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​​​​​​​​Citrus Killer: Battling Back Against HLB

Chen, J., Dewdney, M., and Gleason, M., 2017. Citrus Killer: Battling Back Against HLB. The Plant Health Instructor. 10.1094/PHI-T-2017-0111-01

CAST OF CHARACTERS

Derek Parker: the owner of DP Citrus and Nursery
Ken Moore: the owner of Natural Juice Company
Maria Johnson: the manager of Citrus Growers Association, Inc.
Robert Wilson: researcher and extension specialist at the University of Florida

BACKGROUND INFORMATION

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CLASSROOM MANAGEMENT

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THE CASE

The state of Florida ranks second in the world (after Brazil) in growing juice oranges, and produces about 80 percent of the orange juice that comes from the U.S. Ken Moore, the owner of Natural Juice Company in Florida, is deeply worried about the increasing cost of Florida oranges and grapefruit for juice production. Recently he received a chorus of complaints from large grocery store chains, all saying the same thing: more and more of their customers are switching from Natural Juice Company’s brand of orange juice to other types of juice due to the rising price of orange juice and the recent tendency of Florida orange juice to have a watery and flat taste.

Finally, Ken called Maria Johnson, the manager of their citrus supplier, Citrus Growers Association, Inc., to negotiate a lower price. “Hi Maria, our customers are complaining that price of our orange juice has gone up, while the quality is going down. Can you suggest how to hold down the cost of producing oranges without sacrificing the quality? We can’t keep absorbing these higher prices.”

Maria was very anxious about this situation because the Natural Juice Company was her largest customer and she dreaded losing them. So she turned to her largest orange grower, Derek Parker, the owner of DP Citrus and Nursery. Maria was blunt: “Customers are complaining about our juice prices. They said if we don’t lower our price, they’ll drop orange juice and switch to other beverages. You need to figure out some approaches to cut the price of your oranges, such as increasing the yield.” 

Derek knew this was coming. “You know, this year I lost thousands of boxes of oranges in my groves because of a destructive disease. The average cost of production per acre of oranges is nearly double what it was 10 years ago. I really cannot cut the price; otherwise I would lose any profit margin and I’d be out of business.”

Central and Southwestern Florida are ideal places for growing juice citrus because of their subtropical climate, reliable rainfall, and abundant hours of sunshine. Citrus trees are evergreen and reach an average height of 3.7 to 4.6 m (12 to 15 ft). Commercial citrus trees are always produced by grafting a known scion cultivar onto a suitable rootstock that is propagated from a seed. Trees take around 2 to 3 years to start bearing fruit; trees normally continue to produce good crops for 20 years or more.

Derek owns 364 ha (900 acres) of orange groves in southwest Florida. One third of his trees are about 20 years old, another third are 10 years old, and the rest were planted this year. The young trees were mostly interplanted among the older trees in a practice of replacing removed trees called resetting. Derek grew up on the family citrus farm, and has a lifetime of experience in juice orange production. In his first 10 years as the owner of DP Citrus and Nursery, his earnings from citrus sales were far above average, which gained him a reputation as one of the top growers in Florida.

Now, however, he is facing a crisis unlike any he has handled before. For at least the past 7 years, his citrus groves have been suffering from a devastating disease called HLB.

This citrus disease has a long history internationally. In the 18^th century, a citrus syndrome in India called “dieback” was described but not identified as a disease. In 1927, dieback was associated with psyllid feeding in India. Similarly, as early as the 1870’s, a ‘yellow-shoot (or dragon) disease’ was reported by farmers. Fast-forwarding to 1943, it was definitively demonstrated by Dr. Lin Kung Hsiang that huanglongbing (HLB) was caused by a graft transmissible pathogen. It took many more years to figure out the actual causal agent but it was thought to be a virus at the time because of the disease’s systemic nature. The disease has had different names in different countries, such as dieback in India, likubin in Taiwan, and citrus greening in South Africa, but “HLB” has been widely adopted as the common name.

Since 2005, when this deadly disease was discovered in Florida, it has become the single largest threat to Florida’s $9 billion citrus industry besides freezes or hurricanes. HLB, a slow, insidious disease that requires about 5 years to make a mature tree unprofitable, has now has been reported in 37 of Florida's 67 counties and all major citrus production counties in Florida.

HLB is spread from infected to healthy trees by a tiny insect called the Asian citrus psyllid (Diaphorina citri) (Figure 1), which is about the size of a flea. The psyllid itself does not cause the disease, however. Instead, the HLB-causing pathogen is the Gram-negative bacterium, whose scientific name is Candidatus Liberibacter asiaticus; the psyllid carries the bacterium from tree to tree. Inside the tree, Ca. L. asiaticus moves within the phloem and disrupts the movement of photosynthates and plant hormones. The psyllids pick up the bacterium when they feed on the phloem of an HLB-affected tree, then spread it when they feed on the phloem of another tree. Once the psyllids pick up the bacterium, they can carry it for the rest of their lives, infecting tree after tree. A female psyllid lays eggs on the new leaf growth of expanding tree shoots, in the folds of recently unfurled leaves, and under developing leaf buds. Five nymphal developmental stages occur over 10 to 14 days in Florida depending on the temperature, so psyllid numbers can build up very quickly.

Figure 1.	Figure 2.

The early symptoms of HLB on leaves include an asymmetrical chlorosis, also termed blotchy mottle (Figure 2). This symptom is the most diagnostic for HLB, regardless of tree age. Other symptoms include vein yellowing, vein corking, small upright leaves, and leaf drop (Figure 3). The disease causes sectored yellowing (in other words, more symptoms are noted on particular branches or sectors of the tree than on others) in newly infected trees, especially young ones. The yellowing can spread throughout the tree after it has been infected for at least a season. Affected trees have stunted growth and produce fewer and smaller fruit than normal. The fruit that do develop have a thick, pale peel that can remain green at the bottom (Figure 4). They also become lopsided, with one half smaller than the other (Figure 4). This is particularly noticeable when the fruit is cut longitudinally. As the disease progresses, it becomes common for approximately 30% of the fruit to drop a month prior to harvest. This to occurs within a few years post-infection, resulting in significant yield loss. Eventually the tree will have canopy dieback and deadwood will be visible in the trees.

Figure 3.

Figure 4.

Derek is extremely frustrated and depressed by this disease. He was forced to remove about 20% of his orange trees during the last five years because HLB had devastated their fruit yield and juice quality. Equally distressing, their removal almost certainly left behind many “silent carriers” - trees that did not yet show symptoms but were infected by the bacterium. Moreover, he had to invest more money to replant orange trees in the affected groves, but even if they survived it would be four or five years before they became profitable.

Ever since HLB was discovered in his groves, Derek has made strenuous efforts to control psyllid populations, since he knows that controlling this insect is a key to halting the spread of the disease. He inspects each tree every two months, invites local agricultural inspectors to scout for symptomatic trees, and removes all of these trees. Currently, Derek applies a minimum of 5 insecticide sprays per year to control psyllids: 1) a dormant spray in January; 2) a petal fall spray in April; 3) a summer spray in July; 4) a fall spray in October; and 5) a dormant spray in November. For young trees, he makes soil applications of systemic insecticides (which can be taken up by the roots) every six weeks all year along.

In addition, Derek applies fertilizers to reduce the effects of HLB, since Florida growers have reported that adding nutrients can maintain productivity of HLB-affected trees. Fertilization can be done in two ways: by applying dry fertilizer granules and by foliar sprays. Dry fertilizers include calcium nitrate, monocalcium phosphate, magnesium, iron and boron, whereas foliar-applied fertilizers contain magnesium sulfate, manganese sulfate, and zinc sulfate. However, recently Derek has begun to doubt whether fertilizer application is working in the long term. Despite nutritional therapy, he often found that the while the trees looked better but yields did not improve. Moreover, applying so much additional fertilizer was becoming unaffordable. Therefore, Derek has recently cut back on the amount of fertilizers applied to HLB-affected trees.

A few days later, Derek received another urgent call from Maria. “Derek, did you come up with a way to increase the orange yield in order to cut the price?” Derek replied, “Sorry, Maria. My groves have suffered from HLB for such a long time and I’ve tried insecticides and fertilizers but got little effect. This disease is really out of control...” Maria: “But if you are not able to improve the yield, it means that dropping our price is impossible; buyers will turn to other fruit juices like apple or grape as substitutes for orange juice. Before making a new agreement with Natural Juice Company, can you design a detailed plan to improve HLB management?”

QUESTIONS

1. How do citrus trees become infected by HLB?


2. What are the symptoms of HLB at different stages of disease development?


3. Why is it difficult for Derek to detect HLB in the early stages of infection

DISEASE MANAGMENT

In order to save his citrus business, Derek called an extension specialist and researcher, Robert Wilson, from the University of Florida, who confirmed that HLB management starts with psyllid control. There are two main factors controlling the psyllid population: air temperature and the availability of a new flush of young leaves for egg laying and nymph development. The psyllid population grows rapidly in spring and early summer, along with flushes of new growth from the shoot tips. Therefore, Derek needed to pay special attention to this high-risk season. He also read that the most effective way to reduce the risk of psyllids spreading HLB during spring flush was dormant-season spraying of insecticides that targets overwintering adult psyllids. A research article said that the longer psyllids are allowed to feed on infected plant material, the higher will be the percentage of psyllids that test positive for the presence of Ca. L. asiaticus. Therefore, it is critical to control psyllid populations as early in the season as possible. Furthermore, multiple generations of psyllids can occur on the more sporadic summer flushes, and the dormant-season sprays help keep the difficult-to-control summer populations low. Psyllids that survived the winter on infected trees can spread the bacteria to healthy trees when they move around to find new leaves in the spring.

Psyllids can travel at least 2.5 km (1.55 miles) when dispersing, and probably further. Scientists showed that when an insecticide application was made in one grove, the psyllids simply traveled to a nearby grove and then returned once the insecticide was no longer effective. When multiple growers in an area worked together on coordinating the timing of insecticide applications, however, psyllid numbers stayed low for longer periods, disease transmission was slowed, young trees survived better, and costs were reduced. Now Derek knew that he needed to minimize the psyllid populations in his own groves and coordinate with his neighbors to stop the between-grove movement that was making his current control program ineffective. In particular, he needed to find a way to coordinate with his neighbors to get insecticide applications to the whole area, but how? He was still puzzled.

With this question in his mind, he learned about a new voluntary program Citrus Health Management Areas (CHMAs). This strategy worked by bringing together growers in a region to decide what types of insecticides to use at a particular time and coordinate a one-week window to get the application completed. Under the CHMA system, a local grower designated as a CHMA captain brought the group together and worked to get more reluctant growers to coordinate with the others in the area. The captains were supported by the University of Florida Extension Service, which provided the most up-to-date information on psyllid control. Additionally, the state and federal government agents scouted for psyllids in groves and the results were posted on a website (http://www.crec.ifas.ufl.edu/chma/) for everyone to follow. CHMA groups were also able hire aerial (plane or helicopter) applicators to spray the insecticides more rapidly and cost effectively. The coordinated applications would occur at the time when Derek was already making his applications, but they were likely to be more effective because psyllids could not evade insecticide by moving from one grove to the next.

Derek also discovered that he was making only the minimum number of applications and should apply additional sprays in between the coordinated control measures, but he would be able to afford them because of the cost savings afforded by working with his neighbors. As a relatively young grower, Derek considered whether he should become a CHMA captain to improve psyllid control in his area and give his new plantings the greatest possibility of a profitable future.

Heat therapy. An old technique, especially used to clean budwood for perennial plant propagation, known as heat therapy had been used against Ca. Liberibacter spp.-infected budwood or trees in China, India, and South Africa with some success. However, it had been found to be difficult or impractical to use in the field on a large scale. In 2013, a plant pathologist developed a potential way to practically use heat therapy for HLB in the field. HLB-symptomatic trees can be heated with solar energy by encasing them in plastic “tents” to prolong their productivity. But Derek was not completely confident about trying this potentially costly treatment. So he again called Robert Wilson, a researcher at the University of Florida. Robert told him that he could reduce the number of bacteria causing HLB in infected trees by using solar radiation to heat them up for about a week, then removing the tents and trimming off the outer 10 or 12 inches of shoots that were “browned up” by the solar heat. Some other growers had already applied this method to infected trees. Although the leaves stunted by HLB were not cured, the plant began to flourish 4 to 6 weeks later, producing a new healthy flush of leaves after the treatment. In the following year, the treated trees had higher yield and fruit quality was improved. However, would the disease resume spreading in the years after using this approach? Are the Ca. L. asiaticus in the roots affected by the heat treatment, and can they re-infect the rest of the tree from the roots? How many additional years would the heat-treated trees remain productive compared with those that had not been tented? What was the maximum tree size for using the tents, since some of the trees in his grove are 4.6 m (15 ft) tall? Furthermore, should the HLB-affected trees be classified into categories by symptom severity (for example, severe, moderate, or mild symptoms) and then treat differently based on this classification? What about the cost; would it be more expensive than replanting new trees? Does either tree age or severity of symptoms at the time of treatment impact the effectiveness of the treatment? To Derek, tenting trees seemed labor intensive. Robert told him that the benefit could last at least 2 years and the tent could be reused. If he tents 5 trees at a time in each block, the cost estimate is about $45 per 5 trees (based on 5 trees per tent) including material and labor. Should Derek choose this treatment?

Help Derek to determine his best course of action so he can increase production while keeping his costs at a minimum, and thereby address Maria’s concerns. Derek’s DP Citrus and Nursery includes 364 ha (900 acres) and 331 trees per ha (134 citrus trees per acre), for a total of around 120,600 trees. Many remaining trees have mild symptoms, and he has been steadily removing trees that show symptoms. Using this information, compare alternative or companion strategies listed below:

1. Propagation (purchase certified pathogen-free trees from a nursery)
2. Rogueing (removing trees that display HLB symptoms)
3. Scouting for HLB symptoms
4. Insecticide applications
5. Fertilizer application
6. Citrus health management areas (CHMAs)
7. Heat therapy
8. Planting HLB-resistant, genetically modified (GMO) trees
9. Other strategies

QUESTIONS

1. If you were Derek, what strategies would you choose? Explain your choices.
   
2. What other information do you need to know to come up with an effective disease management plan?
   
3. There is a heated debate about whether growers should plant genetically modified (GMO) citrus trees that are highly resistant to the HLB pathogen. A genetically modified organism (GMO) is an organism whose genetic material has been manipulated using genetic engineering techniques. This definition distinguishes GMOs plants from those developed by conventional breeding, which involves either crossing of male and female parent plants with desired characteristics or the induction of mutations by radiation, chemicals, or other means. If you were Derek, and assuming that GMO orange trees were available in the market and approved by the federal and state government for planting, would you choose GMO orange trees to replace regular trees? Why or why not?

ACKNOWLEDGMENTS

We thank Dr. Edward Braun (Iowa State University) for providing access to his PLP 408 class (Introduction to Plant Pathology) laboratory sections for the in-class tryouts, and the undergraduates in PLP 408 class for providing the feedback on this case study.

REFERENCES​

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