By Gary England, Fred Gmitter and Manjul Dutt
In 2014, most groves in Central Florida had citrus greening infection rates approaching 100%, but a Lake County grower reported that some trees in his Hamlin on Swingle blocks seemed to be relatively healthy. Some visual HLB symptoms were observed on these “surviving” trees, but they did not have the massive fall fruit drop occurring in most Hamlin/Swingle blocks in the area at the time.
Several surveys in an approximately 40-acre Hamlin/Swingle block within this grove revealed there were one to three randomly scattered surviving trees in most rows. As harvest time approached in December 2015, the surviving trees were very evident compared to most of the trees in the block due to their healthier foliage and minimal fruit drop (Figure 1).
HLB continued to drastically reduce fruit yields over the next six years. Many unproductive trees were removed and replaced by Hamlin on some of the newly released tolerant rootstocks. During this time, it was evident that there were numerous randomly scattered surviving trees throughout the block.
This block was replanted to Hamlin on Swingle in 1987 following the devastating freezes of 1983 and 1985. Swingle rootstock was in high demand due to the large amount of replanting. Some growers mention that some trees that were thought to be Swingle were something else. The graft union in most of the surviving trees in the block are like Swingle (Figure 2, top panel).
Analysis on the rootstock DNA of several of the selected survivor trees was conducted at the Citrus Research and Education Center (CREC). Researchers compared the DNA markers in the root tissue of the surviving trees with Swingle and Duncan grapefruit, a parent of the Swingle citrumelo. These markers determined that two trees were identical to Swingle. All the rest, excluding one, were most likely Swingle zygotic seedlings (off types) that originated from self-pollination of the seed source tree. The remaining rootstock was a zygotic seedling that originated by outcrossing Swingle with an undetermined pollen source.
A visual rating of the tested trees was created. Three of the top trees were further evaluated for juice quality and yield compared to a tree that represented most of the original trees (check) remaining in the block. Juice quality tests taken on Dec. 15, 2021, resulted in lower end Brix but good ratios. Harvest completed by a commercial crew on Jan. 28, 2022, resulted in two surviving trees that yielded 5.4 and 8.5 boxes per tree compared to 2.4 boxes for the check. Unfortunately, the third survivor tree was harvested before data could be taken.
Fruit samples from each of the survivor trees were analyzed again at the CREC during December 2022. Pounds solids per box ranged between 5.6 to 6.3 in the best-performing tree. Fruit drop has been observed in the surviving trees. However, yield data collected in December 2022 indicated that the surviving trees produced 6.0 to 9.8 boxes per tree compared to 2.8 boxes per tree from a severely HLB-impaired tree.
Additional evaluation of the scattered surviving trees is needed to learn more about their longevity under endemic HLB pressure. The first challenge is propagating shoots that will become rootstock plants evaluated for HLB tolerance from the root system of the surviving trees without sacrificing the trees. In the short term, rooted cuttings can be budded with the Hamlin budwood and planted for evaluation in a well replicated manner. In the long term, these recovered rootstocks can be entered into the Division of Plant Industry’s Parent Tree Program and certified materials made available to the citrus nursery industry, should replicated trials confirm the hypothesized tolerance of the candidate rootstock.
Gary England is a University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) emeritus multicounty Extension agent. Fred Gmitter is a professor, and Manjul Dutt is an assistant professor, both at the UF/IFAS CREC in Lake Alfred.
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