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  • Author or Editor: Sharon Inch x
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A test population consisting of progenies of 92 seed-source genotypes (hereafter called “parent genotypes”) of Citrus and Citrus relatives in the field in east–central Florida was assessed after natural freeze events in the winters of 2010 and 2011. Eight seedlings per parent genotype were planted in a randomized complete block design; however, as a result of mortality, the number of plants assessed in some genotype groups was reduced at some or all sampling dates. The citrus diseases huanglongbing and citrus canker were endemic in the planting and may have influenced tree response to cold temperatures. Unusually low temperatures (near –4 °C each winter) for east–central Florida were experienced during the trial period. Defoliation and dieback were significantly greater in the winter of 2011 than in the winter of 2010. The winter in 2011 was preceded by a period of extraordinarily low temperatures in mid-December with no period of cool temperatures to allow trees to acclimate. In 2010 the average defoliation was 53% ± 28% and less than 13% of the trees exhibited any noticeable dieback, whereas in 2011, the average defoliation and dieback were 93% ± 17% and 51% ± 35%, respectively. Within the genus Citrus, several progenies were identified that had 16% to 24% dieback in 2011 and these were from parent genotypes C. reticulata (CRC 2590) (23%), C. sinensis (CRC 3858) (24%), C. maxima (CRC 3945) (16%), C. hassaku (CRC 3907 and 3942) (16% and 17%), C. aurantium (CRC 628 and 2717) (18% and 7%), C. taiwanica (CRC 2588) (21%), and C. neo-aurantium (C. obovoidea + C. unshiu graft chimera) (CRC 3816) (23%). Within other genera in the Aurantiodeae, Poncirus trifoliata (CRCs 301, 3957, 3549, and 4007), Severinia buxifolia (CRC 1497), Bergera koenigii (CRC 3165), and Glycosmis pentaphylla (CRC 3285) had the least amount of dieback, all at less than 23%. The two species within the Toddalioideae subfamily of the Rutaceae (Casimiroa edulis and Zanthoxylum ailanthoides) had among the least amount of dieback (1% and 8%, respectively). When considered by groups, the Citrons and Australian natives had the greatest amount of dieback in 2011, 68% and 65%, respectively. The trifoliates (Poncirus and hybrids) had the least dieback, ranging from 4% to 40%. The information from this study may be useful in germplasm enhancement and Citrus breeding targeting greater cold tolerance.

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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.

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