The words ‘elicitors, metabolomics, and proteomics’ may sound like a foreign language to many growers, but these names for new experimental detection methods – five all total – could help reduce the Western U.S. citrus industry’s deadliest threat, the disease Huanglongbing (HLB).

One of the greatest challenges facing the citrus industry is HLB (citrus greening) which kills every tree infected with a bacterium vectored by the Asian citrus psyllid (ACP) insect.

Determining whether a suspect tree has HLB takes a long time. The current test called polymerase chain reaction (PCR) requires 1-1.5 years from the actual infection to find out if the tree is HLB positive.

This long period can allow the psyllid to pick up the disease from an infected tree and transmit it to uninfected trees causing new infection.

The new detection tests currently under review could provide answers in as little as two to three months. This could help commercial citrus growers and homeowners detect the disease earlier, remove the trees if HLB positive, and reduce HLB transmission.

This could be a major victory against the spread of HLB.

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“We hope by late 2014 or in 2015 to have some of these experimental methods approved and in place,” said Joseph Morse, entomologist with the University of California, Riverside (UCR).

These methods were developed by University of California and other researchers funded by grower assessments through the California Citrus Research Board.

Morse and fellow UCR entomologist Mark Hoddle discussed the ACP-HLB challenge during the 2013 Fall Desert Crops Workshop held in El Centro, Calif. (Imperial County) in November. The workshop was sponsored by Western Farm Press.

So far, California has only one confirmed case of HLB, but has psyllids in 10 counties. The disease was found in a grafted hybrid citrus tree in March 2012 in the Hacienda Heights residential area located southeast of Los Angeles.

More HLB infected trees?

While only one case of HLB has been confirmed with the PCR test, Morse warned there could be 10 more HLB positive trees. He says the additional citrus trees in question are within a 700 meter radius of the HLB positive find.

“I am pretty convinced that HLB has spread from the area of the original infected tree,” Morse said.

Results from samples of the nearby 10 trees tested with three of the experimental test methods conclude the trees are HLB positive by at least one method, and in some cases by two or three methods. Meanwhile, state regulators are awaiting official PCR results.

Seven of the 10 trees have been removed. Morse says the other three should be cut down.

Citrus was just one of the topics discussed at the 24th Annual Fall Desert Crop Workshop, attended by growers, pest control advisors, consultants, and other industry members representing low-desert agriculture in California’s Imperial and Riverside counties and southwestern Arizona.

One-third of the California citrus crop is grown in Southern California. The balance is grown in the San Joaquin Valley. 

Morse explained that the largest populations of psyllids in California are in residential areas, mostly in the greater L.A. area. Some psyllids have been found in commercial groves over the last few years and are increasing, including finds this year in California’s top citrus-producing county, Tulare.

The adult psyllid not only flies but is wind blown into new areas.

While a variety of different insects can feed on citrus trees, the adult Asian citrus psyllid is easily distinguishable. Its body forms a 45-degree angle against the stem when feeding.

The first psyllid found in California was in the greater San Diego area in 2008. The pest was then found to the east in neighboring Imperial County.

Morse expressed concern for the desert citrus industry; not so much about the potential spread of HLB from L.A. south into the desert. He believes the disease will eventually move north from Mexico where it is spreading into the desert areas.

Prime time for sprays

Morse spent the balance of his time discussing chemical sprays for psyllid control, and the importance of using different modes of action to reduce possible pest resistance.

“We can’t control the psyllid forever with chemicals,” Morse said. “I think we can do a good job with insecticides for the next five to 10 years until the industry develops long-term solutions.”

The winter months are the prime time for psyllid sprays.

“Use the winter spray treatment as one of your big hammers to knock down psyllid numbers,” Morse said. “Based on experience from Florida, if you don’t control the psyllid effectively during the winter, you may never catch up during the summer.”

Insecticides for psyllid control include broad spectrum and more selective materials. The top broad spectrum insecticide chemistries for winter use against adult psyllids are pyrethroids, organophosphates, and carbamates.

Morse offers suggestions to help reduce pest resistance to insecticides:

  • Minimize pesticide use - treat only as needed.
  • Coordinate area wide treatments with nearby growers.
  • Rotate classes of pesticide chemistry - more persistent pesticides have a greater potential for resistance.
  • Know which class of chemistry each pesticide represents.
  • For pests other than the psyllid, consider a less effective chemical if it represents a different class of chemistry.

He encouraged citrus growers to participate in area wide sprays with neighboring growers with all sprays applied within a two-week period.

“It only takes a few people who don’t spray to ruin the entire program,” Morse said.

The entomologist says neonicotinoids are best used prior to the fall flush (growth) period, not the spring flush.

An imidacloprid soil treatment is very effective. One-half pound active ingredient is allowed annually. Morse says apply the full label rate in one treatment.

Commercial sponsors of Fall Desert Crops Workshop included: Platinum Sponsors – BASF and Bayer CropScience; Gold Sponsors – Dow AgroSciences, DuPont, and Westbridge Agricultural Products; Silver Sponsors – FMC, Ocean Agro, and Valent; and Bronze Sponsors – Syngenta and Oro Agri.

California citrus, a $2 billion industry annually, is mostly grown for the fresh market (80 percent). Oranges comprise about two-thirds of the statewide crop – Navels and Valencias (75 percent and 25 percent respectively). A third of the crop is exported. About 90 percent of the nation’s lemon crop is grown in California.

The Arizona citrus industry produces mainly lemons; a combined $60 million-plus industry. Arizona has the psyllid but no confirmation of HLB.

Biological edge

Entomologist Mark Hoddle discussed his efforts to thwart psyllid movement from urban areas into commercial groves through biological control.

Over the last several years, Hoddle and his wife Christina, also a UCR entomologist, jetted back and forth to the Punjab region on the Pakistan-India border to find parasitic wasps which could biologically fight the psyllid, a native to the Punjab area of Pakistan.

After extensive work, the Hoddle’s gained permission from the USDA to bring back several species of parasitic wasps from Punjab to Southern California for further study and possible rearing, including 2,500 Tamarixia radiata wasps to attack the psyllid.

The female wasp, about half the size of a chocolate sprinkle, lays its eggs inside the psyllid nymph. The wasp larva eats the nymph from the inside out killing it.

Hoddle detailed his journey travelling halfway around the world with several thousand wasps in tow. Security checkpoints at international airports were a huge challenge which delayed the entomologists.

Yet with the required paperwork in hand, the Hoddle’s, and the wasps, finally landed on California soil.

After 18 months of mandatory safety studies conducted on the wasps in quarantine at UCR, the husband-wife entomology team received the green light to release the wasps in the L.A. area. Mark says Tamarixia radiata is a good fit for Southern California since the climate is very similar to the Punjab region.

To date, about 170,000 wasps have been released at about 600 release sites in the L.A. area and areas to the East, plus in Imperial, Orange, Riverside, San Bernardino, and San Diego counties. Hoddle is pleased that about 15 percent of the sites now have established wasp populations.

Parasitism, in this case dead psyllids, is found 5-8 miles from the release sites. DNA collected in the field confirms the parasites are those bred from Punjab wasps.

A second psyllid parasitoid, Diaphorencyrtus aligarhensis, is in quarantine at UCR. The safety testing has been completed and the USDA is reviewing the environment assessment report. It is anticipated that this second natural enemy from Pakistan will be cleared for release in California in early 2014.

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