts in under three months qualify as “resistant”? Would this same dieback amount be acceptable for a tree-purchasing consumer looking to plant a “resistant” tree?
  • J. Pinon et. al performed a study of sprouts (see National Oak Wilt Symposium, 1992) on European white oak species and then carried to the next level by W. L. MacDonald on 14-year-old established field-grown European white oaks (same species) – and the results were even worse in the second study! What would happen if a study of field-grown Californian oaks was conducted?!

Reproduced, by permission, from MacDonald, W. L., et al. 2001. European oaks-Susceptible to oak wilt? Pages 131-137 in: Shade Tree Wilt Diseases. C. L. Ash, ed. American Phytopathological Society, St. Paul, MN.

(Follow this link to purchase the book: Shade Tree Wilt Diseases)

For more see:

[1] C.f. www.texasoakwilt.org (from inception of site to current form).
TX AgriLife Oak Wilt Fact Sheet
Appel, D. ISAT Oak Wilt Qualification Training (2019 powerpoint presentation) see page 23.
Texas A&M Forest Service. Forest Health: Identify and Manage Oak Wilt.

Mexican White Oak Image
This Mexican White Oak (AKA Monterrey Oak)
died in less than a year


D. Author’s Response: A Texas Solution to the Quandary

  1. Susceptible – Any and all members of the Beech family are to be included in the ability to contract oak wilt AND die as a result of infection UNLESS DEFINITIVELY PROVEN TO THE NEGATIVE BY NUMEROUS PUBLICALLY PUBLISHED STUDIES AND PEER REVIEWS.
  2. Resistant – I am of the firm conviction that this term must be abandoned – in a large-scale public retraction campaign, and replaced with the term tolerance and accessibility.

    As per the glossary of the premier textbook for plant pathology:

    Resistance: The ability of an organism to exclude or overcome, completely or in some degree, the effect of a pathogen or other damaging factor.

    Resistant: Processing qualities that hinder the development of a given pathogen; infected a little or not at all.

    PLANT PATHOLOGY. 5th Ed. Agrios, George N. Elsevier Academic Press, 2005.

  3. Tolerance – The ability of the tree to withstand infection for a duration of time until mortality occurs. No oaks unless proven during extensive studies to NEVER experience not only mortality, but minimal if any decline, and if minimal – only temporary, minimal decline after infection, are to be given a perpetually tolerant designation. AT THIS TIME IN TEXAS – NO OAK SPECIES IS PERPETUALLY TOLERANT.

Post Oak: Longest duration of tolerance. Likely to die in 3-5yrs as a result of the primary pathogen – the oak wilt infection, but also often combined with other contributive stressors such as drought, and or, hypoxolon, and or phytophora, etc.

Bur Oak: Moderate tolerance of infection displayed in that decline usually progresses slowly over 2-5 yrs until full mortality (untreated), with ample time to, either perform preventative or therapeutic treatment and thereby retain tree indefinitely.

Bigelow Oak, Live Oak, Lacey Oak, Durand Oak, Chinkapin Oak, Mexican White Oak (AKA Monterrey Oak): LOW tolerance as displayed on account of death OR mortality of a majority (51%+) of above-ground tree parts –leaving the tree as a loss and liability in the estimation of most Tree Risk Assesment Qualified (or TRAQ) and Tree and Plant Appraisal Qualified (or TPAQ) assessors. (see image at top of page)

Black Jack Oak, Spanish/Texas/Southern/Shumard Red Oaks, Pin Oak, Water Oak: Very Low tolerance: Rapid mortality from full canopy death within 1-3 months and trunk death in under 1 year in most cases.

4. Accessibility – This new term is an attempt to further distinguish between another muddled aspect of the previous position of the Texas Forest Service. In that all oaks are susceptible – regarding specifically the overland form of transference of infection – all trees must have some level of accessibility, and with certain givens being equal it is likely any oak is accessible as any other.

For example:

Say there is a 10” DBH Red Oak with an active fungal mat with one beetle covered in spores and about to take off in flight. Let’s also say in a perfect circle around said tree 50’ away, on a windless, spring day - there are every variety of oak species of 10” DBH size and similar canopy, all of which were wounded on the trunk at 5’ simultaneously with a 4” x 4” bole cut. What tree has the highest likelihood of infection?

Without definitive study from oak species sap aroma strength and maybe more importantly - most preferred sap flavor of any or at least of this nitidulid beetle, or which direction that particular beetle prefers to go – maybe it believes flights to the right are always the best sap-feeding direction, one must conclude that all are equally accessible. 

Now, that example was theoretical, but it illustrated the point successfully. In the real world though, the quantity of a specific tree, the total mass or size of that tree, the proclivity of that tree to wounds – either naturally inflicted or inflicted by humans, or in the case of underland spread - the proximity of those trees to one another, the size and quantity of root grafts, the weather, soil type, various abiotic and biotic influences – all these things among others can impact the accessibility of a specific tree and species to infection. In general, though, I will proffer the following:

All oak species can and do graft with one another. In some cases, the transmission between different species may possibly at most be delayed. In very rare occurrences, with little to no logical explanation – a tree expected to be grafted may in actuality not be grafted at all or the sick tree - before the fungus is able to move far enough away from the infected tree – it dies within said tree before it makes it to the next tree.


Lacey Oak Wilt in the Central Texas Region

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