“Susceptible” / “Resistant” Quandary
A. Merriam Webster
- Susceptible: open, subject, or unresistant to some stimulus, influence or agency
- Resistant: giving, capable of, or exhibiting resistance; i.e. the inherent ability of an organism to resist harmful influences (such as disease,…etc.)
B. Northern State Experts
- 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.
- 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 grafting.
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.
“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”
“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 
The consistent position by the Texas Forest Service is that “all oaks are susceptible to oak wilt to some degree.” Further, this susceptibility is tiered in the following way:
“Red oaks are extremely susceptible”, “Live Oaks are intermediate in their susceptibility”, and a blanket statement “white oaks rarely die from oak wilt.”
The use of susceptibility here appears to be defined as their position on the probability of mortality once contraction occurs, (which would technically be an aspect of resistance).
- Regarding resistance (which appears to be used more or less as a synonym of susceptibility and tolerance), the Texas Forest Service has repeatedly grouped the entire white oak family in the “resistant” category. “White oaks rarely die from oak wilt. Most only exhibit some canopy loss.” “Generally” the disease will not spread via root grafts. This is a major divergence from Northern states “may survive” to the TFS “may die” regarding white oaks.
There is one quote from one update that clearly shows a remarkable intra-department divergence of position (on at least some white oak species), that much more conforms with the northern state's experts regarding white oak group mortality:
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.
Clarification on this by the Texas Forest Service (and many other issues) is desperately 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).”
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)
- Has a study of the Chinkapin Oak, Durand Oak, Lacey Oak, Mexican White Oak, and Shin Oak matching the excellent second study been conducted? I have spoken with numerous state experts who have answered in the negatory. Obviously one cannot proffer what doesn’t exist. This is quite an unfortunate fact. It is this fact that has greatly contributed to the dire need for the existence of this webpage and campaign of investigation – recording observations, dna diagnostic sampling, and the like…
For more see:
 C.f. www.texasoakwilt.org (from inception of site to current form).
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.
D. Author’s Response: A Texas Solution to the Quandary
- 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.
- 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.
- 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 true tolerant designation. AT THIS TIME IN TEXAS – NO OAK SPECIES IS TRULY 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 TRAQ and or TPAQ assessments. (see images 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.
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.