The Once and Future Chestnut

"Chestnuts roasting on an open fire…"

Dr. Larry Grand on chestnut. August 2004.
Photo: Caroline Vernia

I can count on the burned fingers of one hand the number of times I've eaten chestnuts, but for some of you they may be an important part of your holidays. A century ago – back the last time folks were dating their letters 12/21/12 – the American chestnut, Castanea dentata, was one of the most prominent trees of the eastern North American forest, from New England down the Appalachians and areas to the immediate west and south (range map). Apart from the prized wood, these trees provided nuts that served as food for humans and wildlife. There was a problem, though. From the area around New York City a blight was spreading. By the time Mel Tormé and Bob Wells had penned "The Christmas Song" in 1944, most of the native range of the American chestnut had been affected by the disease. Now most of the chestnuts are gone from their original range. The picture at left was taken in 2004 and shows retired NCSU forest pathologist Larry Grand standing on the remains of a once great chestnut tree (I don't actually know the cause of death in this case.)

Fruiting bodies of Cryphonectria parasitica
on the trunk of a chestnut tree. October 2006.
Photo: Marc Cubeta

So what happened? This was another case of an introduced fungus causing widespread destruction on a plant species that had not evolved resistance to it. See the discussion of coevolution in our Dec 3rd blog, if you missed it. The fungus Cryphonectria parasitica had been brought accidenetally from Asia on seeds or seedlings. This fungus causes branch and trunk cankers that eventually girdle and kill the tree. Of all the tree species susceptible to C. parasitica, the American chestnut is the most seriously affected. The fungus reproduces by spores formed in orange-colored fruiting bodies on the bark (picture at right). Two types of spores are produced: one that spreads by wind and one that is transported by birds, insects, or water splash. When spores reach a wound on a chestnut tree they germinate, and the fungus infects. According to information provided by the American Chestnut Foundation, it takes only 2 to 10 years for a mature chestnut tree to die. For more information on the disease process, see the March/April 2012 issue of the Journal of the American Chestnut Foundation. 

Unfortunately this sort of scenario has repeated itself with increasing frequency and rapidity as efficient transportation systems have allowed us to move plants and their diseases around the globe. In October 2011, the disease called "box blight" was first found in North America. Within months it had been found in nine US states and one Canadian province, and it hasn't stopped spreading. This has implications not only for the nursery industry, but for the cut greens industry this time of year. For more information about this threat specific to boxwood and Pachysandra, see Dr. Kelly Ivors' box blight information pages.

Chestnuts. Western NC. October 2006.
Photo: Marc Cubeta

Returning to the chestnut trees, it is an interesting fact that the roots are not killed by the fungus, so new sprouts can repeatedly develop. Eventually, though, this new growth succumbs to the blight. Back in October of 2006, one of our professors came across a chestnut tree in the North Carolina mountains that had grown large enough to bear nuts, though it was clearly infected with chestnut blight (picture at left). I suspect it is gone by now.

Concerted efforts to bring the chestnut back have been going on for some time. Different strategies have been attempted, the most important of which is to cross the native species with resistant species, principally Castanea mollissima, the Chinese chestnut. Another approach was to inoculate trees with a weak (“hypovirulent”) form of the fungus that prevented the deadlier version from killing them. The main reason these strains are weak? They themselves have a viral infection! The 2004 Annual Review of Phytopathology contains an analysis of this situation. (For those unfamiliar with the term, "phytopathology" is the study of plant diseases.) Sadly, one of the hurdles faced by growers trying to re-introduce the chestnut is yet another disease: root rot caused by the fungus-like organism Phytophthora cinnamomi.

Wishing a safe and enjoyable holiday season to all our readers from the NCSU Plant Disease and Insect Clinic! Note that we will be closed December 24th through January 1st, but looking forward to serving you in 2013.

Nematodes in the Tree Tops

Deodar cedar, NCSU Campus
Photo: M.J. Munster, PDIC

Japanese black pine, Wilmington, NC.
Photo: John Wooldridge, NCCES.

What do the two trees in the foreground of these images have in common? They are both infected with the pine wood nematode. This pest also goes by the name "pine wilt" nematode and by the jaw-breaking scientific name Bursaphelenchus xylophilus. That's a big name for roundworms less than a millimeter long. They also are narrow and nearly transparent, making them difficult to see without a microscope. The micrograph below shows swarms of them emerging from a bit of infected wood that was placed in water. Although they are called pine wood nematodes, they can invade a number of genera of conifers in addition to pines.

Bursaphelenchus xylophilus
Photo: NCDA&CS Nematode Assay Laboratory, Weimin Ye

Let me back up a moment for those not familiar with the Phylum Nematoda. Nematodes in general are extremely abundant on our planet, but relatively few cause economic damage. Most species are marine - ask your children if they've seen them on "Sponge Bob." Most of the terrestrial types are not parasites but are free-living scavengers and predators. A few bad actors besmirch the nematode name by causing human and animal diseases like trichinosis, hookworm, pinworm, and canine heartworm. Most plant parasitic nematodes live in roots and soil, such as the famous root-knot nematode (Meloidogyne species). Pine wilt nematodes are an exception. They don't inhabit the soil at any point in their life cycle and rely on a beetle to carry them around. The story is a bit complicated, but goes something like this…

Monochamus carolinensis,a species of pine sawyer
Natasha Wright, Florida Dept. of Ag. & Consumer Services
Bugwood.org

To get things started, you need a live or recently dead tree or log that is both loaded with pine wood nematodes and infested with one of the longhorn beetles known as pine sawyers. Inside the beetle galleries, the nematodes move into the trachea (breathing tubes) in the sides of the beetle. When the young adult beetles emerge from the wood, their first act is to do what's called a maturation feeding on the bark of healthy conifer twigs. Thus the beetle both carries the nematodes to a new tree and gives them an easy entrance by exposing the wood. There are no recorded instances of the nematodes ever saying "thank you" for this service. Once in the wood, the nematodes mature, reproduce, and colonize their new home. From there, disease progress can be slow or rapid. Tree growth slows down, and needles begin to yellow, wilt, and brown. This can happen on the whole tree or just one branch. Eventually the tree may die. Some of this information - plus the juicy details of the process, involving toxin production and "catastrophic xylem cavitation" - is found in Sinclair & Lyon's book Diseases of Trees and Shrubs, 2nd edition.

Austrian pine (background) and red pine, Wisconsin
USDA Forest Service – North Central Res. Station Archive
Bugwood.org

The astute reader may have noticed something else about the trees pictured above: they aren't native to North America. This is not a coincidence. Although our native pines can become infested by pine wood nematode, most species are not seriously harmed unless they are also under drought stress. The photo at left shows healthy native red pines and pine-wilt-affected Austrian pines in Wisconsin. The native conifers, the pine sawyers, and the nematodes all "grew up together", so to speak. There's been a millennial struggle for the upper hand that's led to a sort of stalemate over the years. Any trees that were too susceptible to the nematodes have died out. Likewise any nematode that could not prosper in the trees didn't survive. We call this process "co-evolution". Newly imported nonnative hosts suffer greatly in this situation, because they never evolved defenses against the local pests. Japanese black pine is a wonderful tree for our coast because of its salt tolerance, but many are lost to pine wilt nematode. Even worse is the situation in Japan and some other parts of Asia. There native pines have been devastated following the introduction of Bursaphelenchus xylophilus.

Sample of Japanese black pine as
received at the NCSU PDIC

If you have a tree that you believe might have pine wilt nematode, contact your County Cooperative Extension Service about sending a sample to the PDIC. Unfortunately, the best sample is rather destructive: a six-inch-long segment of the main stem (2-4" diam), just below the dead terminal. The picture at left shows an excellent sample we received earlier this year. We will forward the material to the NCDA&CS Nematode Assay laboratory. There is no cure for infected trees. The actions you take will be to prevent problems for healthy trees down the road, both literally and figuratively. Trees that test positive should be removed and destroyed. To prevent existing or new beetle infestations from moving the nematodes around, promptly chip the tree. After doing some investigating into the biology of the beetles, our Dr. Chuck Hodges has added the recommendation: "If feasible, it may also help to remove the stump 2-3 inches below ground line." In some quarters the idea seems to have circulated that stumps should be treated with borax. This is actually a practice used to combat annosum root rot - another occasional killer of conifers - but that will do no good against pine wilt. Much more information about pine wilt disease can be found in US Forest Service Publication NA-FR-01-04.