Big Breakthrough in the Fight Against Citrus Greening

A major obstacle in the fight against citrus greening has been the inability of researcher to culture the bacteria that causes the disease in the lab. Researchers from Washington State University (WSU) have overcome that challenge.

Being able to grow the elusive and poorly understood bacterium, Candidatus Liberibacter asiaticus (CLas), will make it easier for researchers to find treatments for the disease that has destroyed millions of acres of orange, grapefruit, and lemon groves around the world and has devastated the citrus industry in Florida.

A critical step in coming up with weapons to fight the disease is being able to study it in the lab, but the CLas bacterium is notoriously difficult to grow. With a small genome, CLas is thought to depend on very specific nutrient availability and possibly compounds secreted by other nearby bacteria.

When researchers used a traditional rich media that they typically use for growing bacteria, they mostly grew bacteria other than CLas. So, in order to conduct research, scientists have had to get bacterial samples directly from the trees themselves or from the insects that spread it, which is time-consuming and cumbersome.

Trying to conduct experiments also has been difficult because, unlike neat lab cultures, bacterial samples gathered from a sick tree vary, depending on where and when the sample is gathered and the level of infection.

Without being able to grow the bacteria in a lab, researchers have been unable to even absolutely confirm that the bacteria, in fact, causes the disease.

In their paper, the researchers for the first time successfully established and maintained CLas bacterial cultures outside of its host.

Using infected citrus tissue as their starting point, the researchers developed a biofilm — a kind of bacterial city that allows a variety of bacteria to thrive. Instead of a rich growth medium that would crowd out the CLas, the researchers severely limited the growth of partner bacteria and created a medium with the specific nutrients, acidity, incubation temperatures, and oxygen levels that are optimal for CLas. The CLas thrived – an important first step.

“We were really excited,” said Haluk Beyenal, a Washington State Postdoctoral Research Associate, “but then we wondered if we could re-grow it.”

The researchers were able to transfer the orange-colored culture and grow new cultures in their biofilm reactors, which they have maintained for more than two years.

“We can do this for as long as we want,” said Beyenal.

Beyenal’s group is now working to purify the culture, which will further help researchers to study it. They also arealso developing genetic-based methods to understand and mitigate the spread of the disease.

Joining Beyenal, the research team includes, Phuc Ha, Paul Hohenschuh Professor in the Gene and Linda Voiland School of Chemical Engineering and Bioengineering, David Gang and Ruifeng He, from WSU’s Institute of Biological Chemistry, Anders Omsland, from the Paul G. Allen School for Global Animal Health, as well as scientists from the University of Florida and University of Arizona, report on their work in the journal, Biofilm.

Manipulating Flowering for Better Grove Management

Manipulating Flowering for Better Grove Management

By Tripti Vashisth, Garima Singh and Megan Dewdney

 

 

Citrus trees grown in the field undergo various types and levels of stress continuously. The stressors can be several things, including heat, cold, drought, soil pH, chemicals, pests and diseases. The constant presence of huanglongbing (HLB) and psyllid infestation adds stress to the trees, which compromises the plant response and makes the trees susceptible to number of other diseases.

Off-season and prolonged flowering is a common response of trees when undergoing various stress conditions. For example, Navel and Valencia are well known to have prolonged flowering periods with sporadic flowering during the fall. Generally, off-season and prolonged flowering is not a great concern, but when combined with heavy rainfall and warm weather, this can increase the threat of postbloom fruit drop (PFD).

POSTBLOOM FRUIT DROP
PFD is caused by a fungal pathogen, Colletotrichum acutatum. This pathogen is usually present on the tree (twigs, leaves etc.) in a dormant stage and rapidly grows on flowers. Upon rainfall, the fungus can quickly disperse throughout the tree in a couple of days. Upon PFD infection, the flower and young (newly set) fruitlet drops. If severe, this infection can cause a complete yield loss. Therefore, the ability to synchronize and suppress off-season flowering can be beneficial for the following reasons:

1) Compressed and synchronized flowering period can be efficiently targeted with fungicides.

2) With a shorter flowering period, fungicide use can be reduced.

3) Reduced inoculum build-up in the tree during off-season

4) Profuse flowering of citrus provides plenty of opportunity for the fungus to flourish; fewer flowers could reduce potential locations for fungal growth.

A further consideration about the profuse citrus flowering is that only approximately 2 percent of flowers are harvested as fruit. Thus, many tree resources are invested in flowering and fruit set which eventually end up on the ground. The ability to conserve these resources to harvestable fruit may be highly beneficial for HLB-affected trees.

The plant hormone gibberellic acid (GA) is known to influence flowering in citrus plants. GA is also involved in many different aspects of citrus growth and development, such as fruit growth, peel senescence, flowering, etc. GA applied externally can suppress flowering as well as reduce the number of buds in healthy trees. Therefore, GA applications can be potentially useful for manipulating flowering for better PFD management.

GIBBERELLIC ACID TRIAL
In 2016, a 3-year trial was initiated to evaluate the effect of GA to synchronize and compress the flowering period. A major aspect of this study was to evaluate the effect of GA on yield. Because GA could potentially reduce flowering intensity, the yield may therefore be reduced. This study was conducted on two PFD-prone cultivars, Valencia and Navel, at two sites known to have PFD history in Fort Meade and Haines City in Central Florida.

In order to synchronize flowering and suppress off-season flowering, the GA treatments were initiated in early September and continued until early January. The GA was applied at 20 grams active ingredient per acre per application with a surfactant (Induce® 0.125 percent), for a total of five GA applications. These GA applications were compared with an untreated control, where only water and surfactant were sprayed on the trees. Flowering intensity was monitored from December to April. At the time of harvest, yield was measured per tree to assess the effect of potentially suppressed flowering. Altogether, there were two cultivars and two sites, treated and monitored for three years.

LESS FLOWERING, MORE YIELD
Trees of both sites and cultivars had similar responses to GA treatment. Flowering intensity decreased by approximately 50 percent in addition to suppression of flowering in February/early March. A shift in peak of flowering intensity was observed for Navel; peak flowering was delayed by at least a week.

In this article, the trial on Valencia in Fort Meade is mainly discussed. The number of elongated flower buds (buds with elongated petals) formed and subsequent flowers on GA-treated trees decreased by almost half compared to control trees (Figures 1a and 1b). As expected, the peak of flowering was two weeks later than the peak of bud activity. Bud growth extended from late January to early April (more than eight weeks) on the untreated control, whereas bud activity was compressed to less than six weeks for the GA treatment.

The flowering (open flowers) was more synchronized in the GA treatment; open flowers were observed for approximately three weeks. Control trees had a flowering period extending for four weeks. This data is consistent with previous years’ observations, suggesting that flowering can be suppressed and synchronized with GA on HLB-affected trees, which leads to better PFD management.

Since there is no bud activity in GA-treated trees until early March (see the blue highlighted box in Figure 1a), a fungicide application will not be needed. However, untreated control trees have a significant number of buds warranting an early fungicide spray, if weather is conducive for PFD.

Researchers also counted the number of persistent calyces (PFD buttons) on the tree before and after a flowering period. No significant differences were observed in PFD button number among GA-treated and untreated control trees. This may be partially due to the fact there was no major PFD event in 2017 and 2018. The button counts have been generally low in the GA-treated and untreated trees.

Since GA reduced flowering significantly (by 50 percent), a major concern was if the use of GA reduced fruit yield. Fruit yields were collected on a per tree basis. Interestingly, we found that the use of GA increased the yield compared to the control, when considering the multiple-year data (Figure 2).

When annual yield was considered separately, there was no difference in yield for 2017 and 2018 in GA-treated and control trees. However, in 2019, GA-treated trees had significantly higher yield than control trees.

It was demonstrated that GA application reduced flowering without reducing yield and may have increased it. In GA-treated trees, the yield consistently increased from 2017 to 2019, going from 165 pounds per tree to 282 pounds per tree with the repeated application of GA. No such increase was observed for untreated control trees.

HIGHER FDF FOUND
As no negative effects of GA on yield were observed, even though the flowering was reduced by 50 percent, researchers tried to find potential reasons for such observations. In 2019, the potential of GA treatments to reduce fruit drop and thereby improve fruit yield was evaluated. The fruit detachment force (FDF) was measured on the GA-treated and untreated control trees. FDF is the force required to remove the fruit from the peduncle of the twig. The idea is that a fruit that is likely to drop will have lower FDF, and fruit that is hanging tight onto a tree will have higher FDF.

GA-treated trees were found to have significantly higher FDF than untreated trees (Figure 3), indicating that GA-treated fruit were less likely to drop. Numerically, the average FDF between GA-treated and untreated trees differed only by one unit. However, that difference was meaningful. According to existing literature on healthy citrus, fruit below 6 kilogram-force (kgf) is considered loose. Fruit above 6 kgf is considered well retained on the tree, suggesting that GA application from September to January improved fruit retention in Valencia, thereby potentially improving yield.

In summary, this three-year study suggests that GA can be effectively used for synchronizing and suppressing profuse flowering without affecting yield in Valencia. In addition, GA treatment has potential to improve yield and reduce fruit drop. This relationship will be explored further to be confirmed and validated.

Acknowledgment: Funding for this project was provided by Citrus Research and Development Foundation project 16-010C.

Tripti Vashisth is an assistant professor, Garima Singh is a research scholar, and Megan Dewdney is an associate professor — all at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.

Putting IPM Back in Citrus

Putting IPM Back in Citrus

IPM

IPM
Determining Asian citrus psyllid sprays based on economic thresholds can help growers reduce control costs and increase beneficial insects.
Photos by Tonya Weeks

By Lukasz L. Stelinski, Jawwad A. Qureshi and L. Gene Albrigo

Florida citrus production has a long and trailblazing history of implementing integrated pest management (IPM). In 1950, the director of the Florida Citrus Experiment Station, A.F. Camp, proposed an “Ecological Survey of Citrus Pests and Disorders” to provide a comprehensive survey of the ecology of citrus groves throughout Florida (Simanton, 1996).

Important discoveries were also made concerning the existence and impact of biological control agents regulating pest populations. It was realized, for example, that excessive sulfur use had a negative impact on natural enemies of the purple scale. Advising growers to use less sulfur allowed populations of these parasitoids to rebound, which decreased the purple scale populations below economic thresholds.

THE GREENING INVASION
From the 1970s to the 2000s, the pesticides applied to Florida citrus were restricted to a few fungicides, herbicides and horticultural oil sprays with notable reduction of contact insecticide or miticide use. This changed dramatically with the arrival of citrus greening around 2005.

Despite efforts to control the disease, greening spread in Florida due to the mobility of the Asian citrus psyllid (ACP), favorable environmental conditions for both vector and pathogen, and the long latent period, during which initial indications of infection went undetected. In the first decade of greening management in Florida, vector suppression with insecticides played a prominent role. Calendar spraying became commonplace, and coordinated spraying among neighboring growers took place at times to reduce psyllid populations on a regional scale. Despite improvements in management outcomes as measured by reduced psyllid numbers in many cases, the disease has spread to all parts of the state and is now endemic infecting virtually all trees.

Currently, there is renewed interest among growers to return management practices to a more balanced IPM approach rather than heavy reliance on insecticides alone. Is such an IPM approach possible under conditions of endemic greening? We believe the necessary information and tools are indeed available to begin putting such a management paradigm into practice.

APPLICATION THRESHOLDS
The foundation of IPM is the economic injury level, which is also referred to as the economic threshold. This is the pest population level at which the resulting damage it causes justifies the economic investment of implementing a control measure. If the pest population (and resulting damage) is below this point, it does not pay to take a control measure.

Economic thresholds are developed by monitoring pest populations, implementing control measures at varying population densities and then measuring yield at the various levels of input. Comparing investment versus yield at varying levels of input will shed light on the action threshold density under various crop scenarios. Such thresholds may take a few seasons to perfect, but they often work reliably to maximize return on investment, particularly when more variables can be added such as crop price.

Initially, application thresholds were not considered practical for ACP, given that even one psyllid can spread the pathogen. However, the situation has changed since the goal of psyllid management in Florida is no longer prevention of disease spread. The new objective is reducing severity for economically viable production of citrus under the conditions of endemic greening.

An investigation by Cesar Monzo and Phil Stansly in 2017 demonstrated that economic thresholds for ACP are possible and can be beneficial. Specifically, by using a threshold of 0.2 psyllids per tap to trigger the need for a spray, the investigators reduced the number of annual treatments per calendar year from 10 to only 4 sprays. Using the threshold system, they achieved returns that were either equivalent or better than those attained using the calendar-based program.

These results show that the investment made in psyllid control costs using calendar-based programs were not justified. The 0.2 psyllids per tap threshold used in this investigation is currently not optimized for every situation in Florida, but it was a useful starting point to illustrate the potential utility of thresholds for guiding spray decisions under greening. Although it can be further optimized and should not be considered a general rule that will fit every situation, it is a good starting point to consider for those who wonder when they should spray.

MONITORING PSYLLIDS

 

Regular scouting for pests is important for making need-based decisions regarding insecticide applications and to minimize unnecessary investment in control measures and potential collateral damage to beneficial insects and mites. Implementation of an economic threshold for psyllid management depends on monitoring the pest’s density in a grove.

Stem tap sampling is probably the easiest method to collect instant data for making management decisions. A tap sample is made by using a white clipboard or laminated white paper sheet held horizontally under randomly chosen branches. Strike the branch three times with a length of PVC pipe for one tap sample (see https://edis.ifas.ufl.edu/pdffiles/IN/IN111600.pdf).

Psyllids can be counted as they fall onto the clipboard. Scouting 10 trees in a particular block at 10 random locations and taking an average of the number of psyllids from those 10 trees on a per tap basis should give a reliable estimate similar to what Stansly and Monzo did. If your average count is less than 0.2 psyllids/tap, consider not spraying. However, when the count goes above that threshold, consider spraying. Again, more work is needed to determine how effective this threshold is industrywide.

TIMING MATTERS
Although the decision to spray for psyllid management may be guided by economic thresholds, which could use more research-based optimization, there is consensus that well-timed, dormant-season sprays prior to the appearance of flush are critical for reducing psyllid numbers. The goal is to kill adult psyllids before new feather flush is available for them to lay eggs and produce offspring.

If a large area is treated in this manner, then subsequent generations can be greatly reduced. Also, movement of psyllids and population re-establishment can be reduced. If treatments continue prior to each new major flush, ACP suppression can be improved as compared with spraying in a fashion that is not coordinated with flushing.

The previous tendency has been to spray on a schedule with intervals somewhat determined by length of efficacy of an insecticide with additional rotation of chemistries to prevent development of resistance. After a dormant winter spray, the first spring spray has often been timed to when the flush became evident using a pyrethroid or organophosphate for adult ACP. Unfortunately, that allows ACP time to lay eggs and begin a new population for the growing season.

Spray timing can be significantly improved by closely following a budbreak phenology model. This strategy times sprays for adults to bud break or the beginning of each new flush before there is feather flush on which the adults can lay eggs. A second spray is then timed on the flush as ACP begins to reappear.

With this technique, we measured more than 60 days of low ACP populations in 2018. Results showed that one to two sprays starting at initial budbreak or shortly thereafter appear to provide a good initial protection period past bloom and bee activity.

For the spring flush, the vegetative development is fairly coincident with flowering. The initiation of growth of the flower buds is identified by the online Citrus Flowering Monitor model (see http://disc.ifas.ufl.edu/bloom/). The model outputs include the date of bud growth initiation and the predicted date of full bloom. The first summer flush usually starts about the time of the summer rainy season in Florida.

RETURNING TO IPM
Florida citrus is a trailblazer for incorporation of IPM in U.S. agriculture. A return to IPM in citrus is not only possible; it could be very beneficial. Returning to a more sustainable paradigm could bring economic reward.

Implementation of thresholds and timing sprays to flushing should maximize the impact of spraying. The above example from Stansly and Monzo indicates that one well-timed dormant spray followed by three additional sprays triggered by a 0.2 psyllids per tap action threshold can provide equal or greater economic return than 10 calendar sprays in the same grove.

Reducing sprays allows buildup of the biological control agents already present in Florida and allows them to work for you. Diversifying selection forces complicates development of resistance for the insect population. Resistance to one tactic can be compensated for by the others. Therefore, the IPM system can be further improved by not only allowing biological control to do its work by reducing insecticide input, but also by incorporating cultural controls.

Psyllid-exclusion technologies, such as protective screening and kaolin clay, are proving effective. Protecting grove borders with targeted sprays can reduce costs of whole-grove sprays. Significant progress has also been made toward reducing psyllid access to young citrus by planting on UV reflective mulches. Finally, IPM-based management will likely benefit from area-wide implementation. Many of the tools for practicing IPM in Florida citrus are already here; however, it takes integration to assemble the puzzle in practice.

Lukasz L. Stelinskiis an associate professor and L. Gene Albrigois an emeritus professor, both at the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) Citrus Research and Education Center in Lake Alfred. Jawwad A. Qureshi is an assistant professor at the UF/IFAS Southwest Florida Research and Education Center in Immokalee.

Courtesy of Citrus Industry Magazine

Individual Protective Covers Are ‘Promising’ for HLB

Individual Protective Covers Are ‘Promising’ for HL

by Susmita Gaire

Individual protective covers (IPCs) have thus far kept citrus trees free of HLB in research plots, according to Susmita Gaire, a presenter at the recent Florida State Horticultural Society annual meeting. Gaire is a graduate student working with University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) researcher Fernando Alferez at the Southwest Florida Research and Education Center in Immokalee.

The IPCs have also kept trees free of damage from Asian citrus psyllids, the vector for the devastating disease HLB, said Gaire. Gaire added that although the research offers “promising results,” it’s too early to recommend IPC use to Florida citrus growers.

Despite the lack of an official UF/IFAS recommendation, many IPCs are being used to protect young trees from psyllids and the HLB they spread in citrus groves around Florida.

Why Orange-Like Hybrids May Secure Future for Florida Citrus

Why Orange-Like Hybrids May Secure Future for Florida Citrus

Hybrid orange juice processing for chemical analysis

The question, ‘What is an orange?’ is certainly not new to Florida. Research institutions, processors, and growers have long expressed interest in exploring the possibilities of what expanding the definition of orange might hold for the juice industry. The one foray Florida made into this arena three decades ago was to open up the orange juice standard to allow the ‘Ambersweet.’ That example frequently surfaces in conversations as a cautionary tale, and there is no longer interest in expanding the definition of orange juice one hybrid at a time.

The driving forces fueling today’s conversation address the industry’s survival in an HLB-endemic environment and the possibility of simultaneously increasing the quality of the final product by addressing the inclusion of hybrids based on fruit characteristics.

1. Sweet orange is thought to be the most widely planted fruit tree in the world, which drove massive investment in infrastructure and product development. Sweet orange is at the heart of Florida’s identity and culture. It is of great economic significance to Florida.
2. The sweet orange is itself a hybrid and is no longer considered a species. What we know to be sweet orange hails from a very narrow genetic background, leaving the industry vulnerable to widespread damage from pest and disease, to which sweet orange is susceptible.
3. Plant breeders have been working for decades to develop new hybrids that may have characteristics similar to sweet orange. Such hybrids may contribute to improving quality and supply. And some are showing commercially useful levels of HLB tolerance.
4. Current regulations, specifically the Federal Standards of Identity for “orange juice” requires the juice come from citrus sinensis. For pasteurized orange juice, 90% must come from citrus sinensis. Only 10% of the juice may come from reticulata or other hybrids.
5. There is great value in what consumers know as orange juice. This must be protected.
6. More supply is needed to produce the product and maintain infrastructure.

It is clear to many the industry needs to come together to begin discussions around these key factors and what, if any, regulatory changes might help secure a future for Florida’s orange juice sector.

New Varieties Development & Management Corporation (NVDMC) organized a meeting in April at the Florida Department of Citrus in Bartow that included representatives from Florida Citrus ProcessorsFlorida Citrus MutualFlorida Department of CitrusCitrus Research & Development Foundation (CRDF)USDA-ARSUF/IFAS, and several processing companies. This group delved into the science of how sweet orange came into existence, current domestic and regulatory definitions of orange juice, and the quality characteristics of some of the new hybrids.

It was agreed some of the hybrids were indistinguishable from orange, though they do not meet the current definition of orange. Other hybrids, though not entirely orange in their flavor profile, may contribute greatly to the quality and consumer acceptance of the final product.

This first meeting concluded that it may be preferable to revise the federal standards to allow for the inclusion of hybrids with key orange characteristics rather than require a classification or regulatory approval for each new hybrid. Draft language was crafted that will be circulated to the industry.

This revised definition approach would allow the brands and processing companies to develop innovative product blends that consumers may prefer while maintaining the integrity of what can be labeled as orange juice.

It also was clear that any regulatory changes would take place over a three- to four-year period, and the industry should be provided ample opportunity for comment along the way. Additional presentations and discussions will take place over the coming months at Florida Citrus Mutual, Florida Citrus Processors, Florida Citrus Commission, and the CRDF.

Much work remains to be done on the horticultural performance of new orange-like hybrids. Growers will need data and observable, replicated field trials before they have confidence they can produce an economically viable crop. Likewise, it will take some time before processors make their own determination of which hybrids work best in their proprietary blends.

This article is not intended to do more than introduce the topic and hopefully whet your appetite for more information on this ground-breaking topic. A whitepaper is available that goes into more depth. Readers may request a copy of the whitepaper by contacting NVDMC at lucy.nieves@ffva.com. Dr. Ed Stover is leading a cooperative effort with Dr. Fred Gmitter and other USDA-ARS and UF/IFAS staff to prepare an academic manuscript article for journal publication that will provide substantially more depth.

It also is timely that there is a funded project through National Institute of Food and Agriculture titled “Accelerating implementation of HLB tolerant hybrids as new commercial cultivars for fresh and processed citrus” (by Elizabeth Baldwin, Ed Stover, Jinhe Bai, Anne Plotto, John Manthey, USDA-ARS Horticultural Research Laboratory; Rhuanito Ferrarezi, Fred Gmitter, and Yu Wang, UF/IFAS Citrus Research and Education Center; Mike Rouse, University of California, Riverside; and Goutam Gupta, New Mexico Consortium). The results of this project will undoubtedly prove invaluable through this process.

UF Researchers to Study Integrated Approaches to Protect Young Citrus Trees from Greening

While citrus growers continue to look for best management approaches to deal with the deadly greening disease, some scientists at the University of Florida will take an integrated look at how to protect young trees, by using tools growers already can use.

Five scientists from the UF Institute of Food and Agricultural Sciences will compare insect management tools, including insect-proof netting. Researchers also will study reflective mulch, kaolin clay and chemical-based insect pest management. Kaolin clay is a powdery white compound used to conceal citrus trees from psyllids by confusing their visual sensory system.

“All of these tools are aimed at insect management, but it is unclear how they influence other aspects of grove care, like plant growth rates or water use,” said Lauren Diepenbrock, a UF/IFAS assistant professor of entomology and research project leader.

UF/IFAS researchers have earned a $665,471 U.S. Department of Agriculture grant. The grant draws funding from several sources, including the National Institute of Food and Agriculture — an arm of the USDA — and from the U.S. Environmental Protection, the  Florida Department of Agricultural and Consumer Services and industry groups.

With the funds, they plan to study citrus greening control methods from an integrated perspective. When they complete their research, the UF/IFAS researchers hope to develop new recommendations for existing tools that growers can use to fight citrus greening, also known as Huanglongbing, or HLB.

Until now, researchers have evaluated citrus greening prevention methods by studying how they affected one aspect of production, rather than the entire agricultural operation.

“For Florida growers, we hope these tools can help them be more profitable when planting individual new trees or entirely new groves,” Diepenbrock said. “These plants may eventually become impacted by HLB. But we are learning more about living with HLB from the research being done by UF/IFAS researchers and our colleagues globally. That research may give us new tools to use in the long-term management plan for this field, once this initial project is completed.”

Right now, the team has far more questions than answers. Some of them include:

  • What kind(s) of pest management challenges does each tool – for example, insect-proof netting — present?
  • Does the use of reflective mulch (plastic ground cover) impact root diseases?
  • Do these management tools impact the pathogens that cause the disease known as greasy spot?
  • How efficient are these tools at preventing or delaying infection by the bacterium that causes HLB?
  • How will the use of reflective mulch, exclusion bags and/or kaolin clay alter water and nutritional needs?

To help growers answer those questions, Diepenbrock will work with Megan Dewdney, a UF/IFAS associate professor of plant pathology; Evan Johnson, a UF/IFAS research assistant scientist in plant pathology; Davie Kadyampakeni, a UF/IFAS assistant professor of soil and water sciences; and Christopher Vincent, a UF/IFAS assistant professor of horticultural sciences. All the researchers are faculty members at the UF/IFAS Citrus Research and Education Center in Lake Alfred, Florida.

In addition to helping Florida growers, Diepenbrock believes her team’s research results should help scientists around the world.

“For our colleagues in other states who have not been as heavily impacted, we hope that what we learn can be used in their regions to reduce the devastating impacts of this disease,” Diepenbrock said. “And for our colleagues who are in a similar condition of trying to grow citrus in endemic HLB areas, we hope this adds a tool or two for them as well.”

Greening has spread to 40 countries worldwide. In Florida, citrus production volume declined by 71 percent from 2000 to 2017, primarily due to losses from greening, according to UF/IFAS economists. The disease entered Florida in 2005.

Working Toward Better Orange Juice in the HLB World

Working Toward Better Orange Juice in the HLB World

BreedingOrange JuiceVarieties

By Jude Grosser, Fred Gmitter, Yu Wang and Bill Castle

oranges
Figure 1. Left: Juice from UF sweet orange OLL-8; right: commercial not-from-concentrate juice. Note the color difference.

It’s no secret that huanglongbing (HLB) has challenged the industry to maintain the outstanding quality associated with Florida orange juice. Moreover, increased prices and competition from new juice products and blends have reduced Florida orange juice consumption. We believe that improving the quality, especially flavor and color, of juice products can help battle the declining juice consumption problem (Figure 1).

The University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) citrus improvement team at the Citrus Research and Education Center (CREC) has been engaged in sweet orange improvement since the mid-1980s. The complex biology of sweet orange makes it difficult to directly breed new sweet oranges. Therefore, the program has exploited other reliable sources of genetic variation, in addition to conventional breeding, in efforts to improve numerous quality factors.

Last year, the citrus improvement team published an article in Citrus Industry magazine describing how HLB-tolerant mandarin hybrids that have been developed could be used to improve Florida orange juice (http://citrusindustry.net/2018/09/17/could-hlb-tolerant-mandarins-be-used-in-florida-orange-juice/). That article focused on research to gain a better understanding of the genetics that control fruit flavors as well as using sensory analyses (taste tests) to decipher flavor perception.

This article provides an overview of progress and strategies regarding the development of true processing sweet oranges with potential to enhance the Florida not-from-concentrate (NFC) portfolio. Also included in this article is the development of sweet orange-like hybrids that could be used to enhance Florida juice quality or to produce new high-quality, stand-alone juice products.

SOMACLONAL VARIATION
The initial effort at processing sweet orange improvement exploited somaclonal variation, a tissue culture technique that produces trees that sometimes exhibit useful variation from which improvements can be selected. It is a shot-gun approach, requiring large populations of trees to identify improved clones.

We created and evaluated more than 1,000 clones each of Hamlin and Valencia selections through this evaluation process. From these, several improved sweet oranges have been released for commercial production. Since the earlier work, additional somaclones of Orie Lee Late (OLL) have been developed and released that exhibit improved quality factors. The new cultivars include:

  • Hamlin N13-32, selected for improved juice color
  • EV-1 and EV-2, early-maturing Valencias that reach 15 ratio around Thanksgiving
  •  Valquarius, an earlier-maturing Valencia that can be harvested from mid-January through February
  • Valencia B9-65, a standard-maturity Valencia with superior yield and soluble solids production
  • N7-3 (ValenFresh), a nearly seedless Valencia
  • OLL-8 and OLL-4, very productive Orie Lee Late selections that exceed Valencia for flavor and color (Figure 2).

The UF/IFAS team is continuing to produce and evaluate sweet orange somaclones, and more releases are on the way. These include OLL-20, selected for its superior flavor profile (Figure 2), and a new mid-season Vernia selection that appears to be more tolerant to HLB.

oranges
Figure 2. Results from the University of Florida/Institute of Food and Agricultural Sciences Feb. 19, 2019 Juice Display. Subjective mean scores from display attendees are on a scale of 0 to 10 with 10 being a perfect score. 50/50 is a 1:1 blend of pasteurized Valquarius and LB8-9 Sugar Belle® juice; 90/10 is a 9:1 blend of the same varieties.

SEEDLING INTRODUCTIONS
Florida Citrus Hall of Fame grower/researcher Orie Lee and UF/IFAS’ own Florida Citrus Hall of Fame team member Bill Castle have traveled the world to collect seed of sweet oranges that might have potential to improve the Florida Industry. They have collected and brought back many candidates which they have spent many years evaluating.

This pioneering work led to the commercial release of Vernia, now a mainstay mid-season clone in the industry because of its productivity and Valencia-like juice quality. Early Gold, Itaborai and Westin were also products of these efforts.

We continue to seek genetic variation found in sweet orange nucellar seedlings, with focus now on the OLL series. There may be a common genetic phenomenon that leads to a greater frequency of genetic variation occurring in the OLL oranges. The original OLL tree, produced from budwood irradiation more than 60 years ago, was unstable. This instability could be due to the movement of a transposon, also known as a “jumping gene,” induced by the radiation process. Transposon movement in the genome can turn genes on or off, and such transposon-induced genetic variation can be exploited for variety improvement.

We previously planted a population of more than 200 OLL seedlings on their own roots in a commercial grove near Dade City, Florida, in an area with very heavy HLB pressure. In fact, this grove has since been destroyed due to HLB. However, after four years, we identified 2 OLL seedling clones that were completely healthy. They exhibited no HLB symptoms and were PCR-negative for Candidatus Liberibacter asiaticus (CLas), the causal agent of HLB. These two seedlings were recovered by grafting and will be evaluated on a large scale as part of a U.S. Department of Agriculture and Consumer Services Animal and Plant Health Inspection Service Multi-Agency Coordination (MAC) project.

A second trial of OLL seedling trees is underway in Saint Cloud, Florida, with no formal psyllid control. (It receives only one standard oil spray per year.) This trial included 350 individual OLL seedlings. After five and a half years, we identified 17 individual seedling-derived trees that were PCR-negative (no HLB infection) and several more with very low CLas titers. These OLL seedling clones are also showing significant variation in juice quality, maturity date and seed content. Continued evaluation could lead to the development of additional HLB-tolerant clones with improved juice quality.

BUDWOOD IRRADIATION
Exposing budwood to high doses of radiation to induce mutations has been used most commonly to develop seedless selections from high-quality but seedy selections. There have been several successes, including Tango mandarin (from seedy W. Murcott) and Star Ruby (from seedy Hudson).

We have used this approach with sweet oranges as well, leading to the release of the superior Midsweet 11-1-24, a nearly seedless selection with earlier maturity, greater yields and higher solids than other midseason selections. Two seedless Valencia clones from an irradiation experiment have shown notable tolerance to HLB and will be evaluated in the MAC project. We are evaluating irradiated budlines of Vernia and the OLL series. We have identified additional selections of Vernia with earlier maturity and excellent juice quality, and OLLs with improved color and flavor, along with potentially improved HLB tolerance.

CONVENTIONAL BREEDING
Although conventional breeding of sweet orange is difficult, we have produced several HLB-tolerant, orange-like hybrids. They could be classified as sweet oranges, with some minor changes to the “standard of identity” regulations currently used in the industry. Some of these are seedless triploid hybrids. In making such crosses, we focus on combining sweet orange genetics with increased HLB tolerance.

oranges
Figure 3. Data from the University of Florida/Institute of Food and Agricultural Sciences Oct. 30, 2018 Juice Display. Subjective mean scores from display attendees are on a scale of 0 to 10 with 10 being a perfect score. EV-1 and EV-2 are early-maturing Valencia clones, Hamlin is the standard early-season cultivar, and C2-1-5 is a seedless triploid orange-like hybrid that is only one-third sweet orange.

For example, we included juice from a new unreleased triploid hybrid of mandarin with orange, C2-5-1, in a UF/IFAS October 2018 Fruit Display. The results were very promising (Figure 3). Juice color of C2-5-1 was superior to that of even the Early Valencia clones, and flavor was like that of Hamlin and the EVs. This hybrid looks, smells and tastes like an orange. So, is it an orange? At minimum, it could be blended at 10 percent to increase the color and Brix of the early-season juice supply. What’s more, the original C2-5-1 tree is showing very good HLB tolerance! Juice from a second such hybrid was included in the UF/IFAS January 2019 Fruit Display in Vero Beach. It also was highly regarded as a juice product, as well as an attractive fresh fruit selection.

BLENDING
Through blending, we have identified several mandarin hybrids with notable HLB tolerance and potential for improving Florida NFC quality and flavor. Sugar Belle®, clearly the most HLB-tolerant commercial variety currently available, has led the way. It can be reliably used to improve the Brix, flavor and color of processed orange juice (Figure 2). Sugar Belle® also has potential as a stand-alone, high-quality juice product.

In a UF/IFAS December 2018 Fruit Display, we included a juice blend that was 80 percent hybrid C4-16-12 (a Hamlin-like, high-Brix triploid hybrid that is one-third sweet orange and one-eighth trifoliate orange) and 20 percent C4-10-42 (a highly colored, high-Brix and rich flavored triploid hybrid that is one-third Sugar Belle®). This blended juice, containing no true orange juice, was perceived as orange juice at the display, and was favored for flavor by nearly two to one over Hamlin and the Early Valencia selections. C4-10-42 produces richly flavored juice with up to 17 Brix and a 42 color score. It has tremendous blending potential.

Research to pursue blending opportunities is well underway at the CREC. We have also produced and planted hundreds of additional hybrids of sweet orange with more HLB-tolerant mandarin parents that should be fruiting in the coming years to create even more opportunities. Hopefully, the major orange juice processors will team up with the UF/IFAS breeding/flavor chemistry team to fully exploit these opportunities. As the industry replants citrus for the future, we believe that at least 10 percent of the trees should be superior HLB-tolerant hybrids that will improve the flavor and quality of Florida juice products.

For more information on available UF/IFAS sweet oranges, visit the Florida Foundation Seed Producers website (http://www.ffsp.net/varieties/citrus/) and the New Varieties Development & Management Corporation website (http://nvdmc.org/).

Jude Grosser and Fred Gmitter are professors, Yu Wang is an assistant professor, and Bill Castle is an emeritus professor, all at the UF/IFAS CREC in Lake Alfred.