The U.S. Department of Agriculture’s (USDA) U.S. Horticultural Research Laboratory in Fort Pierce, Florida, has been home to a citrus rootstock and variety breeding program for many years. Kim Bowman, a research geneticist, has been with the program for more than 30 years. More recently, Matthew Mattia, a geneticist, joined the breeding team. They provide an update on the program and new rootstocks and varieties that should be considered.
(Photos by Frank Giles)
What have been some of the key areas of emphasis in your breeding program at USDA?
Bowman: I have been working in the USDA citrus breeding program in Florida since 1992 and have focused mostly on the development of superior new citrus rootstocks for Florida. Initially, the new rootstock hybrids are evaluated to identify the most promising for propagation and then are entered into replicated field trials with Valencia and Hamlin scions. We now have about 400 new hybrid rootstocks distributed among 30 different field trials. From these, we collect information on tree growth, health, fruit production and fruit quality.
If new hybrid rootstocks exhibit outstanding performance in several field trials over multiple years, they are released for commercial use. Rootstocks released by our program are all identified with the prefix “US” and now offer some good options for growing citrus in Florida.
An exciting new component of our rootstock program is the development of genetic maps and molecular markers to allow genomic selection for superior new rootstocks. This is a new tool being brought online now that will greatly accelerate and improve the progress in the development of superior new rootstocks for the future.
Mattia: I have been with the USDA since March of 2021. During my time here, I have been active in evaluating a broad diversity of existing USDA hybrids in the field, as well as taking steps to modernize the scion breeding program.
We believe that a thorough understanding of the plant material is crucial for achieving our breeding objectives, since citrus is a crop that presents unique complexities. While genetic tools can be helpful, they are only effective when coupled with a deep knowledge of the citrus germplasm and the traits that are important in each commodity group (such as sweet orange, grapefruit and mandarin). Without this understanding, genetic tools alone cannot fulfill our goals.
To address the needs of our stakeholders and overcome the complexities associated with genetic and plant improvement in citrus crops, we have developed specific pipelines that focus on each of the commodity groups important in the Florida citrus industry. These pipelines are tailored to the unique requirements of each commodity within the citrus family. We recognize that citrus scion breeding is distinct from breeding other crop species, and it necessitates a combination of art and science. By combining our expertise in citrus genetics, plant improvement techniques and a comprehensive understanding of the crop itself, we strive to develop improved citrus varieties that meet the demands of our Florida stakeholders and contribute to the overall advancement of the citrus industry.
What are some of the USDA rootstocks that are already available to growers?
Bowman: Five new rootstocks that are available to Florida growers offer good HLB-tolerant options for new plantings. US-942 and US-812 are two of the most well-known. They provide consistent, good performance from a standard-sized tree with many scions and under a broad range of soil conditions. US-942 has been the most used rootstock for new propagations in Florida over the last five years, and for good reason. US-802 and US-1516 are good choices for a situation where a more vigorous tree is desired. These two rootstocks have performed well in a wide range of soils. US-897 is a semi-dwarfing rootstock that induces good yield and fruit quality on a smaller tree.
What are some of the USDA rootstocks that are in the development pipeline that appear the most promising?
Bowman: Several new rootstocks look very exciting for the future. Three of them are already released and available in limited number to growers from some nurseries. US-1283, US SuperSour 2 and US SuperSour 3 have looked outstanding in trials. They provide good fruit quality with fruit yield that is superior to the normal commercial standard rootstocks. As commercial availability improves, I expect these three will become widely used.
Two other new hybrids that were just selected for release (and will probably be named SuperSour 4 and 5) also look great in field trials and have demonstrated outstanding multi-season yields with Valencia and Hamlin scions. They have demonstrated good performance on both Flatwoods and Ridge sites. I hope that these two will be approved for release soon and can be available for use by nurseries later this year.
What are some new fresh and processed USDA varieties that growers should consider?
Mattia: When it comes to new fresh and processed USDA varieties that appear promising, there are a few options available and some currently in the pipeline. Let’s look at three:
US Superna mandarin variety has been released and is known for its exceptional fruit taste maturing in December. It is a cross between Lee and Nova, offering low seed content and easy peeling. It has shown tolerance to HLB in Florida plantings and is beginning to be used for commercial plantings in California. There are some small, replicated trials at the A.H. Whitmore Foundation Farm where trees range from three to seven years old. The oldest trees on the farm are grafted on US-812 and seem to crop well on that rootstock.
In the pipeline, we have FF-1-43-1 and FF-1-34-2.
FF-1-43-1 is very much like a sweet orange. The female parent is a sweet orange, and it tastes like an orange. It is an early- to mid-season variety, appears to be tolerant to HLB and has produced a heavy fruit crop. However, information about its processing capabilities is currently unknown.
FF-1-34-2 is a cross between Fortune and Kishu and belongs to the seedless mandarin category. It is easy to peel and boasts a large fruit size of approximately 3.25 inches by 2.75 inches. It has good flavor and is typically available from November to December. The tree shows few HLB symptoms, has dark green leaves and a high fruit yield. A little dieback has been observed on its own roots, but it still produces a heavy crop.
The above varieties offer different characteristics and benefits for growers to consider. It’s important to evaluate them based on specific needs, regional conditions and market demands to determine their suitability for cultivation and utilization in fresh or processed forms.
How important is the A.H. Whitmore Foundation Farm in Florida citrus breeding efforts?
Bowman and Mattia: The Whitmore Farm plays a crucial role in Florida citrus breeding efforts and is an invaluable resource for the development of superior citrus varieties and rootstocks. It holds more than 100 years’ worth of unique plant material that is not found in other breeding programs across the United States. This wealth of genetic diversity is vital for creating new varieties with desirable traits.
As the primary farm for making citrus scion selections, the Whitmore Farm allows researchers to assess fruit quality and other important characteristics of various citrus varieties. This enables the identification and advancement of superior varieties that are well-suited to meet the needs of both the Florida processed and fresh industries.
Furthermore, the Florida Citrus Research Foundation, which owns the Whitmore Farm, contributes significantly to the breeding efforts. The foundation provides valuable grower insight and directs commercial research, ensuring that the breeding program aligns with the practical needs and preferences of citrus growers in Florida.
What is the progress on learning more about the Donaldson tree and duplicating it?
Mattia: The Donaldson tree, which was identified at the Whitmore Farm, has been the subject of extensive study. We have gathered as much data as possible on this single tree, focusing on various phenotypic traits such as fruit count, canopy density, leaf area, canopy volume, fruit color and fruit quality. To confirm the rootstock of the Donaldson tree, SSR markers were utilized by Dr. Bowman, and it was determined that the rootstock is the normal commercial Swingle citrumelo.
In terms of fruit processing, the fruit from the Donaldson tree has been commercially squeezed, pasteurized and finished using JBT equipment. Additionally, chemical analysis and taste panels are currently underway, conducted by USDA colleagues Jinhe Bai and Anne Plotto. These analyses will provide valuable information regarding the chemical composition and sensory attributes of the fruit.
We are committed to sharing the findings of our research, and a publication detailing the results will be submitted this fall. The publication will contribute to the broader knowledge base of citrus research and highlight the progress made in understanding the characteristics of the Donaldson tree.
Considering the long-term nature of growing citrus, we have propagated more trees from the Donaldson tree as quickly as possible. A replicated field trial, involving both certified and uncertified Donaldson trees, will be planted this summer. This trial will allow for further evaluation and comparison of the Donaldson performance, providing valuable insights into its potential for commercial cultivation. By conducting thorough research and sharing our findings, we aim to contribute to the advancement of citrus knowledge and potentially introduce superior varieties that benefit the citrus industry.
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