The United States remains one of the world’s top citrus-producing nations, trailing only Brazil and China in annual production. However, total production has fallen more than 50 percent since it hit record levels in the mid-1990s, and the primary reason for that decline is the severe damage that the Florida citrus industry has suffered in the last few decades from a bacterial disease known as citrus greening. In fact, all the major citrus-producing countries have been hit by this disease.
U.S. utilizable citrus production peaked at 17.3 million tons in the 1996/97 growing season. In that year, Florida accounted for 76 percent of the crop, followed by California at 21 percent, Texas at 2 percent, and Arizona at 1 percent. As of the 2018/19 crop year, Florida’s share had fallen to 44 percent of the total U.S. citrus crop, which was just under 8 million tons. It is estimated that the disease causes more than $500 million worth of damage to Florida agriculture every year.
Citrus greening is thought to have originated in China about 100 years ago, where it is known as yellow dragon disease (or Huang Long Bing (HLB) in Chinese). It is transmitted by two species of psyllid insects (small winged insects also known as flying plant lice). The Asian citrus psyllid is one of the carriers, and was first detected in Florida in 1998. The Asian strain of the bacteria, Candidatus Liberibacter asiaticus, was first found in Florida citrus groves in early September of 2005. That same strain of bacteria was also detected in Texas for the first time late in 2013. It was also detected in Arizona in 2015, and in Louisiana and Nevada in 2017. It has been present in a few counties in California since 2014, but has not yet affected commercial citrus production there.
Citrus greening is not harmful to humans but is devastating to citrus trees. The psyllid insect feeds on the stems and leaves of the trees, infecting the trees with the bacteria that causes citrus greening. Greening impairs the tree’s ability to take in nourishment, ultimately resulting in fewer and smaller fruit over time. As the disease progresses, the fruit the infected trees still do produce are smaller and more sour-tasting, making them unacceptable to citrus processors. It affects all known species of citrus trees, including oranges, grapefruits, lemons, limes, and clementines.
To date, no effective treatment has been developed for this disease once a tree is infected. At first, citrus farmers diligently sought to identify infected trees in their operations and destroy them as soon as detected, similar to a livestock producer who culls diseased animals to keep them from infecting the rest of their herds or flocks. However, they could not keep up with the rate of spread--it is now estimated that up to 90 percent of citrus trees in Florida are already infected.
Some Florida citrus farmers have tried to spray their trees with antibiotics such as streptomycin to slow the disease spread in their groves, but the practice is not reaching the infected roots of citrus trees in many instances. In addition, some public health scientists fear that this practice might contribute to the development of antibiotic resistance among humans.
There are also concerted efforts to control the pests that spread the disease. Chemical pesticides have been used to try to reduce the psyllid population that carries the greening bacteria, but unfettered use of this practice increases the risks that these insects may develop resistance as well. Florida citrus farmers now have access to biological control techniques as well, with a tiny wasp known as Tamarixia radiata, a known predator of Asian psyllids, being made available to the farmers free of charge through the state’s Department of Agriculture. This practice was first utilized in India in the late 1970’s.
Research is underway at universities in Florida and California to develop root-stock for citrus species that are more resistant to the disease. Some progress is being made on this front, but it could cost farmers hundreds of thousands of dollars to destroy their existing trees and purchase and plant new HLB-tolerant trees on their operations. In November 2019, the University of Florida offered the state’s citrus farmers to taste the fruit from the HLB-tolerant citrus trees they have been developing over the last several years. Research to develop truly resistant strains through use of genetic engineering or gene editing is also being pursued at the Universities of Florida, Connecticut, and California-Riverside. Prior to 2019, research protocol barred most researchers in California from working on live HLB bacteria because the disease was not yet present in the state’s commercial citrus sector. With the opening of a new BSL-3 lab near the U-C Riverside campus with tight biosecurity earlier this year, that constraint has been lifted.
The federal government established an inter-agency task force in 2014 to coordinate activities addressing citrus greening between various USDA agencies, EPA, and state departments of agriculture in the affected states. Federal funding for research on this topic has been provided through the Specialty Crop Research Initiative in the farm bill, about $25 million annually.