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Citrus Killer: Battling Back Against HLB

BACKGROUND INFORMATION

Disease background

Huanglongbing (HLB) is a disease that is fatal to citrus trees (Figure 1). HLB, caused by a Gram-negative genus of bacteria called ‘Candidatus Liberibacter’, destroys the productivity, appearance and economic value of citrus trees. Diseased trees produce bitter, hard, misshapen fruit and become unproductive within a few years of being infected. HLB is one of the most economically damaging fruit crop diseases in the world, and currently there is no cure.


Figure 1.

Symptoms and Signs

HLB is difficult to detect because symptoms may not show up for more than a year after the tree has become infected, depending on the age of the tree when infected. In addition, some of the symptoms (such as yellow leaves) look almost identical to symptoms produced by certain nutritional deficiencies. For example, zinc deficiency results in flushes of mottle yellow leaves with prominent green veins. One distinction, however, is that almost all trees in a grove will show the problem if it is caused by nutritional deficiency, whereas for HLB only some trees will show symptoms. In addition, leaves of HLB-affected trees show a symptom called “blotchy mottle” that is the most characteristic symptom of the disease. Early HLB-affected symptoms can also appear as one to several yellow-leaved shoots in a tree on which the other branches still have normal green leaves. Eventually, the yellowing will spread from these initial branches throughout the tree. This yellow shoot symptom during early stages of symptom appearance is a very characteristic symptom of HLB.

Fruit symptoms include lopsided, small fruit, and premature and excessive fruit drop. At the time when the fruit normally changes color from green to yellow or orange, HLB-affected fruit can show color inversion: the peduncular (stem) end of the fruit turns yellow while the stylar (blossom) end is still green. On normal fruit, in contrast, coloration during ripening progresses from the blossom end towards the stem end.


HLB Infection

This disease is caused by a the genus of Gram-negative bacteria called ‘Candidatus Liberibacter’. The Candidatus indicates that no one has been able to culture the bacterium in artificial media and complete the technical requirements needed to identify a new species. There are three species, 'Ca. L. asiaticus’, 'Ca. L. africanus', and 'Ca. L. americanus’. The Asian species, Ca. L. asiaticus, was found in Florida in 2005 and has caused enormous losses; it is the predominant form of the pathogen in Florida. It is also the most severe and aggressive species of the Ca. Liberibacters. The pathogen can be transmitted by psyllids and through grafting with diseased budwood. The Asian citrus psyllid (ACP), Diaphorina citri Kuwayama, is a Hemipteran insect measuring 3 to 4 mm in length with piercing-sucking mouthparts that allow this pest to feed on the phloem of Citrus species and related plants. There are five nymphal stages; together, going through these stages requires 10 to 40 days depending on temperature (Fig​ure 2). Two main factors regulating psyllid populations are air temperature and availability of young growing shoots for ovopositioning (egg laying). ACP damages citrus directly by feeding on new shoots and leaves; young leaves twist and curl. In addition, transmission of Ca. L. asiaticus weakens and kills citrus trees.


Figure 2.

Disease Management

There are no cures for HLB. The best way to protect citrus trees from HLB is prevention. Common practices include inspecting the trees monthly for the Asian citrus psyllid and removing symptomatic trees to eliminate as much Ca. L. asiaticus as possible, as well as planting of certified, pathogen-free nursery stock. Additionally, a great deal of effort is directed toward keeping psyllid populations as low as possible. Without adequate psyllid control, it only takes 5 years for an orchard to reach 100% incidence of infection. Reducing pathogen spread maybe achieved in part by targeting overwintering adult psyllids. Compared to psyllids at other times of year, psyllids that overwinter on infected trees are more likely to carry the pathogen to healthy trees. Chemical control by means of insecticide sprays is the most common way to suppress psyllid populations. A typical annual insecticide spray plan for a Florida citrus grower is as follows for mature trees:


Month
Coordinated sprays1
Insecticide Trade Names
JanuaryDormant sprayDanitol, Mustang,
Baythroid
AprilPetal fallDimethoate, Imidan,
Lorsban
JulySummer sprayDelegate, Portal*, Micromite*
SeptemberFall sprayDanitol, Mustang, Baythroid
NovemberDormant sprayDimethoate, Imidan,
Lorsban

*Only if nymphs are present.
1 Coordinate sprays: citrus growers in neighboring area (CHMAs) coordinate insecticide sprays for psyllid control.


Heat therapy has been utilized for decades to prevent the spread of diseases from propagation materials in other plant species, for example, sugarcane or perennial fruit trees. In citrus, the technique was first used on infected budwood and small trees in China and India, respectively. R.E. Schwarz and G.C. Green were the first to test heat therapy on whole trees it by enclosing infected tree in a fiberglass enclosure. The fruit greening symptom severity decreased from 94% to 4% after the treatment. The technique was later on modified by plant pathologists and engineers in Florida in 2013. Several methods have been tried but the most similar to previous work was the application of solar heat to kill the pathogen with minimal damage to the host by encasing several infected citrus trees together in plastic “tents” for about a week, then removing the tents and trimming off the outer shoots that were burned by high temperatures.


There are five additional ways to manage HLB include the following:

  1. GMO citrus trees. A scientist from Texas A&M University generated genetically modified oranges by copying certain genes from spinach into an orange tree’s DNA, resulting in potential resistance to HLB.  If it is commercialized, it will be an alternative to other management approaches. However, the cost of commercialization and consumer acceptance should also be taken into consideration. It is hard to say if consumers will purchase GMO orange juice or not until it becomes available on the market. Other genes are being tested for efficacy but face similar hurdles for release.
  2. Foliar antibiotic applications. Recently, the EPA has allowed the foliar application of the antibiotics streptomycin and oxytetracycline on an emergency basis to suppress populations of Ca. L. asiaticus. To date, there is limited data that show efficacy of this strategy but it is being used on a wide-spread basis in Florida. The first harvest after the initiation of these applications will be the 2016-2017 season which will hopefully give more clarity on the efficacy of this strategy.
  3. Sprays of horticultural mineral oils (HMOs) to suppress egg laying by adult female psyllids. These oils are less environmentally toxic than chemical insecticides.
  4. Biological control of psyllids. In experimental trials, the parasites Tamarixia radiata was found to suppress populations of Asian citrus psyllids. This is used in residential areas but is not sufficient for population control in commercial settings.
  5. General management. Besides HLB, other diseases or disorders, such as Brown rot on citrus fruit infected by Phytophthora spp. which mainly target mature or nearly-mature fruits and eventually affect the yield, should also be paid attention. Water quality and soil pH should be monitored and adjusted to be optimal for the rootstock planted in a grove. Micronutrient deficiencies can be problematic on infected trees and levels should be monitored via leaf sampling. The fertilizers containing those micronutrients that were found to be deficient should be applied to maintain the tree in an as healthy condition as possible.