Managing laurel wilt disease in Florida

Jan 01, 18 Managing laurel wilt disease in Florida

By: FFVA

Although laurel wilt disease continues to fell avocado trees and their wild relatives in the Southeast, a number of Florida growers who have adopted aggressive management programs have reduced avocado tree mortality.

The programs, which involve frequent scouting for diseased trees, removing and grinding them as soon as they’re found, and treating adjacent healthy trees with antibiotics, is admittedly a stop-gap measure, say University of Florida researchers.

“We’ve found that the best way is to make sure groves are as fit as possible,” said Mary Oslund, director of marketing for Homestead, Florida-based Brooks Tropicals. “But it’s a big challenge and a challenge that definitely right now we have no answer for. It’s an on-going challenge that we have to face.”

For the state’s avocado industry to survive and flourish in the long run, Drs. Jonathan Crane and Randy Ploetz agreed that breeding resistant varieties would provide the most sustainable outcome.

The two are part of a multi-discipline group of researchers at UF’s Tropical Research and Education Center in Homestead studying the laurel wilt-ambrosia beetle complex. The work is being funded in part by a $3.45 million U.S. Department of Agriculture National Institute of Food and Agriculture grant.

Hurricane Irma throws a curve ball

Unfortunately, only a handful of growers have embraced the aggressive disease-management program, said Crane, a UF Institute of Food and Agriculture Extension professor of horticulture and tropical fruit specialist.

Others have given up because the disease has gotten out of hand and they can’t afford control measures.

“It’s a very, very fluid situation, and of course Hurricane Irma has complicated everything,” he said. Irma toppled 30 to 40 percent of the state’s avocado trees. Historically, growers would heavily prune the trees, stand them back up and they’d regrow in a couple years.

But Crane said the storm damage also gives growers an opportunity to top work the trees to more profitable varieties or remove the grove entirely and replant with different avocado varieties. He said he should know better what options growers pursued by the 2018 summer.

Going back into avocados isn’t as risky as it sounds, either, because of ambrosia beetle habits researchers have learned about during orchard trapping.

It seems the pests, which spread laurel wilt, prefer large canopied, shady orchards where they can easily fly between trees. Ambrosia beetles also prefer mature trees with large diameter trunks compared to the 3-inch trunks of young trees.

“Most of the groves with between 25- and 40-year-old trees are perfect habitat for these beetles to move freely about,” Crane said. “We’ve noticed through trapping information that these beetles are less active in groves that have been top worked or in replanted areas because of more sunlight.”

The laurel wilt disease itself also can spread from tree to tree through root grafting, where large roots of old trees fuse together and share bark, phloem, cambium and xylem.

An unwelcome intruder

The redbay ambrosia beetle – native to India, Japan, Myanmar and Taiwan – was first detected in 2002 in southeast Georgia and presumably hitchhiked into the country on wood crates and pallets.

Since the beetle was first discovered in South Florida in 2010, laurel wilt has killed about 28,000 avocado trees. “That’s a lot of trees, but 4 percent (of overall production) puts it into perspective,” Crane said.

The beetle itself is not the main concern among avocado growers – it is the laurel wilt fungus the beetle transmits that causes more worries. Although the redbay ambrosia beetle was first implicated in spreading the laurel wilt disease, researchers have since identified at least nine other native ambrosia beetles capable of carrying the pathogen. The redbay ambrosia beetle is unique in a lot of ways, adding to management challenges. Unlike other most native ambrosia beetles that colonize dead and dying trees, the redbay ambrosia beetle also seeks out healthy trees.

Bark beetles typically colonize tree phloem, the vascular tissue responsible for transporting sugars created through photosynthesis and other nutrients to flowers and roots. But ambrosia beetles have evolved to colonize the xylem – plant tissue that carries water and nutrients from the roots upward through the trunk and branches.

The laurel wilt fungus, Raffaelea lauricola, and the redbay ambrosia beetle also have a symbiotic relationship – each organism relies on the other. Adult females, about 0.07 inches long, bore into the wood just below the bark and construct galleries, or tunnels. At the same time, they act as kind of fungal farmers, inoculating the galleries with laurel wilt spores. It is the fungus, and not the wood, on which adult beetles and larvae feed.

As the fungus grows in the galleries and adjacent sapwood, it prompts defensive reactions within the trees that disrupt water and nutrient flows. Visible wilt symptoms appear within a few weeks of infection.

Most of the redbay ambrosia beetle’s life cycle, including mating, egg laying and larval development, is completed within the galleries. As a result, the beetles are protected from most insecticides.

The dying tree also attracts a host of other ambrosia beetles, which will colonize it, further spreading the laurel wilt fungus.

In addition to avocados, laurel is lethal to many other members of the laurel family, including redbay and sassafras. In fact, UF research has found that laurel wilt has killed about 320 million redbay trees alone – or 30 percent of the species’ population – since its introduction in 2002. The disease has spread throughout Florida, north through Virginia and west into Texas.evans-crane-uf

Remove infected trees ASAP

Because the pathogen spreads quickly within infected trees, Crane said trying to prune ahead of the infection usually is ineffective. Instead, he and Ploetz recommend frequent scouting and removing and chipping the tree, including the roots, as soon as wilt symptoms are visible.

“These guys want to save as many trees as possible, and they tend to look at trees and wait,” said Ploetz , a professor of plant pathology. “But that can be disastrous with this disease. We’re having a tough time convincing most growers to really take this seriously. “There are some growers who have taken our recommendations seriously and that’s to remove trees as soon as you see the first symptom. It’s so, so important. If you wait and let that tree become systemically infected, the disease will move very rapidly. Vascular wilt diseases are so, so difficult to manage.”

At the same time as tree removal, they recommend treating surrounding trees with antibiotics to prevent infection. Because of the way the antibiotics work, they are preventive and can’t cure an already infected tree.

Ploetz spent time during the early years of laurel wilt testing fungicides and found triazoles to be effective. Currently, propiconazole, sold under the brand name Tilt, is available under a Section 18 emergency use permit. Tebuconazole, which worked better in trials, is a bit further behind in registration and is not yet approved for use. The challenge with fungicides is getting enough volume into the tree to be effective, Crane and Ploetz said.

Some growers also have found that the beneficial fungi, Beauveria bassiana and marketed as BotaniGard, applied at and after tree removal helps control beetles dispersed during the process. The goal is to make repeated applications so the beneficial fungi can build up to reproductive populations within the grove.

None of the treatments discovered so far are sustainable in the long run, Crane and Ploetz said. As with most plant diseases, the long-term answer is to develop tolerant or resistant varieties. “I’m a big fan of (breeding for) resistance, but the issue with avocados is we have some varieties that have lower susceptibility but no good resistance,” Ploetz said.

In search of resistance

U.S. Department of Agriculture researchers at Miami and Fort Pierce are working to characterize the avocado genome and possibly identify where different traits – including disease resistance – are located. But Ploetz described the work as a long-term project, and many growers can’t wait.

He and Crane said they’re hopeful a shorter-term effort can provide growers at least some interim relief.

Most Florida avocadoes are the West Indian race, which are the most susceptible to laurel wilt because the xylem vessels are large diameter and allow the pathogen to spread quickly throughout the tree. Mexican and Guatemalan races tend to have smaller xylem vessels.

“One of the things we’ve started looking at are different rootstocks and scions to see if we can change that xylem architecture,” Ploetz said. “That would be a much more rapid way – to plant trees that are really, really tough.”

A nursery in California is currently propagating grafted trees, and he said they expect delivery in early to mid-2018. Researchers can then begin to inoculate the trees in the laboratory to measure disease interaction.

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