By Bill Castle
Today, at least three things seem to be true about Florida citrus rootstock field trials:
- There is an unprecedented number of them underway or soon to be planted. The sponsors are many, including the University of Florida, U.S. Department of Agriculture, Citrus Research and Development Foundation, Citrus Research and Field Trial Foundation and the Multi-Agency Coordination Group. Thus, the field has literally become quite crowded.
- The interest in a closer examination of all trial data has increased.
- Endemic field conditions such as HLB, ownership changes and development have made it more challenging to conduct a trial long enough to collect useful data.
Because there are so many rootstock field trials now across a broad spectrum of rootstocks, it is likely that comparing and interpreting the results will also become challenging. Therefore, it is timely to introduce an overlooked but key performance factor in evaluating rootstocks: consistency.
Trees on a given rootstock can perform consistently well, consistently poorly or respond to specific site conditions. In this discussion, only those that perform consistently well are being considered. Further, it should be noted that current and future trial data from Florida likely reflect performance in the presence of HLB, but the principle would be the same if comparisons were among healthy trees.
Consistency is a relative and absolute concept. As an example of relative consistency, consider a properly designed, replicated trial with Valencia scion and Rootstocks 1–10 in a trial on the Ridge. After collecting 6 years of good yield and juice quality data, statistical analysis indicated that Rootstock 5 (R-5) stood alone at the top of the list in pounds solids per acre. In another trial elsewhere in Florida, R-5 ranked third. In another trial, R-5 ranked fourth, but not statistically different from the top three rootstocks.
Such outcomes are what actually happen in rootstock field trials not because there is anything wrong with the trials or with R-5. They happen because inherent variability is a natural part of complex biological systems like citrus trees, and it is rare that field trials involve exactly the same sets of rootstocks over a range of sites and cultural operations. Thus, R-5 would likely be compared to different rootstocks in each trial with perhaps some common standards. In the first trial above, R-5 could have been compared to nine other average, mediocre rootstocks. In the other trials, there might have been a different mixture of rootstocks leading to the shifting position of R-5 among them. That is the relative aspect of consistency.
The relative component of consistency does not, however, create a dilemma because the absolute performance of a rootstock also has meaning and is arguably more important. In all the trials above, what if the trees on R-5 always produced more or less 7 pounds solids per box and yields of 300 to 400 boxes per acre, regardless of what the other rootstocks produced? That absolute performance would be sufficient to continue treating R-5 as a promising selection.
Another way to assess rootstock consistency is to include in every trial the same common standards like Swingle citrumelo and US-942. In doing so, the standard rootstocks serve as a kind of internal check. The performance and ranking of these standard trees will naturally vary because of site and other factors. However, if the difference between a common rootstock and an experimental one remains steady from trial to trial, then that strengthens the characterization of the experimental tree’s performance. But including common standards comes with a cost. Those trees take up space that could be used for other experimental rootstocks, a design consideration especially important at the initial screening stages of rootstock development.
It is prudent to interpret rootstock data with an eye on both relative and absolute performance to grasp the full measure of a rootstock. When HLB is factored into this scenario, consistency of performance may be even more critical. Tree response to HLB is less predictable in any context. Thus, a consistently tolerant rootstock (no matter how tolerance is defined) may be particularly noteworthy.
Bill Castle is an emeritus professor at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.
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