Avocado Trees in Peril

Tom Siddons, the owner of Sleepy Lizard Avocado Farm, prunes some of the plants on his farm. SLEEPY LIZARD AVOCADO FARM/ COURTESY PHOTO

Florida is a top producer of avocados, but agriculturalists fear the spreads of a 20-year-old disease called laurel wilt will continuecontinu to negatively impact the fruits’fru harvest in commercial cial industryin areas throughout the ststate.

DueD to the intricacy of combating bati the invasive redbay ambrosia bro beetles that carry the fungus,fu avocado groves and othero flora continue to decline, falling victim to laurel wilt despite the ongoing development of solutions.

“This insect disease is complex,” said Jonathan Crane, University of Florida’s Associated Center director and fruit crop specialist.

The fruit and laurel wilt

Avocados are fruits that originated in Mexico and South and Central America. Avocado trees were first planted in Florida in 1833, followed by California in 1856. Florida is the second biggest producer of avocados in the United States, accounting for the majority of production, with California being first and Hawaii being third. Florida has more than 56 varieties of avocados.

An employee at the Sleepy Lizard Avocado Farm in Miami-Dade County gets up high to pick the fruit off the trees. SLEEPY LIZARD AVOCADO FARM/COURTESY PHOTOS

According to the National Agricultural Statistics Service per state provided by the United States Department of Agriculture, “In Florida, most early season varieties of avocados are West Indian types, whereas midseason and late varieties are mostly Guatemalan-West Indian hybrids or Guatemalan types. Florida avocados have a lower fat content than those from other states and countries, are typically larger than avocados from California, and are available from June through the end of February.”

Laurel wilt, Raffaelea lauricola, is a fungal symbiont that was introduced by the invasive beetle, Xyleborus glabratus. The beetle was first discovered in the United States in 2002 and found in Florida by 2005. Multiple different species of ambrosia beetles can transmit the disease, which targets and affects trees of Lauraceae plants (Laurel family), such as avocado trees.

“(The beetles) were introduced into Port Wentworth, Georgia, in infested wood packing material from Asia during 2002,” said Crane. “It spreads through natural areas by redbay ambrosia beetle movement and anthropogenic movement of infested wood products.”

Sleepy Lizard Avocado Farm owner Tom Siddons gets some new plants ready.

Such products include firewood, wood-turners and barbeque smokewood.

“It is believed (that) while going through customs, products within the packaging were inspected but not the packaging pallets themselves,” said Tom Siddons, owner of Sleepy Lizard Avocado Farm in Homestead. “Laurel wilt has caused the loss of over half a billion trees in nature.”

Beetles are wood-degrading insects that were once considered beneficial for accelerating the decay of dead trees and the natural cycle of healthy forests. However, laurel wilt is now considered a highrisk invasive disease equal to Dutch elm disease or chestnut blight.

“Laurel wilt has impacted me,” said Siddons. “It was really heartbreaking during the first loss of our trees. I have been in the industry for 12 years and 13 seasons, owning about five acres of groves and running an additional 21 acres that we lease out and experienced the damage and devastation from laurel wilt.”

Florida’s avocado groves continue to decline due to the disease called laurel wilt. COURTESY PHOTO

Sleepy Lizard Avocado Farm grows Florida avocados, also known as the green avocados, which are grown in the Everglades and Homestead, Fla.

After spending a year in the tree, the fruit matures and hits the market in November. The local farm sells commercially, with multiple 12,000-pound packages of perishable fruit shipped, daily online sales, sales with retailers and fruit stands.

“We lose about seven to 10 percent of trees every year,” Siddons said. “It is a challenge in the industry especially with the market of avocados growing in popularity. The average age for commercially producing avocado trees was 55 to 80 years old and now they are dying as early as 15 years old. It is a gambling game now; farmers used to depend on years of tree production that is now being cut off early.”

“We are now selling to break even and losing profit,” Siddons added. “We are often replacing the plant with other tropical fruits.”

Large populations of the redbay ambrosia beetle commonly inhabit avocado groves. Other prone trees include the redbay, swamp bay and sassafras, all of the laurel family.

“The beetles are extremely small, and you cannot see them with your bare eyes,” said Siddons.

The invasive beetle is said to be about two millimeters in length.

“By February 2010, redbay ambrosia beetles were detected in a natural area, 21 miles north of the south Florida avocado production area (125 square miles) in Miami-Dade County,” said Crane. “In 2011, the first confirmed swamp bay tree to succumb to laurel wilt was documented in this natural area, and by 2012, laurel wilt was detected in a commercial avocado grove in Homestead, Florida.”

Avocado farmers are prominent in southern and tropical climate regions of Florida, with findings of the beetles now in all 67 counties in the state.

“There are two variations factors: the beetle and the fungus. Laurel wilt is a disease caused by an invasive fungus carried by an invasive beetle,” explained Siddons. “The beetle is flying around with the fungus around its neck, and it burrows into your tree and deposits the fungus. That fungus infects your tree.”

“Avocado groves are planted in rows, we dig outlines of trenches and then fill it with dirt with the planted tree,” Siddons continued. “Over the decades, the trees within the trenches form root graphs. Roots grow together and intertwine. So, one row of trees is actually one big living organism sharing a common root system beneath the ground. The problem with laurel wilt is all you need is one tree to get infected, the disease goes down into the roots and travels from tree to tree.”

The impact

The Avocado Administrative Committee stated, “Hundreds of thousands of commercial avocado trees have died due to laurel wilt since 2012 when it was detected in the commercial avocado production area (in Miami-Dade County.)”

“The laurel wilt-ambrosia beetle continues to threaten Florida’s avocado industry despite 14 years of research to find an entirely effective control program,” said Crane. “To date, the loss of over 140,000+ commercial avocado trees can be attributed to laurel wilt, worth an estimated $46.2 million. This is because little to nothing was known about the fungal phytopathogenic symbiont.”

Jeff Wasielewski, a commercial tropical fruit extension agent with the University of Florida, said, “A battle is being waged in the avocado groves of South Florida where the region’s largest and most economically important fruit crop is under attack. Avocados account for approximately 4,200 acres in Miami-Dade County and have an economic impact of $54 million on the regional economy. The avocado industry has already lost over 300,000 trees due to laurel wilt since the introduction of the disease to Miami Dade County.”

Root grafts spread of laurel wilt move at a rate of three to six new trees per month, causing the loss of over 90 trees within a six-month period. Visible external symptoms of laurel wilt in avocado trees include green-leaf wilting and turning brown and the sapwood developing black-blue stained streaks. As the infection progresses, the plant leaves turn black and continue to die off throughout the groves.

“It will look like sawdust, have toothpick looking hairs sprouting out and will you see large missing spots of dying trees,” said Siddons. “My wife and I walk around our groves daily and look for symptoms of laurel wilt.”

The beetles are known to have a short flight time frame. Their activity heightens one hour before sunset. The beetles prefer shady spots and are more vigorous during late winter and early spring, consequently the peak of the avocado season.

The studies found that having higher levels of light reduced the infestation of laurel wilt in avocado groves.

According to the University of Florida Institute of Food and Agricultural Sciences, “Areas where trees have been recently stumped (trees cut to 4-foot) and in recently top-worked areas of the grove compared to highly shaded avocado orchards with dense canopies. Implementing or reestablishing a pruning program to enhance light penetration within grove trees is recommended to reduce redbay ambrosia beetles’ activity and new laurel wilt outbreaks. Alternatively, rejuvenating or top-working large trees to reestablish high light conditions in the grove may be an option.”

The beetles primarily colonize stressed trees. However, the fungal organism harboring laurel wilt hosted in root-grafted avocado trees in groves rapidly grows in the well-protected insides of the tree. Laurel wilt is highly virulent, and trees with as few as 36 spores can incubate the disease, blocking off the water-conducting tissues of the tree and impacting the tree’s defense mechanisms.

Growers began stumping or cutting back infected avocado trees. Still, they found the plant would regrow and eventually die off again until the plant was officially deceased, with only a few cases of the plant growing disease-free with fruit production after three to four years.

“If you get to it early, you might be able to save the tree,” Siddons added. “Two methods I have used are cutting and grinding the tree down to the trunk and letting it regrow and another is cutting off the infected areas. Some trees have flowered and produced crops by doing this. However, that doesn’t always work, resulting in losing 12 to 15 trees within a trench.”

The research for solutions

The University of Florida Tropical Research and Education Center conducted experiments with different variations of avocado trees. They found that diverse genetics reacted differently to the infection of laurel wilt and symptoms correlated with plant size and age.

According to the experiments, “Larger plants declined more rapidly to infection by the laurel wilt pathogen than smaller plants.”

“I have noticed that younger trees are not at risk, and I cut our avocado trees to stand about six feet tall,” Siddons agreed.

“In commercial groves, mature trees of 32 avocado cultivars of various genetic backgrounds have been documented to succumb to laurel wilt,” UFIFAS studies affirmed. “No mature avocado trees of any cultivar have been discovered to be tolerant to the laurel wilt pathogen so far.”

Findings and recent research discovered that prophylactic systemic propiconazole (Tilt) fungicide infusion protected avocado trees from laurel wilt for 12 to 18 months but required reapplications that could harm the plant and its fruit. The infusions were costly, making economic control tactics difficult.

“These treatments are expensive, and some require monthly applications,” Siddons said. “These chemicals can get into the fruit, which isn’t good. Researchers are looking at how to stop the disease scientifically and farmers are taking our own hands-on actions in trying to fight back and find solutions.”

Other rheostat systems tested included insecticides for the beetles. However, this method was only effective for redbay ambrosia beetles outside of the plant and most of the infected beetles spend the life cycle within the tree.

Both Siddons and Wasielewski stated that cutting and burning tree wood helps prevent the spread of laurel wilt, as trees are used as breeding sites for the beetle.

“Keep groves pruned,” said Wasielewski. “Ambrosia beetles prefer shady areas and are less likely to go to groves that are well-pruned.”

The Florida Department of Agriculture and Consumer Services suggested scouting groves weekly for laurel wilt infestation, using local wood, not moving unprocessed wood and sourcing from registered nurseries throughout Florida. Florida has approximately 10,000 bonded nurseries and stock dealers.

“Don’t transport avocado trees or any other trees in the Laurel family unless you purchased them from a registered nursery,” said the FDACS. “Dealers in agricultural products, including nursery plants, must be licensed and bonded. All plant nurseries and nursery stock dealers are required to register annually. If you are producing or growing plant material for sale or distribution, you must register as a nursery.”

According to the Avocado Administrative Committee, the estimated final production totals for Florida in the 2023 to 2024 season will be 21,235 metric tons, a decrease of approximately 6.6 percent compared to the 2022 to 2023 harvest of 22,743 metric tons.

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