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The citrus disease huanglongbing (HLB) has become endemic in Florida, with estimates that greater than 80% of citrus trees are currently infected. Although there are no commercial citrus varieties with strong HLB resistance, some field tolerance has been observed in trees exposed to the disease after they were mature. There is great urgency to identify citrus which may permit economic citrus production where HLB is endemic. Therefore, the objective of this study was to assess field tolerance to HLB. To expedite the trial due to urgency, nursery trees were purchased on rootstock varieties as available. The trial included the following unbalanced scion/rootstock combinations: ‘Hamlin/ Kinkoji’, ‘Hamlin/Cleopatra’, ‘Temple/Cleopatra’, ‘Fallglo/Kinkoji’, ‘Sugar Belle/Sour Orange’, ‘Tango/Kuharske’, and ‘Ruby Red/Kinkoji’, with most comparisons based solely on scion/rootstock combinations. A randomized complete block experiment was established at Fort Pierce, FL, in Sept. 2010. All trees exhibited symptoms of HLB and tested positive for the Candidatus Liberibacter asiaticus (CLas) bacterium by Oct. 2012, with similar titers [directly assessed as cycle threshold (Ct) using quantitative polymerase chain reaction (qPCR)] measured for all scion/rootstocks at most sample dates, but early titer development in ‘Ruby Red/Kinkoji’ was significantly lower than several other scion/rootstocks. Across all time-points, ‘Fallglo/Kinkoji’ had the lowest rating of distinctive HLB mottling and ‘Ruby Red/Kinkoji’ had the highest rating, but ‘SugarBelle/Sour Orange’ had the highest percentage of leaves affected. After 5 years, ‘SugarBelle/Sour Orange’ and ‘Tango/Kuharske’ had the greatest overall increase in trunk diameter, and were among the healthiest in overall appearance. In Oct. 2015, ‘SugarBelle/Sour Orange’ and ‘Tango/Kuharske’ trees had significantly greater fruit load (80–88 fruit/tree) followed by ‘Temple/Cleopatra’ and ‘Fallglo/Kinkoji’ (31–35 fruit/tree) while ‘Hamlin/Kinkoji’, ‘Hamlin/Cleopatra’, and ‘Ruby Red/Kinkoji’ produced less than 20 fruit per tree. Despite becoming infected by CLas in less than 2 years after planting, the trees continued to grow and all scion/rootstocks displayed increasing fruit production, although very low in ‘Hamlin/Kinkoji’, ‘Hamlin/Cleopatra’, and ‘Ruby Red/Kinkoji’. Growth and fruit production in the highest performing scion/ rootstocks were likely less than would be expected for healthy trees, but these results are promising, with markedly better response of some scion/rootstocks with mandarin hybrid scions compared with trees with sweet orange or grapefruit scions. Larger fully replicated trials are underway. It is noteworthy that the most pronounced HLB symptoms and higher early pathogen titer, which are the two criteria most widely used in assessing HLB resistance, were not associated with the lowest growth and cropping, and focus on early symptomatic traits when screening for resistance may obscure important disease tolerance. The citrus disease HLB has become endemic in Florida. HLB in Florida is associated with the phloem-limited bacterium CLas, which is vectored by the asian citrus psyllid (ACP) (Diaphorina citri) (Bov e, 2006; Hall et al., 2013b). It is estimated that greater than 80% of Florida citrus trees are currently infected with CLas (Singerman, 2015), with many displaying marked declines in production, increases in fruit drop, and somewhat reduced fruit quality. Sweet orange and grapefruit, which constitute >95% of Florida citrus production, appear to be especially compromised by HLB. Largely as a result, Florida citrus production in 2014–15 was the lowest it has been in 50 years (NASS, 2015). Of interest is that some mandarin hybrid cultivars appeared to have substantial tolerance to HLB when trees were exposed to the disease after maturity (Stover and McCollum, 2011; Stover et al., 2015). If tolerance is verified, through sustained growth and economic fruit production when newly planted trees are exposed, it is possible that some Florida citrus growers could opt to transition away from traditional sweet orange and grapefruit cultivars. Therefore, the objective of this studywas to assess tolerance to HLB under field conditions of some scion/rootstocks to gain inferences about possible HLB tolerance. Materials and Methods Although a replicated, full factorial of standards and putatively tolerant scions each on the same rootstock would have been ideal, when this research was started there was no source of scions on the same rootstock and it would have taken several years to generate such plants. However, the following scion/ rootstock combinations were available from commercial nurseries: ‘Temple/Cleopatra’, ‘Fallglo/Kinkoji’, ‘Sugar Belle/Sour Orange’, ‘Tango/Kuharske’, Hamlin/Kinkoji’, ‘Hamlin/Cleopatra’, and ‘Ruby Red/Kinkoji’. The trees (HLB free) were purchased and a randomized complete block experiment was established. Ten trees of each of the seven combinations were regularly treated with imidacloprid, to try to control the insect vector to delay HLB, and ten additional trees of five combinations (‘Hamlin/Kinkoji’, ‘Hamlin/Cleopatra’, ‘Temple/Cleopatra’, ‘Fallglo/Kinkoji’, and ‘Ruby Red/Kinkoji’) were left untreated (only 10 trees of ‘Sugar Belle/Sour Orange’ and ‘Tango/Kuharske’ were available and all were treated with imidacloprid). This experiment is accurately viewed as comparing specific scion/rootstock combinations rather than scions alone and was essentially two overlapping experiments, one in which all imidacloprid trees were compared and another in which scion/rootstock combinations were compared with and without imidacloprid treatment. The trial included 20 trees each of ‘Hamlin/ Kinkoji’, ‘Hamlin/Cleopatra’, ‘Temple/ Cleopatra’, ‘Fallglo/Kinkoji’, and ‘Ruby Red/Kinkoji’ with 10 imidacloprid-treated and 10 nontreated trees of each scion/ rootstock randomizedwithin each of 10 blocks. An additional 10 trees each of ‘Sugar Belle/ Sour Orange’ and ‘Tango/Kuharske’ were included, one tree of each per block and all receiving imidacloprid. Trees were planted at the U.S. Department of Agriculture research farm, Fort Pierce, FL, in Aug. 2010 on tworow beds typical of the Indian River area, with 6.9 m between rows and 1.5 m between trees within a row. Trees not included in the experiment were planted at the ends of rows. This site was Ankona sand and trees were irrigated using microsprinklers that delivered shallow well water of pH 7.2 and 240 mg·L CaCO3 equivalents from a large bladder reservoir (47 million L), which minimizes seasonal variation in water quality. Except for inclusion of enhanced foliar nutritional treatments that are widely used on HLB-affected blocks (Spann et al., 2014), all cultural practices were consistent with typical Florida production of processing citrus before the introduction of HLB. Details of cultural practices were as follows. Fertilization was applied twice each year to the soil. Hand fertilization occurred from planting with 0.3–0.5 kg/tree of slow release 13N– 4P–9K. In Aug. 2013, ground broadcast of 336 kg·ha of 12N–2P–12K plus 3%Mgwas used. In Mar. 2014, ground broadcast was increased to 449 kg·ha of 12N–2P–14K HORTSCIENCE VOL. 51(2) FEBRUARY 2016 127 plus 2.5% Mg. In Feb. 2015, ground broadcast was increased to 561 kg·ha of 12N–2P– 14K plus 2% Mg. Trees were sprayed 24 times per year (every 14 d) with a low volume foliar application of 9.4 L·ha of 435 spray oil, 4.5 kg·ha of 20N–10P–20K complete greenhouse grade soluble fertilizer, and copper at 200 ppm (Magna-Bon LLC, Okeechobee, FL). Four times per year, with two sprays each timed to coincide with spring and summer flush, Ca, Mg, and Fe were added to the 20N–10P–20K. Foliar fertilization was supplemented three times per year with 9.4 L·ha of N-sure 28–0–0 (Tessenderlo Kerley, Phoenix, AZ) and two times per year with KeyPlex 1400 (KeyPlex, Winter Park, FL) at 4.7 L·ha. Imidacloprid (Macho 2.0; Albaugh, LLC, Ankeny, IA) was applied at 1.89 mL/tree/ application in 240 mL water, three times per year from late March to late September. This was the labeled rate for trees at a commercially typical 210–420 trees/ha, even though experimental trees were at 919 trees/ha. Tree height, canopy width, trunk diameter, fruit number, HLB phenotypic symptoms, and tree mortality were assessed monthly to quarterly beginning 20 months after planting. Tree canopy volume (TCV) was assessed using the assumption that tree shape was one-half a prolate spheroid (TCV = 4p/6 · H · W1 · W2). Disease symptom severity was assessed on a 4-point scale for mottle (0 = healthy, 1 = pale mottling, 2 = mottle but symmetric, and 3 = blotchy mottle), yellow shoots (0 = healthy green, 1 = slightly pale, 2 = yellow, and 3 = very yellow), and rugose leaves (0 = healthy, 1 = slightly leathery, 2 = fairly leathery, and 3 = very leathery), with each tree scored based on the most severe symptoms on the tree for each category. In Feb. and Oct. 2015, trees were also scored for percentage of leaves displaying blotchy mottle. The number of fruit per tree was assessed in Nov. 2014 and Oct. 2015. Monthly to quarterly, we also collected three of the most symptomatic leaves from each tree for qPCR. All leaf samples were stored at –4 C until processed. For each tree, the lower 2 cm from the leaf petiole base into the midrib were pooled. Each pooled sample was macerated in a Fast Prep 24 (MP Biomedicals, Santa Ana, CA)with DNA extracted from 200 mg of macerated tissue using cetyl trimethylammonium bromide (CTAB) extraction followed by column clean up and concentration (Genesee Scientific, San Diego, CA). One milligram fresh weight of petiole and midrib tissue yielded 1 ngDNA. Each sample was assayed for CLas and citrus dehydrin via qPCR using an ABI 7500 thermocycler (Applied Biosystems, Foster City, CA). Each 25 mL qPCR reaction was conducted using 100 ng of DNA template with primers (Stover et al., 2014) specific to 16s ribosomal DN

Conventional Citrus of Some Scion/Rootstock Combinations Show Field Tolerance under High Huanglongbing Disease Pressure