Evaluation of Commercial Citrus Cultivars for Field Tolerance/Resistance to Huanglongbing in East Central Florida

Citrus huanglongbing (HLB) is caused by a phloem-limited gram-negative bacterium, Candidatus Liberibacter asiaticus (CLas). Although there are no HLB-resistant commercial citrus varieties, some resistance or field tolerance to HLB within citrus and citrus relatives has been observed. The objectives of this study were to assess cultivar field tolerance/resistance to HLB compared to sweet orange and to examine the effect of imidacloprid on growth and disease development. To expedite the trial, nursery trees were purchased on rootstock varieties as available.  The trial included ‘Hamlin/Kinkoji’, ‘Hamlin/Cleo’ (sweet orange), ‘Temple/Cleo’ (tangor), ‘Fallglo/Kinkoji’ (mandarin hybrid), ‘Sugar Belle/SourOrange’ (mandarin hybrid), ‘Tango/Kuharske’ (mandarin), and ‘Ruby Red/Kinkoji’ (grapefruit). A randomized complete block experiment with 10-20 trees per scion/rootstock was established at Ft. Pierce, Fl. in 2010. Disease severity was rated on a scale of 0 to 3 (0 = healthy, 1= pale mottling, 2 = mottle but symmetric, and 3 = blotchy mottle), and growth (height and diameter) of trees was recorded starting in April 2012 and continued on a monthly basis. At each sample time, three leaves were collected from each tree for quantitative real time PCR of CLas and citrus dehydrin. All trees exhibited symptoms of HLB and tested positive for CLas. Imidacloprid was applied quarterly to a subset of trees and significantly increased diameter compared to the untreated trees (p<0.05) but did not have a significant effect on height, disease severity, or CLas titer levels. No effects of rootstock were observed between ‘Hamlin’ on Kinkoji vs. Cleo.   There were significant differences (p<0.05) in disease severity, diameter, and CLas titer between the seven varieties screened. ‘Fallglo’ had the lowest incidence and ‘Ruby Red’ the highest incidence of distinctive HLB mottling. The highest CLas titer levels in 2012 were observed in November and December, with ‘Sugar Belle and ‘Tango’ displaying the highest titer levels (70 and 38 CLas per 1000 citrus cells) while ‘Fallglo’ and ‘Temple’ had the lowest levels (~5 CLas/1000 citrus cells). Despite the high titer levels of ‘SugarBelle’ it had the greatest overall increase in diameter, 22.91 cm and was among the healthiest in overall appearance. Initial results indicate that compared to ‘Hamlin’, ‘Fallglo’ and ‘Temple’ appear to display field resistance to HLB while ‘SugarBelle’ appears to have significant tolerance.  These trees are not yet bearing, suggesting that production was compromised in all varieties by the severe HLB pressure at this site, and commercial value of the observed resistance/tolerance remains uncertain.