By Ute Albrecht, Sudip Kunwar and Jude Grosser
Prior to 1865, the only rootstocks used in Florida were sour orange and sweet orange. Use of grafted trees instead of seedling trees became necessary because of the devastating effects of phytophthora on sweet orange. Sour orange was widely used as a rootstock because of its resistance to phytophthora and other positive attributes, until trees started to decline because of the citrus tristeza virus which is still widespread in Florida.
Other diseases and events have shaped the citrus industry and rootstock preference in Florida. Florida Department of Agriculture and Consumer Services Division of Plant Industry budwood records document that from 1953 to 1974, sour orange and rough lemon were the most propagated rootstocks in Florida, followed by Carrizo/Kuharske (1974–1988), Swingle (1988-2018) and most recently US-942.
The rootstock can manipulate a grafted tree in many ways. It can induce tolerance or resistance to soil-borne stresses and diseases and manipulate horticultural attributes such as tree size, yield and fruit quality. Using superior rootstocks to manipulate disease tolerance has gained renewed importance with the arrival of HLB in Florida. This has led to the popularity of US-942, a hybrid of Sunki mandarin and trifoliate orange, which shows strong performance across most Florida production areas. Although traditional rootstocks such as Swingle, Kuharske, X-639, sour orange, Volkamer and rough lemon remain among the top 10 rootstocks, newer varieties are gaining in popularity.
In contrast to most other management strategies, use of superior rootstocks can increase profitability at no additional cost. This article summarizes the performance of more than 30 different rootstocks from breeding programs at the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS), University of California, Riverside (UCR) and Institutio Valenciano de Investigaciones Agrarias (IVIA) in Spain in commercial sweet orange field trials.
FLORIDA FIELD TRIALS SPECIFICS
Four field trials were established in 2015 in a collaborative effort between UF/IFAS Citrus Research and Education Center breeders and Lykes Brothers Inc. The trials are located near Fort Basinger (Highlands County) and Lake Wales (Polk County) and encompass a total area of about 80 acres (Figure 1). Each location contains both Valencia and Hamlin trials.
Each trial is arranged in replicated plots of eight trees for each rootstock with a one-tree gap between each plot. Tree spacing is 8 by 25 feet at Fort Basinger and 8 by 22 feet at Lake Wales. The Fort Basinger trial site consists of two-row beds and furrows, typical for the heavier flatwood-type soil at this location. The Lake Wales trial site is a typical Central Ridge site with a well-drained sandy soil.
The rootstocks were all propagated from seeds. Some of the UF/IFAS rootstocks (UFR-1, UFR-2, UFR-3, UFR-4, UFR-5, UFR-6 and UFR-17) and the Californian rootstocks (C-22, C-54, C-57, C-146 and X-639) are available for commercial use in Florida. Others are still experimental or only available for commercial use in Spain. All Californian and Spanish rootstocks are hybrids of mandarin (Sunki, Cleopatra or King mandarin) and trifoliate orange. The UF/IFAS varieties contain sexual and somatic hybrids and include a large array of different genotypes such as Nova mandarin, Changsha mandarin, Cleopatra mandarin, Succari orange, Hirado Buntan pummelo, sour orange, Benton citrange, Carrizo citrange and trifoliate orange. The rootstocks US-897, Swingle and sour orange are included in some of the trials for comparison.
ROOTSTOCK TRIAL RESULTS
During both the 2018/19 and the 2019/20 season, substantial differences in size and health of trees on different rootstocks across all trials were observed. Trees on some rootstocks were large, had a dense canopy and were undisturbed by HLB (Figure 2). Trees on other rootstocks were compact in size but looked healthy, although the impact of Hurricane Irma was noticeable for some of the trees. Other trees were of medium size and with varying degrees of health.
The average yield in the Hamlin trials was low (18.9 pounds/per tree) during the 2018/19 season because of the spread of canker through the grove after Hurricane Irma. Less canker was observed during 2019/20, and the average yield was 29.3 pounds/tree. However, preharvest fruit drop was unusually high in this season.
The cumulative yields across both production seasons and tree sizes for the Hamlin trials are shown in Table 1. Although there were differences among locations, when averaged, cumulative yields were largest in Hamlin trees on FA-5, UFR-5, C-54 and X-639. These trees were also among the largest trees in the trials. Yield efficiency (pounds of fruit per canopy volume) was highest in trees on Green 3, Amb+Benton and Changsha+Benton, which were among the smallest-tree-size-inducing rootstocks.
Yield and tree size were assessed in the 2018/19 season for the Valencia trials and are shown in Table 2. Valencia data for the 2019/20 season were not available at the time of this writing. The average yield across both locations was 28.3 pounds/tree. Although there were differences among rootstocks depending on the location, C-22 and C-146 produced the largest average amount of fruit per tree despite producing some of the smallest trees. The lowest yields were measured for X-639 and UFR-2. The yield efficiency was greatest for Changsha+Benton and C-22. Interestingly, trees on US-897, which is considered a small-tree-size-inducing rootstock, were among the larger trees in both Hamlin and Valencia trials and among the largest-yield-inducing rootstocks.
Fruit quality was influenced by the rootstock in all trials. In general, fruit from trees on small-size-inducing rootstocks tend to produce a larger amount of soluble solids than fruit from trees on large-size-inducing rootstocks.
Tree size and yield efficiency are important factors for determining planting density and considering long-term profitability. Trees on small-size-inducing rootstocks with a high yield efficiency can be planted at high density and will result in greater early returns. Trees on large-size-inducing rootstocks should be planted at lower density and may not produce the desired yields during the early production years.
However, because of the high vigor of these rootstocks, it is probable that these trees will cope better with HLB and therefore be more profitable in the longer term than trees on some of the small-size-inducing rootstocks. Another drawback of very small-size-inducing rootstocks may be their greater vulnerability to wind-induced uprooting.
More years of evaluation are needed to determine which rootstocks perform best and to determine the economics of tree vigor, planting density and longer-term profitability.
Acknowledgments: The authors thank the Citrus Research and Development Foundation for providing the funds for this project and Lykes Bros. for their continuing support.
Ute Albrecht (firstname.lastname@example.org) is assistant professor and Sudip Kunwar is a graduate student, both at the UF/IFAS Southwest Florida Research and Education Center in Immokalee. Jude Grosser is a professor at the UF/IFAS Citrus Research and Education Center in Lake Alfred.
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