Solving the problem of laurel wilt

Dr. Jason Smith, forest pathologist, University of Mount Union in Ohio, said it took a long time for one of the most recent and puzzling diseases of certain trees and shrubs to get anyone’s attention, primarily because the dead and dying trees were in swampy areas including the Florida Everglades.

The disease is laurel wilt, caused by the fungus Harringtonia lauricola. It’s spread with the help of a tiny beetle, the redbay ambrosia beetle (RAB). Trees and shrubs in the Lauraceae family are affected.

“What’s going on with laurel wilt is completely unprecedented,” said Smith. “It isn’t what ambrosia beetles typically do; the fungi aren’t pathogens. It’s a completely novel relationship due to the introduction of an organism into a new environment. There is no example of an ambrosia beetle fungus causing a systemic vascular wilt like with laurel wilt.”

One RAB successfully boring into a healthy tree and releasing the fungus will result in disease. The fungus becomes established in the sapwood; spores colonize rapidly and are dispersed into sapwood. The vascular tissue initiates a response that results in rapid tree death.

After the tree dies, the beetles use it as they typically would: mass attack, fungus cultivation, then adults emerge and attack more healthy trees. Complicating the issue is that RABs transfer the fungus to at least eight other beetle species, which becomes a problem for management and regulatory control.

“We aren’t totally certain there aren’t other insects carrying this fungus,” said Smith. “Since it’s transmitted into the vascular system and into leaves, we’ve been able to experimentally transmit it by inoculating leaves.”

Dr. Romina Gazis, plant pathologist at the Tropical Plant Clinic & Research Lab, University of Florida, is focused on finding methods to diagnose laurel wilt. Since it was first detected in Florida in 2012, it has killed more than 300,000 avocado trees and many growers have gone out of business. But laurel wilt is more than just a Florida problem.

Gazis said many diseases cause flagging, dieback and rapid leaf desiccation, but laurel wilt kills trees quickly. Trees typically desiccate within one month. While some may resprout, infection of the stump and roots occurs early in the disease cycle. Sassafras is one of the species affected by laurel wilt, and since it’s normal for this tree to lose leaves, field detection is confusing and may take longer to identify.

“Tests and diagnoses are based on samples,” said Gazis. “How well we diagnose something depends on how good the sample is. It also depends on where the sample is taken. One section of the tree may not show signs, while other areas have clear symptoms.”

Gazis’s goal is to detect the pathogen in RAB. So far, polymerase chain reaction (PCR) testing has not been a reliable method of detecting laurel wilt.

“A beetle is its own world,” said Gazis. “When one beetle is crushed, the sample is lost. Reproducibility becomes hard, but we’re working on it and hope to have a tool available soon.”

While the development of diagnostic tools is aimed at avocado because that’s where the money is, Gazis believes the technology will be transferable to other trees in the Lauraceae family. Researchers are working to develop a field-portable diagnostic tool to confirm laurel wilt in minutes rather than the days it takes for lab analysis.

Andrew Lloyd, plant pathologist, Bartlett Tree Care, has been studying the RAB. He believes it’s likely that satellite populations are appearing from moved firewood.

In researching the beetle and pathogen, Lloyd cultured the fungus using logs with active frass and collected beetles from the logs. Beetles were then raised in rearing chambers.

“We were able to isolate the fungus,” said Lloyd. “This fungus produces a lot of conidia very quickly. The goopiness the cultures start off with is full of spores, and they’re easy to see under the microscope for diagnosis.”

The disease is concerning because there have been numerous biological invasions in New York. “Southern pine beetle is wiping out pitch pines, beech leaf disease is killing beech and these are only the most recent,” Lloyd said. “Forest canopy health and mass die-off at a rapid rate shifts the habitat of wildlife that depend on these trees.” Many of the affected trees are important in erosion control.

Although there is currently no quarantine, Lloyd said diseased trees should be cut and processed on site. Chipping is effective because once wood is chipped, it’s no longer in a closed system, the fungus cannot compete and beetles cannot complete their lifecycles. Pruning tools and saws don’t spread the disease.

On high-value specimen trees, preventive root flare injections with the active ingredient propiconazole are a potential solution. Injections should last over the growing season, but the treatment is not effective for trees that are already infected.

“It relies on evapotranspiration,” Lloyd said of the injection. “Trees pull up moisture and minerals from roots and can also move fungicide. We drill into the tree and put plastic quarter-inch tubing in. It hooks up to a spray tank that we can hand-pressurize, usually 10 to 15 psi. The tree does the rest of the work to move material into the tree.”

Since Lauraceae family trees are “gummy” when wounded, part of the injection process involves carefully wounding the tree. The gum makes it difficult for material to move in the tree, so the process takes longer. In most cases, injections are used for highly valued trees.

“There’s always hope for a cold snap,” said Lloyd. “It’s going to have to get significantly cold for multiple days to make an impact. This whole epidemic is potentially from one individual clonal beetle and its symbiotic fungus, so to eradicate them is unlikely.”

Smith said this disease has already had massive impact. “Evidence suggests the beetle and fungus may have come from Taiwan,” he said. “There are other ecological impacts and many other hosts. At least half a million redbay trees were killed in the early years of the disease. We know this disease has the potential to spread across the entire range of sassafras, except maybe very northern areas such as northern Michigan.”

by Sally Colby