Fruitful Feedback From Florida Citrus Rootstock Survey

UFR-4 Vernia citrus rootstock

According to a recent informal survey sent out to select Florida citrus growers, rootstock diversity (for some respondents) was driven by availability. Photo by Peter Chaires

Coffee shop talk, citrus field days, Extension programs, and trade press articles have increasingly focused on tolerance levels of various citrus scion cultivars. What will remain productive in an HLB-endemic environment and provide the return on investment? Growers give weighty consideration to what they are ordering, and nurseries are often hard-pressed to make concrete recommendations until field experience and data combine to substantiate what is currently anecdotal observation with a spicy side of gut hunch. However, when you factor in the rootstock dilemma, the complexity of the conundrum increases exponentially.

As Nate Jameson, Co-Owner of Brite Leaf Citrus Nursery, revealed in his presentation at the Annual Citrus Industry Conference in Bonita Springs, several of the old mainstay Florida rootstocks (such as Swingle) are maintaining their position in the top five, while other newcomers are creeping into the rootstock popularity contest. Availability of seed and tissue culture liners, timing of plantings, nursery capacity, location, conditions of the planting, performance of specific rootstock/scion combinations, and many more factors, are now on the table. However, another consideration has invaded the industry consciousness. The question has been posed by scientists, growers, and nurserymen.

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When it becomes clear which rootstock best addresses the need, should there be any concern about overuse or over commitment to one rootstock?

While Florida’s industry is likely too diverse to establish a true statewide rootstock monoculture, we have seen other industries and companies committing 60% to 90% of their production to one rootstock, only to later discover susceptibility. This has been a challenge with Citrus tristeza virus, sudden death, flood, cold, blight, nematodes, etc.

Should a grower have any concern about committing a large percentage of production to one rootstock? How many times have we heard “I’m all in with US 942, Swingle, Sour Orange, etc.?” What is a reasonable volume threshold? Does it matter?

I reached out to a diverse audience to seek some feedback on this specific issue. While the results are not solidly conclusive, they are interesting and revealing. Here is a condensed summary of what we gleaned.

If you had information from multiple independent sources that clearly indicated one particular rootstock was going to produce the healthiest trees for a given area, and trees that produced the most and highest-quality fruit over many years, what percentage of your citrus acreage would you ultimately put on that one rootstock?

Most respondents made the case that no rootstock works for all circumstances. Even large, contiguous plantings will have variations that call for some diversity of rootstock selection.

Most respondents replied candidly that risk abatement was not a significant factor in their current decision process. However, for most it seemed logical, for the same area and same conditions, to ultimately avoid having more than 50% on one stock. The lessons learned with sour and lemon rootstocks, and seeing other production areas go through similar situations, was impactful. It makes sense to not let history repeat itself.

A smaller but significant group would plant up to 75% on one rootstock. The rationale is that it is so difficult to find the right stock in our HLB environment that once found, they would have a greater propensity to plant larger percentages of their acreage on that one stock. With HLB, they viewed a strong performing rootstock as less risk than diversity for the purpose of avoiding an unknown.

A few would go 100% with anything showing consistent performance in our current environment — anywhere the growing conditions would warrant.

Two growers made the point that most of the scions they are investing in have no appreciable track record, so they are going with two to three rootstocks to spread risk until more is known. When older, familiar scions are being planted (such as ‘Valencia’), their answers were more similar to those above.

Reality check. For some, diversity was driven by availability. They have committed acreage to second- and third-choice rootstocks for this reason. They spent a lot of time determining the best stocks for their situation and then ended up planting what they didn’t want — because it was available, and their timeline would not allow them to wait on the preferred rootstock.

How much of your acreage (in a given area) would you plant on a genetically different, less durable, and less productive rootstock, to protect against the possibility of a future susceptibility (diversity to spread or abate risk)?

Most clarified that even to consider this at all, the difference in performance of the alternate stock could not be significant. Input and production costs would not allow more than a 15% to 20% difference.

About half of the respondents rejected this as a possibility and would never knowingly do this for protection against an unknown.

Other responses ranged from “some” to about one-third of the acreage. Most said that the other one-third might be divided among other rootstocks.

Interesting side note. Though this was not the specific question asked, three respondents said they would plant with a rootstock that had lower productivity and fruit quality if it adjusted their maturity enough to get a fresh fruit variety into a high-value market window.

What drives growers to diversify rootstocks? Is it generally because they are concerned about overcommitting to one or two rootstocks — for the risk-based reasons previously stated — or a lack of confidence due to lack of clear and convincing information about new rootstocks?

I was forewarned by someone much wiser than I — there is no answer to this question. Apparently, there is not. Replies were all over the board.

For some, it is purely a weighing of horticultural and economic performance. They will not commit on any scale until the process of data collection can be satisfactorily completed. This reply was not driven purely by scale. This was the answer of small and large operations.

For many, it is the desire to find something better than what they currently have in production and, as availability will allow, they will plant small quantities for testing and observation.

For others, trial plantings within existing commercial groves adds complexity that they won’t tolerate.

The predominant answer to this question was the diversity of new scions — and a general lack of confidence in data available for new and old rootstocks used with these scions — and the combinations thereof, is driving diversity of planting, not for the sake of diversity (i.e., fear of increased susceptibility), but rather by the need to find the best combination for each set of conditions as fast as possible.

Are growers willing to make rootstock decisions based on their own observations and trials, or are they more likely to want formal research data?

The vast majority of respondents would much prefer to make rootstock decisions based on formal trials and a bank of data from a range of locations and conditions. However, until that data is available for the new processed and fresh-market cultivars, growers will continue to diversify their plantings, talk to neighbors, query researchers and Extension specialists, etc.

Approximately one-third of respondents prefer to make rootstock decisions on the basis of their own observations and/or data. But due to the large number of rootstock and scion cultivars available, this is becoming increasingly impractical. They simply lack the resources and time to trial all of the possibilities. More public and diversely replicated trials are needed for fresh and processed citrus.

If pressed to condense the input from these surveys to get to the heart of whether growers are concerned about increased risk from heavy use of just a few rootstocks, the answer is no. There are too many other rootstock-related concerns in an HLB world. Growers are concerned with new rootstock/scion combinations, market considerations, fruit quality, stretching maturity ranges, etc. Whether from formal trials or private observations, the market and economics seem to be driving the bus. Will the tree maintain canopy, hold fruit, and produce an acceptable quantity of reasonable quality? For most, diversity of rootstocks to mitigate risk appears to be a decision point for another day. It’s a luxury that they don’t currently have.

Their immediate concern is to tackle or manage HLB and become more familiar with the varietal options. They will concern themselves with risk abatement through rootstock choice down the road. It’s a known-unknown and will remain so for the foreseeable future.