By Bill Castle, Fred Gmitter and Jude Grosser
The University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) 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 UF/IFAS trials are conducted cooperatively with citrus growers. When public and private sites are viewed together, a broad range of circumstances are encountered that call for flexibility in designing and managing those trials.
Here are a few tips for a successful nursery-grower-researcher field trial partnership:
OVER-PLAN PROPAGATION
Virtually all trials involve new rootstocks for which their nursery performance is not entirely known or predictable. Therefore, propagation often begins by including a few “extra” rootstocks just in case some do not germinate well and fail to yield the number of trees required for the field trial. Likewise, plans are often made to produce extra trees of particular combinations to ensure the number of needed trees. If at the end of the propagation cycle there are more trees than required, there usually is space in the field trial site where those trees can be planted and used during the first couple of years as replants.
NURSERY OPERATIONS
Occasionally, the nursery trees are produced at the CREC, but more commonly a commercial nursery is provided 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 intellectual property with a material transfer agreement 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 to 50 specific rootstock-scion combinations while continuing the nursery’s regular operations. This requires careful oversight. 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 preselection 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.
PLANNING AND ACCURACY
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 to 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 require careful rogueing to remove zygotic off-types, and some do not produce useable trees at the end of the propagation cycle or the required number for each rootstock is not delivered.
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.
MANAGING TIME
As with propagation, we ensure 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 researchers need to track identity of each tree and planting space. Researchers oversee each planting to ensure planned designs are implemented. They adjust the planting map as changes occur and then verify the whole map as soon afterward as possible.
The website https://crec.ifas.ufl.edu/citrus-research/rootstock-field-data/ provides growers with user-friendly access to collective decades of UF/IFAS rootstock trial data.
Bill Castle is an emeritus professor; Fred Gmitter and Jude Grosser are professors — all at the UF/IFAS CREC in Lake Alfred.
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