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.
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.
Six Key Factors to Consider
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.
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 firstname.lastname@example.org. 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.
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.
By Jude Grosser, Fred Gmitter, Yu Wang and Bill Castle
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.
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.
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.
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.
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.
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.
Callie Walker with the Florida Department of Agriculture and Consumer Services (FDACS) recently gave an update on the citrus black spot quarantine in Southwest Florida. Walker is bureau chief of pest eradication and control for FDACS’ Division of Plant Industry and serves as statewide director for the Citrus Health Response Program. She summarizes the presentation she made at the Florida Citrus Growers’ Institute in Avon Park last month.
“This year we have had two major expansions” in the quarantine area for the disease, Walker says. “One was in Charlotte County. The other was the first time we’ve ever had residential citrus black spot found on a tangerine variety in Lee County.”
She says those expansions added “significant acreage” to the quarantine area. The major quarantine areas are in Collier, Hendry and Lee counties. There is also a very small regulated area in Polk County, along with one citrus black spot find in Charlotte County.
Toward Better Food Plants: The Promise of Gene Editing
By G.B. Crawford
Since the dawn of agriculture human societies have sought improvements in food plants. Over time this quest has involved such objectives as a sweeter fruit or better resistance to a pest.
Traditional plant breeding can achieve a desired result, but the process may take many years of trial and error to complete and nurture unwelcome traits.
A series of scientific breakthroughs that began in the 1990s has created the opportunity to use a new approach. Researchers discovered that nucleases (enzymes) could remove specific sections of genetic material within a plant. They subsequently found that they could activate the expression of certain genes.
As currently applied, the tool relies upon the plant’s natural biology to enhance a preferred trait without introducing material from another organism – much like traditional breeding.
Genetic engineering of a plant can require more than a decade to develop and exact a price tag of above $100 million. By contrast, researchers note, gene editing costs a fraction of that figure and may be completed much more quickly.
Within the past five years, a new editing technology known as CRISPR has given researchers a more precise way to target a particular gene. (CRISPR is an acronym for an appallingly long description of certain DNA sequences: clustered regularly interspaced short palindromic repeats). It is easier and cheaper to use than other editing techniques.
For farmers, editing promises increased harvest volumes on the same acreage at less cost. New plant varieties can reduce the use of water and fertilizer as well as losses due to pests and disease.
Consumers can benefit from larger crop yields with less production costs by boosting the abundance of nutritious foods and moderating their retail prices.
According to the Food and Agriculture Organization of the United Nations, agriculture must increase productivity by 50% within the next 30 years to supply adequate nutrition for the world’s human population. Most experts warn that current agricultural capability alone cannot meet this need.
Susan Jenkins, managing director of the Innovative Genetics Institute at the University of California, Berkeley, has concluded that gene editing offers a key to meeting the challenge. “We have the ability to modify plants to make them have better water use efficiency and also better nutrient use,” Jenkins said.
“I am optimistic about how we use this technology, how it will be applied and the number of people it will benefit,” she added. “Because we can sequence the entire genome very inexpensively right now, we can make a change in a plant by using CRISPR. We can sequence the entire genome of that new organism and we can say exactly where other changes took place.”
The California facility has been a leader in developing improved cassava plants. Jenkins noted that researchers across the nation are working with other crops, including citrus and tomatoes.
Citrus growers in Florida anxiously await the development of new varieties in their long battle with greening disease. Gene editing may develop a tree immune to the malady.
Microbiologist Nian Wang is employing CRISPR to develop such a tree at the University of Florida/Institute of Food and Agricultural Sciences (UF/IFAS) Citrus Research and Education Center in Lake Alfred. Wang, a pioneer in this research, has directed the project for nearly five years.
“We try to optimize gene editing for citrus,” he explained. “We want to identify which genes make the plant susceptible to the pathogen.”
Testing plants is a long-term process. “You want to make sure the genes you modify will not cause unwanted side effects,” Wang said. “You want to make sure you do the right thing by moving carefully.”
A commercially-viable plant will be forthcoming. But he cannot yet offer a timetable for its availability.
“We feel a real responsibility as scientists,” Wang said. “We want to find a solution as soon as possible.”
Geneticist Tong Geon Lee is leading a gene editing project with tomatoes at the UF/IFAS Gulf Coast Research and Education Center in Balm. The goal is to develop a tomato plant that can be harvested by machine.
Lee noted that although this research has already produced new plants for greenhouse testing, he must complete a comprehensive assessment of new varieties before they can be released for general production. “Our first goal is to reduce plant height without damaging the fruit quality,” Lee said. “We don’t want to sacrifice fruit quality and other important traits for Florida growers.”
A number of private firms have launched ventures to utilize the technology. Corteva Agriscience, agriculture division of DowDuPont, has invested in CRISPR research for a number of row crops – especially corn and soybeans.
Senior Research Manager Bobby Williams said the company is moving forward with plans for a commercial pilot project with waxy corn. The research team has already developed an edited plant. Kernels of this corn contain high levels of amylopectin, a starch that can be used to thicken foods and make adhesives for labels.
Waxy corn is primarily a Midwestern farm commodity. But Williams said gene editing may reinforce traits that allow it to thrive in other locations.
He is optimistic that the technology will be widely applied to food crops. “I think there are a lot of opportunities when you get into the fruit and vegetable market,” Williams said. “Those are things that consumers can see and understand their benefits.”
A Durham, North Carolina-based firm, Pairwise, is applying CRISPR to row crops as well as fruits and vegetables. Haven Baker, the company’s chief business officer, said the “focus is to make noticeable improvements to the produce aisle. We’re trying to make produce more convenient, more affordable and possibly healthier.”
CEO Tom Adams pointed out that Pairwise is designed to operate as a food company. “The amazing, exciting opportunity that CRISPR brought was the ability to start doing things with crops that are in the consumer space,” Adams said.
He expects that it will have a large-scale impact on fresh produce by the mid-2020s.
The United States Department of Agriculture has expressed approval of gene-edited food plants. In 2018 Secretary of Agriculture Sonny Perdue announced that the agency “does not regulate or have plans to regulate plants that could otherwise have been developed through traditional breeding techniques.” Other agencies, such as the U.S. Food and Drug Administration, have not yet issued official positions.
Communication with Consumers
Researchers and agribusiness leaders acknowledge the obligation to communicate the value of the technology.
“I do believe that the products out there and the things that are nearing production on the market are safe to consume,” the University of California’s Jenkins said. “I would personally consume them.
“In the past we as scientists made the assumption wrongly that if we just got out and educated people about what we are doing, they would understand how safe this is, how practical it is and what our intentions are, it would be accepted,” she explained. “It is more about having a conversation with the consumer and what the consumer wants or needs.”
Haven Baker of Pairwise is confident consumers will be able to understand the benefits of this work because they will have access to the results in the grocery store.
“We think CRISPR will be one of the key tools of this century in meeting the needs of Florida and the rest of the world – to bring better foods to our markets,” Baker said.
G.B. Crawford is director of public relations for the Florida Farm Bureau.
By Tripti Vashisth, Davie Kadyampakeni and Lushan Ghimire
In the past few years, horticultural citrus grove management practices have changed significantly in Florida. It is a common observation across the state that huanglongbing (HLB)-affected trees respond well to complete and balanced nutrition programs. Therefore, citrus growers are currently focusing on intensive management of irrigation and nutrition.
HLB-affected trees suffer a significant root loss. In some cases, up to 80 percent root loss has been observed. Nonetheless, the roots of HLB-affected plants are functional and efficient in nutrient uptake. But due to reduced biomass and surface area, the amount of nutrient absorbed by roots is limited. Application of large volumes of fertilizer or water are therefore likely to leach out of the root zone before the tree can absorb them. Hence, application of small and frequent doses of fertilizer and water is recommended to improve nutrient availability and potential absorption by the tree.
A fertilizer program should include a balanced supply of all the essential nutrients. Every nutrient is indispensable; an excess or deficiency of any single nutrient can adversely affect tree performance. Foliar application of certain nutrients can be beneficial for the tree, especially when dealing with a known nutrient deficiency, but the focus should be on an all-nutrient soil-applied fertilizer program.
The soil-applied nutrients are absorbed by the roots along with the water uptake, so mobile (e.g., nitrogen and potassium) and immobile (e.g., boron, zinc, iron and calcium) plant nutrients are distributed throughout the plant as needed. It is important to ensure that the fertilizer is available to the trees year-round, since the growth season is long in Florida.
When applying fertilizer, the focus should be on the 4Rs (right source, rate, timing and place) of plant nutrients. However, with a soil-applied fertilizer program, another factor that is as important as the 4Rs is soil pH.
SOIL pH AFFECTS NUTRITION
Soil pH of the root zone is a very critical factor that affects nutrient availability and uptake. Most of the micronutrients become less available at higher, alkaline (pH>7.0) soil pH and reach toxic levels at low acidity (pH<5.0). Many Florida soils and irrigation waters are alkaline, which can limit nutrient availability if not corrected.
In addition, irrigation water in Florida is often high in bicarbonates, which after long periods of use can increase soil pH, affect tree health and reduce yields. The increase in soil pH and effect of bicarbonate-rich irrigation water depends on the bicarbonate concentration in the water, irrigation timing and quantity, the buffering capacity of soil and the rootstock variety.
Field studies done by Jim Graham and Kelly Morgan suggest that in groves where soil pH is neutral to alkaline and the irrigation water contains high concentrations of bicarbonates, trees often exhibit increased HLB symptoms and decline severely. Leaf nutrient analyses of such groves exhibit multiple nutrient deficiencies. In another survey, Graham and Morgan found that groves with soil pH higher than 6.5 and bicarbonates greater than 100 parts per million in irrigation water suffered from increased feeder root loss as well as reduced yields. A clear correlation of yield loss was observed in trees under bicarbonate stress and with lower root density when compared to low-bicarbonate stress trees. (For more information, see https://crec.ifas.ufl.edu/extension/trade_journals/2015/2015_May_bicarbonates.pdf.) Therefore, most growers are currently using some method to reduce the soil pH.
The interaction between soil bicarbonates, pH and HLB is still unclear. Whether the bicarbonates increase the soil pH and therefore reduce the nutrient availability to the tree, or whether the presence of disease causes physiological changes to the tree remains to be answered. It is noteworthy that the soil bicarbonates in Florida have always been present, but pre-HLB tree performance was acceptable, even with alkaline pH-susceptible rootstocks. To better understand this interaction, we conducted an irrigation experiment with healthy and HLB-affected plants.
IRRIGATION WATER pH EXPERIMENT
The experiment was performed in a greenhouse because it is nearly impossible to keep trees free of HLB under open-field conditions. Healthy and HLB-affected Midsweet orange trees grafted on Kuharske rootstock were grown in potted grove soil. Plants were irrigated with water at pH 5.8, 7.0 or 8.0 every two to three days.
The pH of irrigation water was adjusted with sodium phosphate buffers. The tree performance was monitored over a period of 60 days. HLB-affected plants watered with pH 8.0 showed more than 80 percent leaf drop; healthy plants dropped about 60 percent of leaves (Table 1). About 40 percent of HLB-affected trees irrigated with pH 8.0 water died within 60 days. The leaf chlorophyll index decreased with higher pH (Figure 1) in both HLB-affected and healthy plants.
No significant differences in total root biomass were observed with different irrigation water pH. However, Figure 2 shows that plants irrigated with higher pH water had less visible feeder roots compared to pH 5.8. At the end of the study, it was observed that the HLB-affected plants irrigated with pH 5.8 water showed more growth than the healthy plants.
When irrigated with water at pH 7.0, leaf drop and stem dieback were apparent in HLB-affected plants, but healthy plants were less affected. With irrigation water at pH 8.0, both HLB-affected and healthy plants showed leaf drop and stem dieback, but these observations were more pronounced in HLB-affected plants.
Leaf nutrient analysis revealed that all the nutrients were in the optimum range for healthy and HLB-affected plants irrigated with water at pH 5.8 and 7.0. HLB plants irrigated with pH 8.0 showed deficiencies for magnesium, calcium and zinc, while other nutrients were in the optimal range.
Even though plants were watered at three different pH levels every time, the soil pH did not change dramatically, and remained close to 7.0. As the experiment progressed, the soil pH changed slightly depending on the irrigation water pH. This suggests that the soil buffering capacity and the rootstock have a major role to play in soil pH adjustment. So any efforts to manipulate soil pH should be for the long term and will require patience.
Overall, HLB-affected plants perform better when irrigated with low pH (moderately acidic) water. In the study, there was an interaction between HLB-affected plants and soil pH. The HLB-affected plants tended to perform better when soil pH was close to 6.0. When pH was above 6.5 to 7.0, the HLB plants began to decline. Healthy plants performed well at a wider pH range (6.0 to 7.0). Currently, we are conducting in-depth analyses to understand this interaction of soil pH and disease severity.
Acknowledgements: This work was supported by the Florida Citrus Initiative and the U.S. Department of Agriculture’s Specialty Crop Research Initiative.
Tripti Vashisth and Davie Kadyampakeni are assistant professors, and Lushan Ghimire is a horticultural sciences graduate student — all with the University of Florida Institute of Food and Agricultural Sciences.