Scientists On A Mission To Build The Perfect Citrus Tree

Photo courtesy of New Varieties Development and Management Corp.
Photo courtesy of New Varieties Development and Management Corp.

The pressure is on to develop new scion and rootstock options for the Florida citrus industry. While the process of identifying new citrus rootstocks with varying levels of tolerance to HLB is under way, another challenge is pressuring industry and researchers. The evaluation of new scion candidates (fresh and processed) in today’s environment presents a myriad of new challenges. Researchers face budgetary constraints that limit their ability to maintain healthy blocks through multiple crop sets and physiological changes to trees and crops make it difficult to evaluate the true characteristics of each potential commercial cultivar. Variability in size, color, juice content, quality, productivity, and flavor must be evaluated. When HLB infects the tree, each of these factors can be impacted to some degree — exponentially increasing the complexity of the task. Outdoor evaluation provides some measure of performance in real-world conditions, but may not represent the true characteristics of the selection under more “normalized conditions.” Trees can be moved indoors or under screen where tree health can be maintained but again, crop characteristics and variability may not be representative of outdoor production.
In order to better understand these challenges, I sought feedback from USDA and UF/IFAS on this issue. Dr. Jude Grosser (University of Florida, CREC, Lake Alfred), Dr. Bill Castle (professor emeritus, University of Florida), Dr. Ed Stover (USDA USHRL, Ft. Pierce), and Dr. Fred Gmitter (University of Florida, CREC) offered input from the front lines. Each was asked to comment on the challenges of breeding and evaluation in the presence of HLB.

Dr. Grosser:
“As you can imagine, this is a major problem. It is expensive and time consuming to produce new hybrids with cultivar potential. Keeping them alive and happy in the field or in a rapid-evaluation structure (RES) structure is difficult, along with being more expensive and time consuming than before. Good psyllid control with timely pesticide applications is essential. Using leafminer as an indicator is useful to determine if adequate pesticide is present. I am trying several things to improve young tree health and vitality, including applications of Harrell’s UF mix and biochar (2½ pounds per tree). The biochar seems to be helping, but it is no silver bullet. I also am testing different rootstocks to improve the response against HLB. This will require more time to determine whether or not it helps. I expect this will be more successful after we identify and propagate rootstocks that can actually mitigate the disease. One of our big problems is that our new material is generally planted in the middle of blocks full of HLB and thus high levels of inoculum. Isolated sites away from the bad neighbor effect would be great, but at present, this is not possible for us. The silver lining in the dark cloud is that all of our material is now being screened naturally for HLB and canker. This is true for both the cultivar candidates and the breeding parents. Certain parents and crosses are rising to the top somewhat guiding the future direction of the breeding program. But, it takes a lot of time in the field to determine any emerging patterns. This, indeed, is a work in progress.”

Dr. Castle:
“Before, field trials offered an opportunity to evaluate many scion and rootstock traits, often with no serious interference from disaster or disease. Blight was about the only factor that was sometimes problematic, but it essentially didn’t ruin a trial. Then came canker, weevils, and other things. HLB has changed all that so now we establish field trials and wait to see what survives. Maybe we get to collect some horticultural data in the meantime. In some sense today, life in the field is much cleaner. It has been reduced to those that do survive and those that don’t.
“Personally, I’m OK with noting survivors. But of course, combinations that survive are not acceptable if a grower doesn’t make money. So, we must have other data. Maybe the new algorithm is we must start with some level of genetic contribution to tree performance/survival after which comes the possible contributions of other non-genetic factors.”

Dr. Stover:
“At many sites in Florida, response of any new hybrid or selection is compromised by response to HLB. Whether we like it or not, HLB-tolerance has become an integral part of the field evaluation process. If we are going to live with existing HLB pressure, then HLB susceptibility will remain a limiting factor, with reasonable tolerance being necessary for any new cultivar to be useful. However, if biotechnology or therapeutic treatments markedly change the importance of general HLB-tolerance, then we are making a mistake to emphasize HLB, and must evaluate new material in an appropriate HLB-free environment. In the USDA-ARS citrus breeding program, this requires us to follow two parallel paths, one in which HLB dominates and a second in which potential is assessed without HLB pressure. It is easy to find sites where HLB pressure is provided, and there are few sites with more aggressive HLB-pressure than the USDA Ft. Pierce farm. To create a HLB-free environment, the Florida Citrus Research Foundation (Whitmore Foundation Farm Board), the Citrus Research and Development Foundation (CRDF), and USDA have joined forces to build a one-acre screenhouse at the USDA Whitmore Farm.
When evaluated in the presence of HLB, the data shows just that, ‘how the selection performs in the presence of HLB.’ It is difficult to extrapolate how it would perform if HLB were not present.”

Dr. Gmitter:
“In addition (to the points raised above), there are other complications that arise from the imposition of HLB on the breeding processes. Nearly all of our parental trees have become infected, and several are so severely impacted that they cannot be used to make crosses any longer. Even some that are in reasonably good condition have been found to yield fewer normal seeds and therefore we recover fewer hybrids from crosses, decreasing the odds of finding superior new selections. We have begun to clean up some of our parents through the DPI PTP program under Dr. Peggy Sieburth, and now we are growing these under screen. Such trees will be smaller than field trees, and therefore the number of hybrids we can recover from any cross combination will be reduced as well. Keeping the young seedlings alive and healthy for a long enough time that we can see fruit is an obvious challenge. The RES helps to some degree as it is easier to keep systemic insecticides at adequate levels to minimize HLB incursions, but even this approach is not fool-proof. And again, these smaller trees produce fewer fruit, making display days and postharvest assessments difficult or impossible. Top working new selections to quickly increase fruit supplies for advanced evaluations requires very careful psyllid management as such trees flush nearly constantly. The breeders in Florida remain committed to the future of the industry and we appreciate the support and efforts of the New Varieties Development and Management Corp. (NVDMC) and CRDF to allow us to continue our work in service to the industry.”

Industry Support Essential
NVDMC is in the business of evaluating experimental selections and varieties for commercial viability in Florida. Some of these were Florida bred, while others are from California or other international sources. In most cases, the trees can be adequately protected through fruiting for evaluation in multiple crop years. However, it is increasingly difficult to ascertain whether off flavors, fruit size, uniformity, productivity, and fruit quality (juice content, color, ratio, flavor) are true to type or some variation of normal as a result of tree stress and other HLB-related factors.
As already mentioned, it would be beneficial to maintain duplicate trees free of HLB for comparison. Budget constraints have heretofore precluded this from happening.
It is imperative we maintain support for the Florida breeding and plant-improvement programs. Scion and rootstock development appears to be yielding promising short and intermediate tools in the fight against HLB, and the genetic diversity under evaluation (if maintained) can only strengthen this industry’s long-term prospects.

The author would like to thank Drs. Grosser, Castle, Stover, and Gmitter for contributions to this article.

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