The Western citrus industry is drawing upon research and the lessons learned in Florida, Texas, and foreign countries to better prepare against the threats of the Asian citrus psyllid (ACP) pest and citrus greening disease.
California's citrus industry remains on high alert after the ACP plant-sucking, aphid-like insect was detected in San Diego County last August and several months later in Imperial County. The California Department of Food and Agriculture (CDFA) imposed a quarantine which mandates washing fruit before it leaves a quarantine area.
First found in Florida in 1998, the psyllid is the primary carrier of the incurable Huanglongbing (HLB) disease or citrus greening. HLB was confirmed in the sunshine state in 2005 and is now found in all 32 citrus-producing counties. Over 60,000 citrus trees have been removed statewide. HLB was found recently in Louisiana.
HLB is a bacterial disease that infects the tree's vascular system. It renders the fruit inedible and eventually kills the tree.
Where ACP is typically found, HLB generally follows. There's no confirmation of HLB in California. Arizona has no ACP or HLB confirmations. The psyllid is in Texas, Louisiana, and Mexico. HLB has not been confirmed in Texas and Mexico.
The Western citrus industry is working fervently to prevent a potential citrus industry fiasco. The U.S. Department of Agriculture pledged $5.8 million last November to help halt psyllid spread in California.
About 80 percent of California's $1.3 billion commercial citrus industry is located in the San Joaquin Valley with additional citrus grown in Southern California from Ventura to Imperial and Riverside counties. Arizona's $63 million citrus industry is found primarily in Yuma and Maricopa counties.
“If the psyllid is not in Arizona now, it soon will be,” said University of Arizona Citrus Extension Specialist Glenn Wright. He shared the latest ACP and HLB developments with growers during citrus workshops in Phoenix and Yuma, Ariz.
Wright said, “We'll probably find the psyllid in Arizona this spring because it loves new citrus flush growth.” He predicts the Yuma area is where the first psyllid will be found. Wright is based at the Yuma Agricultural Center in Yuma.
“There's almost no doubt in my mind that the psyllid can survive in the extreme low desert heat,” Wright said. “The insect has survived high heat conditions in citrus groves near the city of Hermosillo in the Mexican state of Sonora.” The psyllid survives in 117 degree temperatures in central Pakistan.
In cooler weather, preliminary Florida research suggests that temperatures in the 23 to 24 degree range for seven hours can kill 100 percent of psyllid eggs.
Other psyllid research suggests that more nymphs contract HLB than adults. After 21 days of feeding on infested flush, 56 percent of the nymphs had HLB compared to 17 percent for adults. Only 1 percent of the adults landing on infested flush contracted HLB. About 70 percent of the adults which grew from the egg stage to nymph to adult on the same flush became HLB positive.
“It's best to keep the insect from growing and maturing on the same piece of flush,” Wright said.
The psyllid's stylet sucks phloem from the plant. The insect's digestive juices then distribute the bacteria in the leaf. The psyllid prefers feasting on young flush since the stylet penetration is easier. Feeding on the leaf's underside is preferred.
In trapping efforts about three-quarters of psyllids are caught during the day. Research shows higher psyllid catches in new citrus flush-colored sticky traps compared to traditional yellow traps.
Spraying groves with insecticides can reduce psyllid numbers. Psyllids rest on the leaves at night. Sprays should target overwintering adults before new flush develops to reduce the nymph populations, Wright said. Several generations can mature each year.
Repellents are one method to keep trees off-limits to the psyllid. A report concluded that Vietnamese citrus growers who interplanted guava between citrus rows didn't find psyllids. Damaged guava leaves release a strong repellent called dimethyl disulfide that wards off insects. The ingredient, also found in leeks, could be commercialized as a spray in the future.
The parasitic wasp Tamarixia has reduced some Florida psyllid populations by up to 50 percent.
Meanwhile scientists have almost completed mapping the psyllid's genome. “Knowing the genes mean determining the insect's Achilles heel and opens the door to developing ways to block the insect's proteins and enzymes resulting in death,” Wright said.
Beyond the U.S. HLB disease is found in citrus groves in China, the world's largest citrus producer, plus in Cuba, India, Pakistan, the Far East, and Brazil.
U.S. Sugar, a Florida company with 16,000 acres of citrus, has lab tested almost 65,000 samples for HLB. Results suggest that trees six to nine years old are more susceptible. Symptoms are more visible in cooler weather.
ACP-HLB-related scouting, psyllid control, and tree removal add an estimated 40 percent to citrus production costs in Florida, Wright says.
While officials have supported tree removal and scouting for disease control, some growers are thinking twice. Some growers believe tree removal financially pushes them out of business faster than the disease taking its natural course. Trials utilizing minerals, nutrients, and a natural form of aspirin are showing surprising results in slowing HLB.
Are all citrus species equally sensitive to HLB? In a Florida greenhouse study, sweet orange, Minneola, ruby red grapefruit, and Nules Clementine were the most sensitive. Lemons were very tolerant.
“That might be good news for lemon growers,” Wright said. “For the orange grower located next to lemons, that's not a good situation because the pathogens in the lemons might still move to the orange trees.”
One way to combat HLB is chemotherapy, according to a Taiwanese study. Meanwhile, Wright says heat in the low desert provides a version of thermotherapy that could possibly reduce the disease's titer level.
Wright calls genetically-engineered citrus the “ultimate control for HLB.”
“Hundreds of segregated, genetically-modified citrus plants challenged by HLB are growing in greenhouses around the world and surviving very well,” Wright said. “This issue opens up regulatory concerns from the Food and Drug Administration, Environmental Protection Agency, and USDA.”
Another disease answer is developing resistant rootstock. “In the next 10 to 15 years a selection of trees could be available that are genetically HLB resistant,” Wright said.
Wright urges Arizona and California growers to monitor groves for psyllids. The University of Florida Extension Service recommends the following method:
place a sheet of paper 1 foot below a branch;
tap the branch by hand or PVC tube;
count the insects - both beneficials and pests;
be sure to save the results on a computer spreadsheet.
Any psyllid finds should be promptly reported to agriculture authorities.
The CDFA, University of California, the California Citrus Research Board (CRB), and other groups offer online resources on ACP and HLB. The CRB recently ramped up ACP detection efforts by hiring former CDFA Plant Pathologist MaryLou Polek to manage a new operations department.