An avocado tree shows signs of laurel wilt disease. Photo courtesy of the University of Florida
ORLANDO, Fla., March 5 (UPI) -- Florida's avocado industry is bouncing back from hurricane damage, but still struggles to find new, effective weapons in a war against laurel wilt fungal disease.
The state's industry hopes for a crop of about 900,000 bushels this year, up from 800,000 bushels last year, mostly thanks to recovery from Hurricane Irma damage in 2017.
Florida is known for the large, green-skinned avocado, which is produced commercially only in the southern part of the state around Miami. Also known as the West Indies avocado, the Florida variety is larger than the darker-skinned Hass avocado that comes primarily from California or Mexico.
With the damage mostly behind them, avocado growers still wage warfare against the disease, which first appeared in South Florida's groves in 2011. Tree loss is now estimated at over $42 million, with lost avocado sales well over $4 million, according to the University of Florida.
"Pathologists are working on a vaccine to inoculate against the fungus," said Jonathan Crane, a horticulture scientist with the University of Florida.
"We just got the funding for that," Crane said. "We're at the very early stages. Developing a vaccine for something like this is very difficult. We hope to have results of current tests in about three years."
He said additional research is focused on finding varieties of avocado trees that are resistant to the disease.
After the disease wiped out large tracts of older, more vulnerable avocado trees, the spread of laurel wilt isn't as bad now, but trees still succumb to it, Crane said.
"Even when we've gone through and removed all the laurel wilt trees, in a few weeks we're going to see another one," said Medora Krome, a third-generation grower near Homestead, south of Miami. "There are very few groves where you don't see dead trees from this now."
Laurel wilt has killed half a billion laurel trees, which include avocados, on farms and in woods in the southeastern United States over the last 18 years.
The best defense once a tree is infected is to cut it down, grind it into chips and apply chemicals to kill larvae of invasive redbay ambrosia beetles that spread the disease.
Avocado trees aren't the only species of laurel that are attacked. The disease also kills redbay laurel, which is common in many wooded areas, and several other types of laurel -- but not every type.
The industry also is struggling to contain the spread of the disease when people carry infested firewood or even furniture made with redbay laurel trees into new regions, allowing hidden larvae to escape and begin spreading.
Krome remembers the day in 2011 that field inspectors told her they'd spotted the disease in a single tree amid groves first planted by her grandfather, William Krome, an engineer who helped build Florida East Coast Railway.
"I called my grove manager and said nobody goes home at 4 p.m.," Krome said. "We went out there with chainsaws. I cut it down that day."
At the same time, demand for avocados -- most recently for avocado spread -- caused prices to double over the past year. The current price is just over $3 each, according to the most recent information from the USDA based on market prices in Florida.
The trees can be saved by injecting them with a fungicide. But it is expensive and time-consuming and only lasts a year or two, said Jonathan Crane, a horticulture scientist at the University of Florida. Many growers, especially those who produce organic avocados, are reluctant to go that route.
"It showed up in South Florida much faster than we thought after it was first detected in Georgia in 2002 and then in Jacksonville," Crane said. "We thought it would take 20 years, but it only took a few years."
Research has shown that the beetles like bigger, older tree trunks in shaded areas that are stressed by drought or poor soil. That allows them to bore holes into the trunk, where they cultivate the Raffaelea lauricola fungus inside the tree to feed their larvae.
Older trees also pass the fungus to other trees because their roots often grow together, Crane said.
In response to this research, best management practices to contain the disease now include keeping trees healthy with proper irrigation and fertilizer, and cutting them off at the stump to be grafted with new cuttings to grow newer, healthier treetops.
"We learned that the large groves of older trees were most vulnerable," Crane said.
So-called dooryard avocado trees -- individual trees growing on someone's private property -- generally are not as vulnerable because they are usually smaller, isolated and grown in bright sunlight, Crane said.
The new understanding of the disease means growers have some hope, said Alan Flinn, manager of the Florida Avocado Administrative Committee. The state-authorized trade group includes growers and packers to promote Florida avocados and set quality standards for the industry.
"Many of the acres lost weren't properly managed and fertilized and topped. If you manage it correctly, you can still be a successful grower," Flinn said.
For now, optimism reigns.
"We've got a problem with laurel wilt, so what are we doing?" grower Krome asked rhetorically. "We're replanting. We are getting grants to replace and to remove the sick trees. And we trust the biologists are working on a better solution."