Lessons Learned While Conducting Rootstock Field Trials
This blog is authored by:
Bill Castle, professor emeritus, UF/IFAS Citrus Research and Education Center
Fred Gmitter, professor, UF/IFAS Citrus Research and Education Center
Jude Grosser, professor, UF/IFAS Citrus Research and Education Center
“The notion that field trial data are of little value in the early years and trials must run for 15 years is outdated.” [Lesson 7]
“Our new website https://citrusresearch.ifas.ufl.edu/rootstock-field-data/ provides the grower community with user-friendly access to our collective decades of rootstock trial data……” [Lesson 9]
The University of Florida [UF] Citrus Research and Education Center [CREC] Plant Improvement Team has long been engaged in field research to evaluate new scions and rootstocks. That effort continues and has involved trials on public and private property. While public sites are valuable, they are limited in number. Thus, most of our trials are conducted cooperatively with citrus growers. When public and private sites are viewed together, it is clear that a broad range of circumstances are encountered that call for flexibility in designing and managing those trials. Here are a few of the lessons we have learned from our field work.
Our rootstock pipeline has three levels of assessment. Periodically, introduced rootstocks and those from our breeding program are assembled, propagated and set out in a screening trial for their initial evaluation in proper statistical formats, often at one location. When sufficient data are collected to reveal those rootstocks best suited for the next level of testing, usually smaller trials are planted, possibly each with a different scion and at two or three locations. Next, the “winners” are put into advanced trials that involve primarily larger, more grove-like plot sizes [20-100 trees] and fewer replicates. Sometimes, we establish “plantings” which are designed to be even more grove-like with larger plots and few replicates. These are as much demonstration projects as experiments and are comprised of well advanced rootstocks. Trials also originate by grower request. In some cases, progressive growers have “out of the box” ideas for trials on their own property that are worthy of pursuit. We vigorously pursue opportunities leading to a broad range of field situations.
Planning. This is where it all starts and communication, timing and coordination come into play.
Lesson 1. Meeting with the cooperator as often as needed to convey and agree on the plan and the contributions of each partner helps reduce misunderstandings later. Discuss dates, who pays for what, harvesting, access, fruit income, field days and a host of other things covered in the UF Material Transfer Agreement (MTA) to protect intellectual property (IP). Among the researchers, good communication is also essential. In our case, with three investigators, each having their own sources of new rootstocks, it can be challenging to assemble a collection of new materials for testing in a timely, coordinated manner. However, as opportunists we are eager to seize offers from interested growers to plant what we can, when we can.
Design. A standard field layout is simple when the space is not restrictive. Typically, there are plots of 1-6 trees on each rootstock, each replicated 3-12 times in a randomized complete-block design, so that site variability is spread across all rootstocks. From this point on, it can become more complicated. What to do when an irregular flatwoods site has double-row beds, each of a different length with obvious soil variability and at different distances relative to drainage features? In other cases, there is space for 15 rootstocks and three standard commercial stocks with nearly equal plot sizes, or somewhat unequal plot sizes and 20 rootstocks with no comparison stocks [checks]. What are the tradeoffs from adding more rootstocks versus evaluating fewer with checks?
Lesson 2. Among the tradeoffs encountered, we often choose more rootstocks even if it results in unequal plot sizes. The time and expense involved in originating and conducting a field trial makes it advisable to test potentially outstanding rootstocks as soon as they are available. Treatment assessment in such a trial then becomes a matter of data analysis combined with the observations of the researchers and growers together, and their ability to assess performance using best professional judgment.
Nursery operations. We occasionally produce the nursery trees at the CREC, but more commonly provide a commercial nursery with seeds and/or liners. Providing the propagation material directly to a nursery invokes several responsibilities. If the plant material is unreleased, we protect the potential of the IP with an MTA with the nursery as well as the field trial cooperator. The nursery owner will be requested to produce perhaps 20 trees of each of 25-50 specific rootstock-scion combinations while continuing the nursery’s regular operations. This requires careful oversight as there can be no mistakes or the integrity of the trial is diminished. Therefore, once a trial is planned, we meet with the nursery owner to explain the project and review timelines and other aspects of the propagation cycle.
Nursey operations are also impacted by HLB. Since pre-selection of rootstock candidates is now focused on ability to mitigate or eliminate HLB from trees grafted with commercial scions, new rootstock hybrids may be selected before source trees are available to provide the seeds needed to establish new trials. Moreover, to significantly expand the rootstock germplasm base, abundant production of nucellar seed is no longer a requirement for a rootstock candidate because there are alternative methods of propagation now available, such as rooted cuttings or tissue culture micropropagation. This adds another layer of complexity to producing adequate numbers and uniformity of trees for advanced trials.
Lesson 3. Work closely and regularly with nursery and/or micropropagation company personnel to avoid mistakes in such critical activities as labeling and inventory. In planning, add 10-30% extra rootstocks and plants on each rootstock because shortfalls will occur. Many rootstocks, especially at the screening level, have unknown nursery characteristics. Some rootstocks do not produce useable trees at the end of the propagation cycle or the required number for each rootstock is not delivered.
Field operations. An absolute requirement for field trials is accuracy. Once that is established and maintained during propagation, the next steps requiring close attention are movement from the nursery, planting and mapping.
Lesson 4. As with propagation, we insure in advance that the cooperator knows what is expected during planting and that it is likely to be a slower operation for the commercial planter, since we need to track identity of each tree and planting space. Our folks oversee each planting to ensure planned designs are implemented. We make adjustments to the planting map as changes occur and then verify the whole map as soon afterward as possible.
Lesson 5. How many trials for a given set of rootstocks? There are four aspects to that question:  when a trial is planned, should it be repeated in each major growing region of the State? Our pre-HLB research has shown that a rootstock’s attributes in one trial at one place remain largely the same when the same rootstock is included in trials elsewhere in Florida. However;  we have observed that the variability in HLB severity in trials from site to site, or HLB interaction with other site variables, means the best assessments of rootstocks occur when excellent performance is repeated in multiple trials;  HLB further complicates the picture as different scions have different levels of HLB tolerance, and a rootstock proven to work for Valencia might not work for Hamlin, grapefruit or Murcott. Thus, rootstock candidates showing exceptional promise with any sweet orange should also be trialed with these more challenging scions; and  the nutrition program has a profound interaction with scion/rootstock combination suggesting that additional trials may be required to fully understand the potential of any new rootstock.
Data. One of the beauties of a cooperative field trial is that most data aside from yield can be obtained at a time selected by the researcher. The main data are yield, juice quality, tree size [height and canopy volume] and overall health [tolerance to HLB and other factors]. Other information might be gathered from a soil survey, measuring leaf nutrient concentration, and noting precocity. In all trials, distinguishing greening-affected from unaffected fruit is important.
Lesson 6. In a grower-trial where harvesting is to be done by a commercial crew, we design with harvesting as a primary consideration. For example, in one of our young trials on 10 acres, the rows are 79 spaces long. A plot is 1/2 of a row randomly repeated 3 or 4 times. This trial is formally designed, but for easy commercial harvest with yield recorded by the harvesting company. A statistically elegant trial is of no value if we cannot efficiently harvest fruit and collect valid yield data.
Trial duration. Data are collected annually beginning when the trees start producing fruit [2-4 years after planting]. Those early years are more important than generally recognized. Our experience has shown that the relative performance among rootstocks that presents itself in the first few years often continues through subsequent years. In our long-term trials where we collected 10 or more consecutive years of yield data, the results consistently show that the yields among rootstocks after 4 years of data are highly correlated to the all-years data set.
Lesson 7. The notion that field trial data are of little value in the early years and trials must run for 15 years is outdated. Begin data collection at age 2-4 years, collect 4-6 years of yield data and select the winners. However, that scenario does not mean terminating a trial early, but rather placing it in the inactive category, recognizing that rootstock long-term behavior may reveal additional useful observations and data about such things as HLB, blight, delayed incompatibilities and response to unusual weather events. Our point is that the first 8 years may tell the major portion of a trial’s story. Meanwhile,the cooperator continues to harvest the fruit rather than replace the trial.
Lesson 8. Trial duration also encompasses the idea that after the first few years, it may be evident that certain rootstocks were underperforming and those trees could be removed. The space created is an opportunity to establish a second trial embedded within the first trial.
Lessons 7 and 8 illustrate the issue of best use of resources particularly given the current urgency imposed by HLB in generating scion and rootstock assessments. What is the tradeoff between a single long-term trial versus several shorter-term ones?
Data interpretation. The bottom line with all field data is financial analysis. This was evident in examination of our long-term trials. They were mostly conducted before the HLB era, but were affected by blight. In an Indian River fresh fruit trial with ‘Marsh’ grapefruit, all fruit in the trial were sized in the field for three successive years. Seasonal FOB prices were used to compare rootstocks. The financial outcome among rootstocks was primarily determined by one factor: YIELD. The same conclusion was reached in every instance for trials of Valencia or Hamlin orange grown for juice. We also recognize that rootstocks significantly affect tree size, leading to tree spacing considerations as one factor in maximizing grove income. Thus, YIELD EFFICIENCY [an index of the amount of fruit/unit of tree size or grove space occupied by the tree] can be a useful calculation to predict the full economic potential of any scion/rootstock combination. This concept can be used to estimate optimum planting density from trials where all trees are planted at one spacing.
How and when do we declare any rootstock candidate to be a winner? The rootstock development cycle, from creation to 8 years in the field to release, is presently about 10-15 years; however, if multiple trials are involved, then the time required is increased. The criteria used to advance new selections through the system are commercially based. Rootstocks for juice fruit are assessed on pounds-solids/acre. For fresh-market fruit, rootstocks are evaluated on the yield of flavorful, high quality fruit of profitable sizes. However, the environment in which those criteria are employed has changed largely because of HLB. New rootstocks are formally evaluated over a shorter period of time because of the urgency imposed by HLB. As a result, the distribution of performance risk has shifted with a greater portion now assumed by the grower with the release of a new selection. New rootstocks are released with smaller data sets meaning they may not have been evaluated over 15 years with all commercial scions under a range of site or management conditions. Today, in our system, we identify candidates for release essentially at any time or place in the assessment process as long as there is evidence of superior, consistent performance. Such an approach requires responsibility to insure that data and observations are sufficient to support growers’ long-term financial investment decisions regarding rootstock choices. Balancing responsibility with urgency is a constant challenge.
We have always viewed the last step, the final experiment, in the rootstock development experimental pipeline to be what happens after release, when the industry begins to propagate and plant, and gains real world experience with the host of rootstocks that have been made available. This actually is the true nature of fruit tree breeding reality. Of the thousands of new cultivars of apples, pears, citrus, peaches, name-your-fruit, that have ever been released, most were released by breeders based on the best data they could generate over time, with the expectation and hope that they would be commercially successful. Yet of these literally thousands of released cultivars, an extremely small number actually achieve commercial significance and success.
Grower interactions. In one sense, all trials are public enterprises. It has been and remains our desire to share resulting information. Many growers who were acquainted with us and the work we were doing, would frequently call or visit with us, to discuss progress and results. We have come to recognize that even though that is the best way for us to communicate with growers, not everyone has the time or inclination to engage personally, and that a better mechanism accessible to all was needed for broad access to trial information.
Lesson 9. Our new website https://citrusresearch.ifas.ufl.edu/rootstock-field-data/ provides the grower community with user-friendly access to our collective decades of rootstock trial data, and we must remain committed to expanding and improving what is there.
Bottom line. Past, present and future citrus rootstock field trials have only one goal: Help our industry identify and exploit superior rootstocks. The goal is simple: productive trees that survive with sustained profitability. Built into that goal is the concept that further improvements always are possible and, indeed, actively pursued and tested.
Many thanks to the diligent nursery owners and progressive grower-cooperators who have been the mainstay of our field program.
Bill Castle received his B.S. degree from Rutgers University followed by 3 years in the U.S. Army. He attended the University of Florida, completing his Ph.D. degree in 1974 followed by a career in rootstock research at the UF/IFAS Citrus Research and Education Center (CREC) on the plant improvement team along with fellow authors, Drs. Gmitter and Grosser. He retired in 2009 as Professor Emeritus.
Fred Gmitter received B.A. and M.S degrees from Rutgers-The State University of New Jersey, and completed his Ph.D. degree in 1985 at the University of Florida. He has focused on citrus scion and rootstock breeding in collaboration with Drs. Castle and Grosser, beginning in 1985. In addition, he has conducted research in citrus genetics and genomics, to increase the knowledge base and to improve the likelihoods of success in achieving citrus genetic improvement objectives through more efficient breeding approaches.
Jude Grosser received his B.A. degree from Thomas More College, M.S. degree from Morehead State University, both in Kentucky, and his Ph.D. degree in 1984 from the University of Kentucky. He began his career at the UF/IFAS CREC in 1984, focusing on adapting emerging plant biotechnologies to citrus improvement. Dr. Grosser joined the citrus improvement team with Drs. Castle and Gmitter, with the role of developing and integrating relevant biotechnologies with conventional breeding, including somaclonal variation, somatic hybridization and cybridization, ploidy manipulation, embryo rescue, and genetic transformation.