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.

Fungal frustrations

I've been wanting to write about this situation for quite a while, but only now have found the time: nuisance fungi in landscape beds. I'm going to zero-in on the two most aggravating groups: the stinkhorns and the artillery fungus. Other nuisances include the bird's nest fungi and certain true mushrooms. The "dog-vomit slime mold" is a very different sort of organism and was covered in our June 2011 blog post.

Stinkhorns. Wake Co., NC. Oct 2012. Photo: Greg Florian

Most stinkhorns are very conspicuous horn-shaped fungal fruiting bodies several inches tall that produce strong odors, unpleasant to humans. It's a bit hard to write about the stinkhorns without losing our PG rating, so I'll leave it to you, the reader, to look up the meanings of some of the Latin names if you want to know more. We received the image at left from a homeowner in Wake County. These are almost surely Phallus ravenelii, and they appeared in his mulched bed in early October. The submitter said he could smell them from two houses away. Is there a purpose to this odor, you ask? Take a close look at the picture and you can see that flies have been attracted to the stinkhorns. Why, you wonder? Well, the sticky green portion at the top contains the spores of the fungus. When flies visit, they pick up some of the spores and carry them away to new locations, so they're a means of dispersal for the fungus. You may have heard of a similar raison d'être for the foul smell of skunk cabbage, though in that case it's to attract pollinators. Fortunately for farmers and home gardeners alike, the odor does not persist once the fungi are gone.

Clathrus columnatus. Wake Co., NC. Jan 2008

Over several weeks in October and up until about our first frost, there were large numbers of stinkhorns in a the landscaped median of a major road near campus. Traffic didn't allow me to get a good look, but I think they were Mutinus elegans. This genus has a more tapered apex than Phallus. Back on January 18, 2008 I found a specimen of Clathrus columnatus on a south-facing slope here on the NCSU Campus. It had the smell of spoiled fish wrapped in a wet diaper. The fruiting body of this species has four "arms" fused at the apex.

Sphaerobolus gleba on siding. Jackson Co., NC.
Oct 2010. Photo: Christy Bredenkamp

Sphaerobolus gleba on gardenia leaf. Wake Co., NC. Oct 2012.

A very different dispersal mechanism is used by fungi in the genus Sphaerobolus. Their fruiting bodies on the mulch are small and inconspicuous, but they launch tiny dark spore balls for distances up to several feet, earning them the name "artillery fungus" or "cannonball fungus". The spore masses, known as gleba, stick fast to whatever they hit: plants, siding, and vehicles. Even when scraped off, they leave a stain, causing great frustration for the owners of affected homes and cars.

There's a lot more to these fungi than meets either the eye or nose. Their real "body" consists of a network of fine threads called hyphae that grow throughout the soil and mulch, where they decompose dead organic matter such as the mulch itself. The good news is that they do no harm to trees, shrubs, or bedding plants. The bad news is that there's no easy way to get rid of them. There are certainly no fungicides or disinfectants you can use in these situations. Removing stinkhorns as soon as they appear will help with the odor problem. Turning under the existing mulch and replacing it with composted mulch or a coarse pine bark mulch may help. Dr. Harry Hoitink of the Ohio State University has a very interesting fact sheet that includes a discussion of the microbial ecology of composts. One of his conclusions: "… water applied at the right time during composting, storage, and mulching can solve most of the fungal nuisance problems." Another very informative resource comes from Dr. Donald Davis at Penn State University. His Artillery Fungus FAQ gives details about this organism, how to deal with it, and suggestions from readers about how to remove the spots. It appears from reading his page that Dr. Davis has dealt with everyone from homeowners to attorneys about this issue.

Most fungi outdoors in North Carolina are going to be hidden from view over the winter, but keep the nuisance fungi in mind when planning your next landscaping project. You may be the next one to notice an unusual smell in the neighborhood.

Don’t Move Firewood!

Photo: Larry Grand

With a chill in the air and leaves turning brilliant colors here in Raleigh, our thoughts turn to cozy firesides, big bonfires, and warm campfires after a long day’s hike. With this in mind, we asked Rob Trickel of the North Carolina Forest Service for a timely reminder about the dangers of moving firewood. 

Invasive Pests and Firewood Movement

Non-native invasive forest pathogens and insect species have potential to cause great harm to North Carolina’s forests and landscapes.  The fungus that causes laurel wilt is killing redbays and swampbays in the Coastal Plain and we think it may also devastate sassafras across the state.  Other invasive diseases and insects that have the potential to cause great harm to walnuts (thousand cankers disease), ash (emerald ash borer), and a variety of hardwoods (gypsy moth) are found in counties in Tennessee and Virginia adjacent to our state (see current invasive monitoring map).  In addition, Asian longhorn beetle is devastating a wide variety of hardwood tree species in the Northeast and Midwest, and has the potential to do the same here.   All of these pests have the following in common:  they move naturally from place to place at a very slow rate, but can be spread rapidly if moved in firewood.

Emerald Ash Borer Galleries in firewood. Photo: Troy Kimoto, Canadian Food Inspection Agency

Moving firewood can introduce invasive pests to new ecosystems, where they can cause ecological, economical, and social problems in our forests.  In response to the rapid spread of emerald ash borer via firewood, some states have even enacted laws regulating the movement of firewood to slow the spread of invasive species.  North Carolina is approaching the problem with public awareness and education campaigns and has joined with other southeastern states to promote the use of local firewood or firewood that has been treated or certified to be pest-free.  

As part of the effort, the Changing Roles program of the USDA-Forest Service has developed two fact sheets about firewood movement as a means of spreading invasive species to new areas. These fact sheets help equip our partners who work with various audiences (consumers and producers of firewood) with information to combat the spread of invasive pests.

Fact Sheet 5.4 (Invasive Species and Firewood Movement) is firewood/invasive species 101 and covers:  What is firewood?  What types of invasive pests are transported in firewood? Why is the movement of firewood a pathway for the spread of invasive pests? What are the ecological, economical and social effects of invasive pests?  

Fact Sheet 5.5 (Preventing Firewood Movement) concentrates on how to engage a variety of audiences on firewood issues including:  How do we (foresters, extension staff, natural resource professionals) communicate with and engage different audiences on this important topic? How do we work with homeowners and outdoor enthusiasts (also parks and campground staff, arborists and green industry professionals, wood processors and producers) to combat the spread of invasive pests?  How is the movement of firewood regulated? And, where can I find more information about preventing firewood movement?

You can find more information about firewood movement and forest health, excellent images of the insects and diseases that can be spread by moving firewood, some very entertaining videos at www.dontmovefirewood.org   

Photo: Rob Trickel

 

Prepared by Rob Trickel, Forest Health Branch Head, North Carolina Forest Service

Super Sad Sunflowers: Alternaria leaf and stem spot

Field of sunflowers (Photo: B.B.Shew)

Sunflowers are becoming an increasingly popular alternative crop in North Carolina. A field of sunflowers in bloom is beautiful, but sunflowers are most important as an oil crop. Sunflower oil is lighter in taste and supplies more Vitamin E than other vegetable oils. Sunflower oil is particularly appealing to the food industry because it is trans-fat free and stable without hydrogenation, making it excellent for frying and increasing shelf life. Since most sunflowers are produced in the Great Plains region of the U.S., little is known about which diseases and insect pests may become problematic or limit sunflower production in North Carolina.

As the growing season came to an end, we received several sunflower samples that we diagnosed with Alternaria leaf and stem spot. In the traditional growing areas of the Great Plains, Alternaria is usually of minor importance because conditions are not favorable to severe disease development. Unfortunately for us, North Carolina has hot and humid summers perfectly suited for Alternaria disease development.

Alternaria disease symptoms (Photo: E.C. Lookabaugh)

Alternaria leaf and stem spot is caused by the fungus Alternaria. Two species of Alternaria cause disease on sunflowers, Alternaria helianthi (now known as Alterniaster helianthi) and Alternaria zinniae with A. helianthi being more common. Symptoms include irregular leaf spots, stem lesions, and dark brown spots on the seed heads. Leaf spots are dark brown with grey centers. Yellow halos around spots are seen on younger foliage. The stem lesions begin as dark flecks that enlarge to form large blackened narrow lesions. Severe infections result in defoliation and stem lodging.

Flower head symptoms (Photo: E.C. Lookabaugh)

Stem lesions (Photo: E.C. Lookabaugh)

The pathogen survives on infested plant debris and on the alternate hosts safflower and cocklebur. Cocklebur is native to the U.S and is found throughout NC. Safflower is not present in NC. The fungus is spread by wind and splashing water. Spores splash from infested crop debris onto the lower leaves and stems, where symptoms first appear. Warm, humid conditions favor disease development and spread.

Control of this disease can be achieved through a strict sanitation program, crop rotation, and the use of preventative fungicides. Sanitation is key. Disease plants should be removed and destroyed. Cultural methods that limit splash dispersal and long periods of leaf wetness probably will help to reduce disease. These include avoiding overhead irrigation and using proper plant spacing to promote air circulation. This disease can be seedborne. Use certified, disease-free seed that is produced in dry regions where Alternaria is not a problem. Do not save seed from diseased plants. Do not grow sunflowers two years in a row in the same field because the fungus overwinters on plant debris. Tillage operations promote debris decomposition and reduce the chances of splash dispersal of the fungus. Preventative fungicide applications can be started when symptoms first appear or at flowering, since this is when plants become most susceptible to the disease.

Happy Sunflower (Photo: B.B. Shew)

Post by Emma Lookabaugh and Barbara Shew

Southern Stem Rot on Peanut

Written by Barbara Shew 

The fungus Sclerotium rolfsii has over one thousand plant hosts and causes diseases on field, vegetable, and ornamental crops grown in North Carolina. On many hosts, the first sign of infection are profuse strands of white, fan-shaped fungal growth at the base of the plant. Later, the fungus produces large numbers of round, tan-to-brown sclerotia that resemble mustard seed.

Stem Rot on Peanut ( Photo: B. Shew)

On peanut, the disease caused by S. rolfsii is known as southern stem rot, stem rot, or white mold. White fungus growth appears when the environment is right, often just following a rain (Figure 1). However, southern stem rot can be very damaging on peanut even when the distinctive fungus is not apparent; the damage often is hidden below ground. In addition, above-ground symptoms of stem rot can be hard to distinguish from other important peanut diseases, including CBR, Sclerotinia blight, and Rhizoctonia limb rot. The best way to determine the severity of a stem rot problem is to check the plants within an hour or two of digging. It is a good idea to check peanuts for symptoms of stem rot, CBR, and nematode damage immediately after digging. Growers should note problem areas so that appropriate controls can be used the next time peanuts are grown. I prepared a short video pointing out the distinctive features of stem rot on freshly dug peanut plants.



For more information about peanut diseases and their control, check out these links:
http://www.cals.ncsu.edu/plantpath/extension/clinic/fact_sheets/index.php?do=plant&id=1
http://www.peanut.ncsu.edu/

Box Blight Variety Trials

Box blight is a new disease to the U.S. and is caused by the fungus, Cylindrocladium buxicola.  Box blight can seriously affect the appearance and aesthetic value of boxwoods. Typical symptoms include circular leaf spots with dark brown to purple margins, dark brown to black stem lesions, blighted foliage, and eventual leaf drop.  Under humid conditions,  white fuzzy spore masses can be found on infected stems and leaves.  Disease develops rapidly in warm, humid conditions and is more severe in shade conditions.  Boxwood varieties differ in their susceptibility to box blight.  A recent study determined that B. sempervirens types are more susceptible to box blight.  B. microphylla var. japonica 'Green Beauty', B. sinica var. insularis 'Nana', B. harlandii, and B. microphylla 'Golden Dream were all found to have some level of resistance to box blight.  Be sure to remember that some boxwood varieties are limited in their optimal plant hardiness zones.  

Box Blight Symptoms

For more information on Box Blight, click here.  For more information on the boxwood variety trials, click here.  

Honey Mushrooms Galore

Last week NC State hosted the 18th Ornamental Workshop on Diseases and Insects at the Kanuga Conference Center in Hendersonville, NC.  Ornamental pathologists, entomologists, and diagnosticians from across the country spent the week discussing all things related to ornamental diseases and insect pests.  Lucky for us, we were also able to carve out some time to do a little hiking/ mushroom hunting in Pisgah National Forest.  Over the course of our 5 mile hike, we saw one black snake, one timber rattlesnake, and thousands of honey mushrooms! Apparently Pisgah National Forest is a hotbed of Armillaria.  With all of the recent rain, you should start seeing Armillaria honey mushrooms popping up in your neck of the woods as well.  For more information on Armillaria root rot, see our previous blog post here.

Honey Mushrooms (Photo: E.C. Lookabaugh)