This article is mostly a summary of a report published by Jennifer Juzwik, Thomas C. Harrington, William L MacDonald and David N Appel titled “The Origin of Ceratocystis fagacearum, the Oak Wilt Fungus.” David Appel is the head of Department of Plant Pathology and Microbiology, Texas A&M University, College Station. He is considered by many as the go-to for oak wilt in the state of Texas. And, there is strong argument that central Texas is the oak wilt capital of the world. The epidemic is at it’s worst here in our own back yard. You can see the document in its entirety here. Let’s get started…
Reports of widespread oak deaths that resemble oak wilt date back to the 1930’s. The disease was first described by a scientist in 1942 where oak wilt was killing red oaks in the upper Mississippi river valley of Wisconsin. However, it is believed that oak wilt was likely killing trees as early as the 1890’s. By the 1960’s oak wilt was confirmed in all the mid-central, mid-western and mid Atlantic states. According to this research report, oak wilt is known to exist in 860 counties in 23 states. The increase in disease range is believed to be a product of increased recognition of oak wilt rather than actual spread of the disease; they say it is likely that 100 years ago the disease existed everywhere is does today. Oak wilt was first discovered in Texas in Dallas in 1961 and is currently known to exist in 65 counties in Texas.
Many trees naturally die in any forest ecosystem annually, and the relative few that contract oak wilt in such forests could easily be discounted by anyone not familiar with the disease or intentionally surveying for it. Indeed, the confusion over the casual organism of live oak mortality in central Texas in the mid-twentieth century supports this point.”
– Juzwik et al.
*This quote caught my attention for two reasons: a)many oaks die naturally, and b) the confusion of what kills live oaks. Oak wilt has gained such a foothold in the public eye that nearly all dying oaks are deemed oak wilt. 9 out of 10 times when I am called out to a property suspected of oak wilt there is some other problem, not oak wilt.
Most fungi in the same genus (ceratocystis) as oak wilt are known to colonize plant or tree wounds, but oak wilt is the only one known to develop into the vascular system and cause a wilt. This is a very unique feature of the oak wilt fungus; a scientific analysis has shown there are no other fungi that have a close relation to oak wilt, evolutionarily speaking. Juzwik et al also believe the fungi may have evolved with a very close relationship to one very specific nitidulid beetle: Colopterus truncatus.
There is no confident date or place of the emergence of oak wilt into the US. However, they suggest the greatest possibility is the disease was introduced and did not evolve here naturally. Here are their four main points suggesting the likelihood oak wilt is an introduced species:
- The disease has very little genetic variation. Had oak wilt evolved here there would be a large variation in its DNA that occurred over thousands of years.
- There is no close relative that it could have evolved from.
- The oak wilt range continues to expand. *This is the only piece of info in the document that I found a hole in. At one point in the report they say it is likely that oak wilt has been where it is for 100 years or more and the increase in range is probably just an increase in recognition. And then, here, they say its expanding range is a hint towards its origin. Hmmmm?
- Oak wilt’s aggressive behavior toward red oaks might have extinct red oaks many years ago had it evolved here naturally.
If oak wilt was introduced, they think it might have originally come from the cloud forests of Central America, South America or Mexico. And, the only idea pitched about how oak wilt made its travel was via a yellow bellied sap sucker bird (woodpecker). These birds are migratory from South America to Canada. The sap sucker woodpeckers like to poke holes in trees to create sap which attracts bugs for them to feed on. Juzwick et al suggest that maybe a woodpecker had eaten a nitidulid beetle carrying oak wilt spores and then flew to the US and deposited these spores in one of our native oaks.
The report suggests that research in the cloud forests of these regions to the south could offer a breakthrough to the origins of oak wilt. If we knew the origins of the disease we could learn more about how to develop new trees to plant that are tolerant or resistant to oak wilt, or to introduce a biological control. But, for now it’s still a mystery. Mystery is a typical adjective describing most of what we know about oak wilt.