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ARS Home » Southeast Area » Fort Pierce, Florida » U.S. Horticultural Research Laboratory » Subtropical Insects and Horticulture Research » Research » Publications at this Location » Publication #319258
Title: Conventional citrus of some scion/rootstock combinations show field tolerance under severe huanglongbing disease pressure

Author
item Stover, Eddie
item INCH, SHARON - Former ARS Employee
item RICHARDSON, MATTHEW - Former ARS Employee
item Hall, David

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/7/2016
Publication Date: 2/1/2016
Citation: Stover, E., Inch, S., Richardson, M., Hall, D.G. 2016. Conventional citrus of some scion/rootstock combinations show field tolerance under severe huanglongbing disease pressure. HortScience. 51:127-132.

Interpretive Summary: Although there are no known commercial citrus varieties with strong resistance to huanglongbing (HLB), some field tolerance has been observed. We assessed field tolerance to HLB of the following scion/rootstock combinations: ‘Hamlin/Kinkoji’, ‘Hamlin/Cleopatra’, ‘Temple/Cleopatra’, ‘Fallglo/Kinkoji’, ‘Sugar Belle/Sour Orange’, ‘Tango/Kuharske’, and ‘Ruby Red/Kinkoji’. Planted in 2010, all trees developed symptoms of HLB and tested positive for the HLB pathogen with similar levels measured at most sample dates. Overall ‘Fallglo/Kinkoji’ had the lowest HLB incidence and ‘Ruby Red/Kinkoji’ the highest rating for disease symptoms. ‘Sugar Belle/Sour Orange’ had the greatest overall increase in diameter, and was among the healthiest in overall appearance. In November 2014 ‘Temple/Cleopatra’ trees had significantly greater fruit load followed by ‘Tango/Kuharske’ and ‘Hamlin/Kinkoji’. Production was likely compromised in all scion/rootstocks by the severe HLB pressure at this site, and commercial value of the observed tolerance remains uncertain. It is noteworthy that the most pronounced HLB symptoms and higher early pathogen levels was not associated with the greatest suppression of growth and cropping, thus focus on early symptomatic traits may obscure important disease tolerance.

Technical Abstract: Although there are no commercial citrus varieties with strong huanglongbing (HLB) resistance, some field tolerance has been observed. The objective of this study was to assess cultivar field tolerance to HLB compared to sweet orange and grapefruit. To expedite the trial, nursery trees were purchased on rootstock varieties as available. The trial included ‘Hamlin/Kinkoji’, ‘Hamlin/Cleopatra’, ‘Temple/Cleopatra’, ‘Fallglo/Kinkoji’, ‘Sugar Belle/SourOrange’, ‘Tango/Kuharske’, and ‘Ruby Red/Kinkoji’. A randomized complete block experiment with 10-20 trees per scion/rootstock was established at Ft. Pierce, Florida in September 2010. Disease severity was rated on a scale of 0 to 3 and growth of trees was recorded starting in April 2012 and continued on a regular basis. At each sample time, three leaves were collected from each tree for quantitative real time PCR of Candidatus Liberibacter asiaticus (Las) and citrus dehydrin. All trees exhibited symptoms of HLB and tested positive for CLas by Oct 2012, with similar titers measured at most sample dates. There were significant differences (P=0.05) in disease severity, stem diameter, and CLas titer (at some time points) between the scion/ rootstock combinations tested. Overall ‘Fallglo/Kinkoji’ had the lowest incidence and ‘Ruby Red/Kinkoji’ the highest rating for distinctive HLB mottling, though ‘SugarBelle/Sour Orange’ had the highest percentage of leaves affected February 2015. ‘SugarBelle/Sour Orange’ had the greatest overall increase in stem diameter, and was among the healthiest in overall appearance. In November 2014 ‘Temple/Cleopatra’ trees had significantly greater fruit-load (26 fruit/tree) followed by ‘Tango/Kuharske’ (10 fruit /tree), ‘Hamlin/Kinkoji’ (5 fruit/tree) and all others with 0-1.4 fruit/tree. All scion/rootstocks displayed increased in cropping in the next season (May 2015). For most rootstock/scion combinations, half of trees received regular imidacloprid treatments. The initial CLas titer data indicate a difference between treated and non-treated trees only for ‘Ruby Red/Kinkoji’ and no further differences were observed. Tree growth rate was affected by imidacloprid only in ‘Ruby Red/Kinkoji’ and ‘Fallglo/Kinkoji’. No effects were observed on mortality or cropping from imidacloprid applications. Treated trees were surrounded by non-treated trees, and results might have been quite different with larger blocks of treated trees. Production was likely compromised in all scion/rootstocks by the severe HLB pressure at this site, and commercial value of the observed tolerance remains uncertain. It is noteworthy that the most pronounced HLB symptoms and early pathogen titer were not associated with the greatest suppression of growth and cropping, thus early symptomatic traits may not reflect a scion/rootstock’s tolerance to HLB.