Volume 91 Issue 51 | Web Exclusive
Issue Date: December 23, 2013

Saving The Christmas Trees

Moisture-loving soil microbe continues to ravage firs as researchers seek solutions
Department: Science & Technology | Collection: Life Sciences
News Channels: Biological SCENE, Environmental SCENE
Keywords: fungicide, root rot, pathogen
Phytophthora-induced root rot discolors Christmas trees such as Noble firs (shown) as it works its way up the plants.
Credit: Katie McKeever
Photo of two Noble fir trees afflicted with red-brown discoloration due to Phytophthora-induced root rot.
Phytophthora-induced root rot discolors Christmas trees such as Noble firs (shown) as it works its way up the plants.
Credit: Katie McKeever

The Grinch that has stolen America’s Christmas trees for decades remains at large. This thief is hard to see, though. It comes in the form of a microscopic pathogen that kills Christmas trees by slowly rotting away their roots and then ravaging their innards.

The pathogen that causes this “root rot” condition is a moisture-loving, funguslike soil microbe from the Phytophthora genus. The genus’s most noteworthy species is P. infestans, which helped produce the Irish potato famine in the 1800s. Lesser-known species such as P. cinnamomi have gone after firs for decades. These conifer trees are popular for Christmas because of their long-lasting needles and stiff branches, good for hanging ornaments. Two popular firs in the U.S.—Fraser firs and Noble firs—have been especially prone to root rot. Although some growing practices and fungicides can reduce the severity or incidence of root rot, growers in the U.S. Christmas tree industry—worth roughly $1 billion in retail sales in 2012, according to the National Christmas Tree Association—continue to lose trees.

Although some growing practices and fungicides can reduce the severity or incidence of root rot, growers continue to lose money. Now researchers, aided by modern molecular and genetic approaches, seek to unlock clues about how Phytophthora works and ways to quickly screen firs for root rot resistance. That information could help growers, many of whom have experimented with other less susceptible species. Or it could help them breed more-resistant trees. That could mean a larger batch of trees for consumers.

“It’s research that benefits consumers and growers,” says Katie McKeever, a graduate student at Washington State University (WSU), Puyallup, who’s researching the topic. “It helps people on both ends.”

Researchers still don’t know the molecular mechanisms of root rot infections. They do know that a Phytophthora microbe propels itself through soil water with the aid of a flagellum. When it reaches a tree, the pathogen sheds that microscopic tail, then latches onto and attacks the tree’s root tips. From there it severely damages the root system, often causing discoloration and lesions as it works its way up the plant, until the tree dies.

Given that the pathogen thrives in water, it’s not surprising that more cases are seen after unusually rainy periods or major storms like hurricanes. North Carolina’s Christmas tree industry could endure an increase of root rot after abnormally high rainfall in 2013, says Kelly L. Ivors, a plant pathologist at North Carolina State University’s Mountain Horticultural Crops Research & Extension Center in Mills River.

A number of fungicides can control Phytophthora infections to an extent. Two are especially widely used. Mefenoxam, a phenylamide that Syngenta Crop Protection makes under the trade name Subdue Maxx, targets the pathogen’s RNA synthesis system. The other dominant fungicide is fosetyl aluminum, a phosphonate made by Bayer CropScience under the trade name Aliette; its mechanism isn’t fully understood.

Those fungicides, though effective in nurseries and seedling beds, aren’t economical for large-scale growing operations, Ivors says. She explains that a prohibitively large amount of fungicide must be applied to the soil for it to penetrate deep enough to kill all the Phytophthora.

As for alternatives, fumigation isn’t practical on large scales either, she says. Even letting soils sit fallow for years won’t necessarily work. Phytophthora can be hardy, Ivors says. “Once soils get it, it’s very hard to get rid of.”

Species diversity also plagues the search for effective, practical solutions. Different tree species grow in different regions, which in turn harbor different Phytophthora species.

McKeever, who’s studying under WSU plant pathologist Gary A. Chastagner, is collecting Phytophthora cultures from across the country to determine which species of the pest are found where. The researchers plan to expose nine species of firs to these various species of Phytophthora and see how the trees respond. “By using these combinations, we should get a good idea of which of these firs are displaying resistance,” McKeever says, and to what Phytophthora species.

Studies that compare which genes turn on and off in the various trees could then determine which genes activate in response to root rot, Chastagner says. Armed with that information, he says, researchers hope to identify biomarkers of resistance, making it easier to screen for resistant trees. They can also look for molecular or morphological changes in the affected tree parts to better understand the mechanisms of root rot, he adds.

Meanwhile, some U.S. growers have begun to experiment with Turkish and Nordmann firs, nondomestic varieties known to partially resist contracting root rot. But these and other resistant species—at least so far—have drawbacks as Christmas trees. For instance, some don’t keep their needles as long after being harvested, unless they’re given a constant water supply.

Growers can graft shoots from a nonresistant species such as Fraser onto rootstock of a resistant one, creating a tree with resistant roots and good Christmas tree qualities, says tree geneticist John Frampton at North Carolina State University. But it’s expensive.

Not all hope is lost, researchers say. Perhaps crossbreeding resistant trees with others that have good Christmas tree qualities could yield an effective hybrid tree. Frampton identifies another option: genetic engineering. Someday, he suggests, researchers could transfer a resistance-conferring gene from one species into the genome of a less resistant but otherwise good tree.

At least for now, however, this group of pathogens will continue playing Grinch.

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