Agricultural researchers are making tremendous strides in advancing technologies which combined could help save the U.S. citrus industry from its top nemesis and scourge – the deadly disease Huanglongbing (HLB), also called citrus greening.
Research achievements to manage or stop HLB are so significant, one Florida citrus leader says, that the state’s citrus industry, bedazzled by HLB destruction for more than a decade, is now “cautiously optimistic” about its future.
“We have made great headway and Florida growers are cautiously optimistic,” said Mike Sparks, executive vice president and chief executive officer with Florida Citrus Mutual, in a message to attendees at the 4th International Conference on HLB held in Orlando, Fla.
HLB is caused by the Liberibacter bacteria spread or vectored to citrus trees by the pencil-lead-sized Asian citrus psyllid. Once infected with HLB, it’s a potential death sentence for every tree…so far.
A global disease of citrus, HLB was first found in the U.S. in Florida – the nation’s top citrus-producing state – a decade ago which has since led to massive tree death and financial devastation for the state’s citrus industry.
Yet the future is looking brighter for Florida growers, and others in the nation’s citrus industry, thanks to the bevy of agricultural research advancements.
Besides Florida, HLB has been found in citrus in Texas. A single confirmed HLB infected tree was found in California in 2012 , the nation’s second-largest commercial citrus producer. Many expect the disease to rear its ugly head there again soon.
There are no confirmed cases in Arizona.
Glenn Wright, tree crop specialist with the University of Arizona (UA), attended the International Research Conference on HLB and shared related information during a UA-sponsored Citrus and Date Palm Seminar held in May at the Yuma Agricultural Center in Yuma.
Among the potential HLB cures Wright discussed was advanced research into transgenic citrus trees created through genetic engineering or GMOs. Perhaps the most advanced of these were developed by Erik Mirkov of Texas A&M University in cooperation with Southern Gardens Citrus (US Sugar Corp).
Mirkov first planted a transgenic tree in 2009 which included a gene from spinach. The gene encodes a protein with broad-spectrum antimicrobial activity which allows the tree to fight off bacterial and fungal plant pathogens.
The protein, called a defensin, binds to the HLB bacteria cell wall creating holes in it and kills the HLB.
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“This is a particularly effective defensin,” Wright said.
Today, Mirkov has produced six generations of transgenic trees; each an improvement over the predecessors.
Wright expressed concern over the transgenic method due to potential public outcry over the use of genetically modified organisms in citrus production. He shared results from a GMO study that shows 37 percent of the general public disagree with GMO use. Yet the same study revealed that 88 percent of scientists believe GMOs are safe.
“The regulatory hurdles for transgenic citrus could be high along with consumer resistance,” Wright said. “I believe GMOs are safe.”
Resistant, tolerant varieties
In other HLB-related developments, some citrus varieties show tolerance to HLB including US Early Pride, a low seeded, early-maturing mandarin hybrid developed by USDA. As this tree gets HLB, the yield is not reduced, contrary to many other varieties where yields fall dramatically from HLB.
Another tolerant tree variety is Seedless Snack where the tree can express HLB symptoms, leaf mottling for example, but it recovers, yields well, and does not die.
Other approaches include increasing the production of natural citrus defense proteins already in the plant.
Wright said, “The natural protein route could reduce regulatory hurdles and could be more acceptable to consumers.”
Resistance, tolerance in rootstocks
Researchers are also developing novel rootstocks using cell fusion called somatic hybridization and-or natural crossing to create rootstocks with multiple parents. Most HLB tolerant varieties produce poor yields or fruit quality when used as a rootstock.
However, when the parents are combined with one or more HLB susceptible rootstock varieties which normally produce good yields and fruit, the result can be a HLB tolerant rootstock which delivers good yields and sweet fruit.
Several of these rootstocks have been released to some Florida nurserymen.
A field-based process called thermotherapy involves treating trees with steam inside of a canopy wrapped around several trees at a time. HLB thermotherapy research began in China in the 1990s where budwood was exposed to 104 degrees F. for about three days.
Research conducted since then suggests a steam temperature of 113 degrees F. is needed for four days to reduce the disease titer level. Heat can decrease the disease titer for 6-18 months and reduce fruit drop which is beneficial to growers.
In Florida, some growers are applying thermotherapy in the field with a 136 degree steam temperature. The process can kill HLB in the tree except the roots. The process should be repeated every 2-3 years.
Wright said, “I think we live in an area of thermotherapy in the low deserts in Arizona and California. I believe thermotherapy is our number one defense against HLB.”
Nanotechnology uses the element zinc as a tool for disease control. A zinc-based antimicrobial compound Zinkicide has particles small enough to penetrate the leaf surface.
These particles are much smaller than zinc normally applied to trees through a spray. The tiny particles, about the size of a plant protein, target HLB bacteria. Field and greenhouse trials are underway.
Wright says nanotechnology could be a short-term answer to HLB. He is interested in studying the technology to possibly kill brown wood rot fungus in citrus.
Antibiotics are another short-term path, including the antibiotic Carbenicillin to kill bacteria in the plant. Researchers are developing spray and trunk drench applications.
“Antibiotics, nanotechnology (zinc), and steaming are areas where a many research dollars are currently being invested,” Wright said.
He questioned how the public might react if citrus juice contained an antibiotic.
Other discoveries, developments
Other recent developments include Florida growers using n-phuric or sulfuric acid to condition well water used for irrigation to reduce the bicarbonates in the water. Bicarbonates stress roots by reducing the root’s nutrient uptake.
HLB reduces an already stressed root system’s density. Root loss caused by HLB ranges from 30-50 percent. The roots become a reservoir for the bacteria.
HLB increases root growth yet reduces root longevity leading to less root density. The trees become more susceptible to Phytophthora disease and water stress which can lead to fruit drop. Acid use could create healthier plants.
Another finding is that citrus tree roots become infected with HLB before visible leaf symptoms occur; the opposite of what was previously thought.
Wright said, “It might be worthwhile to occasionally collect citrus roots from a suspect HLB tree to determine if the disease is in the tree.”
One control measure used by some Florida growers is a ‘nutritional cocktail’ of sorts for the tree, along with a vigorous psyllid control program including 20-25 sprays times per year.
“This is an expensive mix and not sustainable over the long term,” said Wright.
It has also been determined that the ACP can travel further than originally thought – up to 1.5 miles without wind assistance. That said, infected ACPs can travel further than un-infected psyllids.
Finally, there is a better understanding on the psyllid’s transmission of HLB, Wright says. The disease increases in new flush immediately after infection thereby infecting the next generation of psyllids.
Since a female psyllid can lay hundreds of eggs, billions of infected psyllids can be in an area months before visible HLB tree symptoms appear. It appears the trees fight hard before finally succumbing to the disease.
However, human-assisted or natural movement of even one of the many positive female psyllids can quickly spread the disease.