Submitted to: Scientia Horticulturae
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/31/2012
Publication Date: 9/28/2012
Citation: Albrecht, U., Bowman, K.D. 2012. Tolerance of trifoliate citrus hybrids to Candidatus Liberibacter asiaticus. Scientia Horticulturae. 147:71-80. Interpretive Summary: Huanglongbing (HLB) is a most destructive disease of citrus in Florida and in other citrus producing countries around the world. The pathogen associated with HLB in Florida is a bacterium called Candidatus Liberibacter asiaticus. Although most commercial citrus varieties show little resistance to HLB, some varieties commonly used as rootstocks appear to show tolerance to this disease. This study assesses the reaction of eight different rootstock varieties to the HLB pathogen. The percentage of plants for which the HLB pathogen was detected within the first six months of the experiment was less than 7 percent in the varieties Benecke, Carrizo, US-802, US-942, and Volkamer. One year after the start of the experiment, the percentage of infected plants was 91-96 percent for the varieties US-802, US-812, Cleopatra, and Volkamer, and 70-78 percent for US-897 and US-942. Lowest percentages (44-52 percent) were observed for the varieties Benecke and Carrizo. The concentration of the HLB pathogen was lowest in the varieties Benecke and US-897 and highest in Cleopatra. Significant growth reductions were observed for the varieties Cleopatra, Benecke, US-802, and Volkamer seedling after one year, but Benecke recovered six months later. Our results suggest classifying Carrizo, US-897, and US-942 as tolerant, US-802, US-812, and Volkamer lemon as moderately tolerant, and Cleopatra mandarin as susceptible to HLB. Low rates of infection and low pathogen concentrations also suggest resistance of Benecke to the disease. Additional greenhouse experiments and field observations on a subset of varieties confirmed these findings.
Technical Abstract: Candidatus Liberibacter asiaticus (Las) is the suspected causal agent of Huanglongbing (HLB) in Florida and other citrus producing countries around the world. Although little resistance to HLB is found within commercial citrus varieties, tolerance has been reported for some varieties that are commonly used for rootstocks. This study assesses the response of eight different rootstock varieties, which include the Citrus x Poncirus trifoliata hybrids Carrizo citrange, US-802, US-812, US-897, and US-942, the trifoliate orange Benecke, Volkamer lemon, and Cleopatra mandarin, to Las under controlled conditions in the greenhouse. The percentage of plants detected positive for Las did not exceed 7 percent in the genotypes Benecke, Carrizo, US-802, US-942, and Volkamer during the first six months after graft-inoculation (mai). Pruning resulted in much increased numbers of plants with detectable Las levels and induced disease symptoms in previously asymptomatic plants. At 12 mai, percentages of Las-positive plants were 91-96 percent for US-802, US-812, Cleopatra, and Volkamer, and 70-78 percent for US-897 and US-942. Lowest percentages (44-52 percent) were observed for Benecke and Carrizo. The number of Las genomes per g of leaf tissue in Las-positive plants was considerably lower in Benecke and US-897 seedlings with 4.5 x 105 and 1.6 x 106 compared with Cleopatra (1.0 x 107), while numbers for the other genotypes ranged from 3.1-4.9 x 106. At 12 mai, foliar disease symptoms, though variable in extent, were prominent in most genotypes, except US-897. Shoot masses were significantly reduced in Cleopatra, Benecke, US-802, and Volkamer seedlings in response to Las at this time. Continued observation until 18 mai found no significant growth reductions in Benecke seedlings. Our results suggest classifying Carrizo, US-897, and US-942 as tolerant, US-802, US-812, and Volkamer lemon as moderately tolerant, and Cleopatra mandarin as susceptible to Las. Despite irregular growth, low rates of infection and low Las numbers indicate some resistance of Benecke to Las. Additional greenhouse experiments and field observations confirmed findings for US-802, US-897, US-942, and Cleopatra, although results for US-802 were more variable.