Tuesday, July 9, 2019

Oak Wilt And Invasive Species Vigilance

leaf and whole tree symptoms of oak wilt in a red oak tree It’s hard to be cheerful in a job where I am expected to keep up on each newly arrived or imminent threat from invasive insects, novel plant diseases, and worrisome trends in the environment. Although I typically deflate everyone’s happy-bubble when I give a talk, I’ve discovered we need not fret that the sky is going to fall.

The National Atmospheric Deposition Program (NADP) is a joint effort of research institutions, government agencies and nonprofit groups; their mission is to monitor stuff which falls to Earth that is not some form of water. Since one of the NADP’s tasks is to study tiny particles of pollutants in the air, they will certainly notice if the sky starts to fall, and give us ample time to take cover.

Kidding aside, when we hear bad news day in and day out we usually get the sense there is nothing we can do about any of it. That helpless feeling is the worst; it breeds anxiety and apathy. However, the up-side of knowing about threats to our forests and gardens is that often we can make a difference. About 40% of the time, new invasive-species infestations are found by folks who knew about such issues, but who weren’t actively looking because it was not their job.

In 2017, a small infestation of hemlock woolly adelgid (HWA) was found by a hiker near Lake George. This little creature (HWA, not the hiker) is lethal to hemlocks, and though it was horrifying to find it in the Adirondacks, that person likely averted a major crisis. He knew about HWA, reported it, and as a result, the infestation was eradicated.

Therefore I don’t feel so bad presenting yet another threat to our forests, oak wilt, because as far as we know, it’s not in northern NY State, and there are several things we can do to help keep it that way. First discovered in Wisconsin in 1944, it is now in twenty-four States, with four outbreaks in New York State. The fungal pathogen that causes oak wilt is thought by many to have originated outside North America, but no one knows for sure.

What is for sure is that oak wilt is a death-knell for red oaks. It kills healthy red, black, pin, and similar “red type” oaks in two to six weeks. It is similar to Dutch elm disease in that is plugs up xylem tissue and blocks water uptake. Symptoms are sudden browning of leaves, usually in July or August, followed by leaf drop. Oak wilt spreads easily, and there is no known treatment. Lovely, I know.

“White type” species such as white, bur, and swamp white oaks can take from one to three years to die. This is not good news because they can infect nearby oaks without showing major symptoms.

Oak wilt is spread in two general ways; below ground and above ground. A tree’s roots extend two to three times its branch length, and as they cross paths with roots from related species, they often form graft unions. We know little about this “tree internet,” other than chemical signals can share news of, for example, the arrival of a pest from one edge of a wooded plot to the other.

Sadly, root grafts also spread dreadful blights like Dutch elm disease, and now oak wilt. Such tree-to-tree spread, while important, is a lot slower than airborne transmission above ground, oak wilt is naturally spread by insects, especially sap beetles in the family Nitidulidae, a detail I mention because the word is endlessly fun to say. At least for me. Nitidulid beetles carry oak wilt spores from infected trees to fresh wounds on healthy oaks. The oak wilt fungus only makes spores on red-type oaks, on which sweet-scented spore pads develop. But all species of oak can be infected by a spore-covered nitidulid. Rarely, birds may carry oak wilt spores, and it was recently found that powerful spring and early summer windstorms (the risk of infection drops significantly after midsummer) can vector the pathogen.

We are the other vector, of course. In 2018, new infestations cropped up in Ontario County, nearly 200 miles from the nearest known oak wilt source. It is almost certain this was due to the transport of firewood.

We can help keep oak wilt away by changing a few habits. The easiest is to not bring firewood home with us when traveling. Trite as this may sound, it is vital. It’s dangerous to think oak wilt is hundreds of miles away. As the residents of Canandaigua recently learned, it is as close as the first idiot who brings firewood back from visiting his (and it’s always a guy who starts these calamities) buddy whose dead tree he helped cut up.

Between April 1 and July 1, the risk of spreading oak wilt is extreme if there is any exposed fresh-cut oak, whether a stump or a pruning wound. From July through September the risk is moderate. Obviously we might need to cut an oak branch or tree in this time. The work-around is to spray-paint the wound or stump immediately after cutting. I emphasize immediately because nitidulid beetles will find fresh oak sap within an hour. If possible, only prune or harvest oak from October through March.

Sorry for the echo, but if this pathogen does find us, our oaks will go the way of the American chestnut. By adopting these guidelines, we can vastly reduce the risk of seeing oaks vanish from our forests. If you suspect you have seen oak wilt, contact NYSDEC at foresthealth@dec.ny.gov or call 1-866-640-0652. Never bring “free” firewood home to the North Country. It’s a small thing to do to keep the sky from falling.

Photo of leaf and whole tree symptoms of oak wilt in a red oak tree courtesy Joseph O’Brien, USDA Forest Service.

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Paul Hetzler has been an ISA Certified Arborist since 1996. His work has appeared in the medical journal The Lancet, as well as Highlights for Children Magazine.You can read more of his work at PaulHetzlerNature.org or by picking up a copy of his book Shady Characters: Plant Vampires, Caterpillar Soup, Leprechaun Trees and Other Hilarities of the Natural World

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