There’s Hope In Citrus Rootstock Ramp-Up To Head Off HLB

Citrus tissue culture

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Nurseries, growers, and just about everyone in the citrus industry with a pulse have been intrigued, if not excited, about the ability of some new rootstocks to remain productive in the face of HLB. Folks know that more research needs to be done. But the obvious question is: If we do find promising new rootstocks, how can we possibly produce enough nursery plants fast enough to meet industry needs?

In search of answers to this perplexing question, I sought input from industry experts. The UF/IFAS Plant Improvement Team (Bill Castle, Fred Gmitter, and Jude Grosser) was particularly helpful in reviewing options and providing relevant history.

There are essentially three techniques at our disposal for rootstock propagation: seed, rooted cutting, and tissue culture (TC). There are many factors that can influence each of these, so we must make some general assumptions. Additionally, the performance of each technique varies by rootstock. Trifoliate hybrids tend to be easier and faster to produce compared to sour orange types. However, in our emerging world of complex hybrids, these lines of distinction are blurring.

Production From Seed

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  • Citrus seed generally germinate in one to two months (quicker if the seed coat is removed). 
  • It takes approximately three months to produce a liner. 
  • The original source material must be true to type when produced from seed. Some of the new tolerant rootstocks may be monoembryonic — so no seed or seed-to-TC option is available. With these, we would have to go to rooted cuttings.

Production From Rooted Cuttings

  • It takes approximately one month to root a citrus cutting, depending on the species and selection. Sometimes it takes two months. 
  • It takes another month to make a decent liner. 
  • The limiting factor in making a large number of plants is the availability of source material. It takes a lot of mother trees to produce a lot of rooted cuttings. Sufficient mother trees may not always be available. 
  • In cases where rooting cuttings is the best option, there would be one original source tree. As this tree ages, the success of rooting may decline significantly. This is a limiting factor for long-term production.

Production From Tissue Culture

  • There is at least one functioning tissue culture facility in Florida (See “Citrus Nursery Source,” July 2011). There are other tissue culture labs serving other woody plant segments that may have an interest in this opportunity. The level of investment and commitment to tissue culture production will be reflected in the results. 
  • Professional woody plant micropropagation companies can produce thousands of liners per day once they establish a protocol and culture for a particular rootstock. 
  • Each rootstock requires a different media and technique.
  • Companies with experienced and skilled tissue culture personnel have a significant advantage in efficiency and quality. 
  • Scientists consulted on this article believe that a well-organized and managed TC facility would be more productive than a well-run operation doing rooted cuttings especially if seed are used as the starting material.

Other Factors To Be Considered

• Do we have seed source trees that are through their juvenility phase and that are fruiting?
• How much fruit do they produce? How seedy is this fruit?
• If fruit seed trees are available, additional trees can be produced quickly that would fruit relatively fast. If not, large-scale production from seed production is years away.
• Trueness to type of the starting
seed material.
• Are the trees monoembryonic, (e.g., do they normally produce seedlings of asexual origin that are essentially true-to-type)?
• Production from TC and by rooted cuttings produces a tree without a tap root. Though some view citrus as not having a tap root, others disagree. The first root that emerges from a germinating seed is the primary or tap root. Some growers have concern that trees without a tap root will be unable to sustain tropical winds. However, the fact remains that hundreds of trial trees are now in the field showing no obvious differences from trees produced from seed. Studies in South Africa compared trees produced from rooted cuttings to trees from seed. They showed the trees from seed performed better initially. But by year two, there was no longer a significant difference. That being said, the absence of irrigation would increase the importance of a tap root.

All things considered, it is likely that the citrus industry will employ a combination of these techniques to propagate promising new rootstocks. The availability of vegetative source material, availability of seed, and trueness to type from seed will dictate the technique or techniques used. For those rootstock selections where we have only one or two juvenile seed plants available, we may choose root cuttings and use TC. In cases were seed is readily available, it is likely all three techniques will see action.

The ingenuity of the citrus industry never ceases to amaze. If the rootstock is promising, the industry will find a way to produce the liners at the nursery level or through a third-party supplying the nurseries. Stay tuned into the latest rootstock research from UF/IFAS and USDA-ARS. New trials are being planned and will soon yield results.

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