Plant pathology researchers with UF’s Emerging Pathogens Institute and IFAS seek to uncover what drives the spread of laurel wilt disease, an emerging forest pathogen responsible for killing more than 300 million redbays. A new study led by IFAS-EPI postdoctoral researcher Robin Choudhury found that the biggest indicator of a Laurel family trees’ susceptibility was having a large diameter trunk and dense clustering.
To eastern forest lovers, it feels like chestnut blight all over again. Only this time, instead of four billion chestnuts succumbing to a lethal fungus, the victims are hundreds of millions of redbay, swampbay and silk bay — trees in the Laurel family. And just as the microorganism that felled chestnuts hailed from South East Asia, so too does the new fungus, Raffaelea lauricola, which is the causal agent of laurel wilt disease.
But whereas chestnut blight fungus was propelled by wind and rain splashes from tree to tree, this time the fungus-farming redbay ambrosia beetle, Xyleborus glabratus, ferries R. lauricola in its specialized mouth pockets from host to host. The beetle plants its food stock in new trees and tends fungus gardens that nourish the industrious insect but simultaneously wreak havoc on a host trees’ vascular system. Only a small proportion of infected trees are thought to survive. Many plants and trees in the Lauraceae family — commonly known as the Laurel family — are at risk.
Laurel wilt disease affects eastern forests in the U.S., and in the sixteen years since it was first discovered in coastal Georgia and South Carolina in 2003, the disease is estimated to have claimed 300 million redbay trees — and that may be an underestimate.
New research by Ariena Van Bruggen’s and Karen Garrett’s laboratories, in UF’s Emerging Pathogens Institute and the Institute for Food and Agricultural Science, plant pathology department, explores the dynamics of laurel wilt disease spread in both coastal and inland sites, and how environmental factors may propel or mitigate the disease’s spread across the landscape. Marc Hughes, a former PhD student in IFAS’s School of Forest Resources and Conservation, and Hong Ling Er of Van Bruggen’s laboratory led the site survey work and performed preliminary data analysis. (Hughes studied under Van Bruggen and also with UF’s Jason Smith, who is an associate professor of forest pathology; Smith is also a coauthor to the paper.) The research team also included Randy Ploetz, who has led a USDA-funded project addressing laurel wilt management in avocado, and who recently led a synthesis of knowledge about the laurel wilt pathosystem. Robin Choudhury, a post-doctoral researcher in Garrett’s lab, led the final data analysis and is the paper’s lead author.
Many previous laurel wilt studies have focused on a few sites, but the new study surveyed 87 plots located in both coastal and inland areas. More than three-quarters of the study plots were affected by laurel wilt disease. The breadth of sites allowed them to assess landscape-level patterns.
“One of our most important findings was that a host’s trunk diameter size is directly related to disease incidence. For me, it was cool to see that come out in the data,” says Choudhury. “It’s a result that has been seen in other studies for laurel wilt and even in other forest pathogens — but to see it confirmed at such a broad scale was both fascinating and reassuring.”
Redbay trees appear both as tall shrubs covered in dense and leathery aromatic green leaves, and as short-trunked trees reaching a height of 65 feet or less. Their fruits are bitter, but they last into winter and provide nourishment to many birds including quail, turkey and generalist songbirds.
Eastern forests could potentially lose the vast majority of their redbays, swampbays, silk bays, and other susceptible shrubs and trees including sassafras, pondspice, and pondberry which are affected to varying degrees. Some species, like gulf licaria and pondberry, are already federally endangered, and the disease may push them closer towards extinction.
Laurel wilt disease is documented in 198 counties in Florida, Alabama, Georgia, North and South Carolina, Louisiana, Mississippi and Texas. The beetles are not choosy, they will propagate their fungus gardens in healthy trees just as often as they may choose sickly or old ones. Laurel tree species dominate forest canopies throughout the world’s tropics; the potential losses are staggering.
Diseased plants and trees choke to death. The fungus triggers a host tree to produce an overabundance of tyloses in its vascular system. Tyloses are the plant immune system equivalent to an emergency tourniquet that seals off affected vascular cells. They shut off the flow of water and nutrients locally to prevent the fungus invader from spreading. But trees affected by R. lauricola create so many tyloses that they cripple their own water and nutrient supply; in essence, it’s not the fungus that directly kills the tree, rather it’s a tree’s own reaction to the fungus that is lethal.
Early symptoms may include a wilted-looking canopy, and later symptoms include loss of leaves, limb dieback, dark streaks and stains in the sapwood, and tiny bore holes that lead to “galleries” where the beetle farms its fungus crop.
Both the redbay ambrosia beetle, and the R. lauricola fungus, are native to Asia where they do not cause disease to plants in the Laurel family. However, the fungus shows a great deal more genetic diversity in Asia which, combined with the lack of disease, suggests a long history of nonpathogenic coexistence with Laurel family plants, or perhaps R. lauricola coevolved with native Asian Laurels.
The redbay ambrosia beetle and its symbiont fungus are thought to have arrived to the United States aboard cargo ships before becoming established locally in coastal forests of South Carolina and Georgia. From there, they spread astonishingly fast considering that the beetle usually flies fewer than 30 feet at a time between trees. People may have sped the disease’s spread by transporting infected firewood across state lines.
TRACKING A FOREST’S DEATH
In their new study, the team examined how a susceptible Laurel tree’s physical characteristics — its species, trunk diameter at breast height, distribution and density on the landscape — contributed to its susceptibility of contracting laurel wilt disease. The team focused on three species in the Persea genus: redbay, swampbay and silk bay. They also examined how environmental factors, such as air temperature, wind speed, and relative humidity affect the incidence of laurel wilt disease. Last, they tracked how laurel wilt disease colonizes previously unaffected areas over time and space to better understand its patterns.
With funding from USDA’s National Institute of Food and Agriculture, they selected a mix of 87 diseased and disease-free study plots spread across seven sites sprinkled along the coasts of South Carolina, Georgia, and Florida plus an inland site at Archbold Biological Station in Florida. Located in south central Florida at the headwaters to the Everglades, Archbold is a longstanding ecological research site. It contains large swaths of varied and mostly intact habitats spanning from Florida scrub to sand dunes and flatwoods.
The most striking physical attribute to predict whether a susceptible tree became infected was the trunk diameter as measured at breast height, or about four feet off the ground. The research team found that trees of five centimeter diameter or greater showed the highest infection frequencies, and that every centimeter increase in trunk diameter equated to a seven percent increase in disease incidence.
This result closely matches other studies which reveal how the beetle employs senses to select a host tree. “Prior studies created silhouettes made from PVC pipes and filled with attractive baits at different diameters,” Choudhury explains. “They found that beetles tended to home in on a specific size. They have a search pattern, and it seems they first largely operate off of olfactory senses, they sense chemicals associated with the hosts they are interested in. And when they get close, they seem to go off of visual cues.”
When larger diameter trees were densely clustered together, they tended to have the highest incidences of disease. In other words, study plots with dense stands of large-diameter trees tended to have much higher rates of diseased trees compared to study plots with smaller diameter trees that were spaced apart and not as closely clustered. These data combined with forest surveys could help identify areas at risk of invasion by the disease in the future.
While the coastal study plots had disease incidence ranging from 43.6 to 86.1 percent, Archbold had a surprisingly low incidence rate of only 3 percent. Choudhury says a few different things may be at play. First, redbay trees were the only Laurel plant found at some coastal sites, and the dominance of this highly susceptible tree in some ecosystems may lead to faster disease spread.
Second, temperature may have also played a role, as the inland site was consistently hotter than the coastal sites. Archbold’s air temperature was, on average, about 5 degrees Fahrenheit warmer than the coastal sites. It’s possible that the fungus, or the beetle, may have an upper heat limit threshold that slows disease spread.
Third, it may be that the region’s natural fire cycle disrupts laurel wilt disease. Just as handwashing prevents disease in people, small but frequent fires may “cleanse” a fire-dependent landscape. Fires may mediate laurel wilt disease by preventing bay trees from growing large enough to become attractive to the fungus-farming ambrosia beetles. By this line of thought, the seasonal cycle of burning and regeneration may keep the bay trees just small enough that they evade the redbay ambrosia’s search pattern, Choudhury says. Or, the frequent low-scale burns may support a diverse ecosystem that prevents one species, such as redbay, from being dominant or closely clustered; and the diversity and density of plants themselves may inhibit laurel wilt disease from spreading.
The team also found a correlation between high wind speeds, high relative humidity, and higher air temperatures that appeared to limit the occurrence of disease within a study plot. Choudhury suggested a possible mechanism was that a combination of environmental factors made it unfavorable for the beetle vector to fly in search of new host trees.
“While I feel very confident about our results concerning the role of the trunk diameter and host density, the data we got from environmental factors — such as temperature, wind speed and relative humidity — seemed to spark more questions than answers,” Choudhury says.
Laurel wilt disease also affects commercial avocados, which are in the Laurel family. The Garrett Lab is involved with studying how to support commercial avocado growers in Florida and smallholder farmers in Haiti. Whereas avocado varieties grown in Central America (which have a buttery consistency) have natural resistance to laurel wilt disease, the varieties that flourishes in Florida’s climate (which tends to taste more watery) are at higher risk.
The disease process that threatens Florida’s “green-skinned” avocado cultivar may differ somewhat from affected redbay coastal forests because additional species of ambrosia beetle may be vectors. The result is that avocado growers in south Florida are entrenched in a multi-vector siege against the laurel wilt pathogen. Miami-Dade County alone has lost more than 9,000 avocado trees since 2011.
Fungicides exist, but they are costly and the injection process causes physical harm to the host tree. Thinning densely planted groves, and maintaining a mix of younger trees versus larger, more mature trees, may give avocado growers an edge against the fungus, according to the new study.
“Unfortunately, in Florida we can’t grow any of the really resistant varieties,” Choudhury says. “One of our collaborators, Bruce Schaffer with IFAS, is interested in the idea of grafting, or using resistant root stock and grafting Florida-specific varieties on top of it.”
But until a laurel wilt disease-resistant avocado variety is ready for Florida’s growers to cultivate, Choudhury’s best advice is for farmers to scout out and remove diseased avocado trees as quickly as possible, a process called roguing.
“Rogue early and rogue quickly,” Choudhury advises. “This will reduce and eliminate threats as quickly as possible.
One significant difference between chestnut blight and laurel wilt disease deserves consideration. Chestnut blight killed its victims before they could reach sexual maturity and reproduce, which circumvented the natural emergence of resistant trees. Species of Persea, however, can reproduce even at very small trunk or stem diameters. This offers a glimmer of hope that over very long time periods some natural resistance to laurel wilt disease may emerge.
“But even if this happens,” Choudhury says, “Laurel wilt disease will still profoundly disrupt and restructure the species composition of eastern forests as we know them.”
By: DeLene Beeland