How to keep avocados from getting toasted

Berea Etherton, a University of Florida doctoral candidate in plant pathology, doing field work in an avocado orchard in Florida.
Berea Etherton, a University of Florida doctoral candidate in plant pathology, doing fieldwork in a Florida avocado orchard. Florida produces around 13% of the country’s avocados, where roughly 1,000 people are employed through approximately 350 growers, according to the UF Institute of Food and Agricultural Sciences. (Credit: Berea Etherton)

Avocado growers in California, Hawaii, and Mexico: Take heed. Florida has some lessons to share about laurel wilt disease, a pathogenic fungus that can kill avocado trees in less than a month.

While the laurel wilt epidemic in the U.S. has not currently spread west of Texas, scientists are concerned it will. With this in mind, a group of University of Florida researchers developed a model to better understand how the decisions made by avocado growers affect epidemic spread. Growers are challenged to protect their orchards from epidemic threats while balancing cost, effectiveness, and a threat’s severity.

Their work was recently published in the journal Agricultural Systems.

What is laurel wilt disease?

Laurel wilt disease was first reported in the U.S. in 2002, and in Florida in 2005. Twelve years later it had spread throughout the state. It is caused by a fungus, Harringtonia lauricola, which infects and kills trees in the laurel family such as red bay and avocado trees.

Ambrosia beetles bring the fungus with them as they burrow into the trees and create tunnels in the wood, called galleries, where they grow the fungus for food. The fungus, however, triggers a response from the tree that effectively chokes the tree rapidly by gumming up its vascular system. The effect is that the tree often dies from the canopy down.

Blooms on a healthy avocado tree.
Blooms on a healthy avocado tree. Avacodo farming in Florida is second in value only to citrus, earning $24 to $35 million in farm and wholesale revenues annually, according to the US Department of Agriculture. (Credit: Berea Etherton)

Modeling how decision-making affects disease spread

Plant pathologists affiliated with the UF Institute for Food and Agricultural Sciences, department of plant pathology, and the UF Emerging Pathogens Institute, wanted to help avocado growers by learning about the effects upon epidemics of decisions they make to manage against laurel wilt disease in their orchards.

“The issue we are trying to address is the lack of a silver bullet treatment that cures laurel wilt disease,” said Berea Etherton, a UF doctoral candidate in plant pathology and the study’s first author.

The main management tools growers have, she said, are to spot and remove diseased trees—a relatively lower-cost process called roguing—or to apply expensive preventative fungicide that deters ambrosia beetles from using the tree as a fungus farm.

A row of avocado trees affected by laurel wilt disease.
A row of avocado trees affected by laurel wilt disease on a Florida farm. The disease was first detected in Florida avocado farms in 2012. (Credit: Berea Etherton)

Etherton and her advisor, Karen Garrett, a UF preeminent professor of plant pathology, who was the study’s senior author, developed a model to evaluate the effects of management choice growers might select depending upon the phase of the epidemic and their information sources. The findings are relevant to other crop disease epidemics, Garrett said.

“The study results are relevant beyond laurel wilt, since it is a common problem to mobilize growers and policies for quick responses at the beginning of an epidemic when there is more opportunity to contain a disease,” said Garrett, who is also affiliated with the UF Global Food Systems Institute .

And it’s the early management decisions in an epidemic that can potentially stop or slow the spread of a contagion.

“We found that if an epidemic threat is not overwhelming, let’s say two or three growers have laurel wilt disease at the beginning of an epidemic, the cost of managing for laurel wilt may not be worth it to an individual grower when the threat is low,” Etherton said. “So at the beginning of these epidemics, when the cost of managing for laurel wilt outweighs the potential benefits, growers may choose not to manage for disease.”

But then these growers who did not invest in management tend to do worse once the epidemic pace quickens, she said.

Social connections influence decision making

UF plant pathologist Karen Garrett
University of Florida preeminent professor of plant pathology Karen Garrett, Ph.D., focuses on researching systems that support food security and conservation. (Tyler Jones
UF/IFAS Communications Photography)

The researchers also looked at how social connections between growers might influence their management choices. Their model assumed that growers were basing their decisions on talking informally to other growers, and not making decisions based on expert advice from extension agents or agricultural groups. They found  that:

  • The more socially connected a grower was during the early phase of an epidemic, the more likely they were to choose an inexpensive management option which may harm regional health over time.
  • Growers that adopted costlier management methods early in an epidemic helped protect regional health over time but they incurred higher costs.
  • Incentives to get growers to use costlier management methods earlier were less effective at protecting regional health than punitive measures.

A road map of lessons learned

The work could help influence agricultural organizations in areas that do not yet have laurel wilt disease, according to Etherton and Garrett. Specifically, they could develop plans to help defray the costs of expensive management choices early in an epidemic to help growers contribute to protecting regional health, before it is too late for effective interventions or avocado orchards are lost.

By: DeLene Beeland