Tolerance of trifoliate citrus rootstock hybrids to Candidatus Liberibacter asiaticus

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 as rootstocks. This study assesses the response of eight different rootstock varieties, which include the Citrus×Poncirus trifoliata hybrids Carrizo citrange, US-802, US-812, US-897, and US-942, Benecke trifoliate orange, 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% in the genotypes Benecke, Carrizo, US-802, US-942, and Volkamer during the first 6 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% for US-802, US-812, Cleopatra, and Volkamer, and 70–78% for US-897 and US-942. Lowest percentages (44–52%) 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 4.5×105 and 1.6×106 compared with Cleopatra (1.0×107), while numbers for the other genotypes ranged from 3.1–4.9×106. 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.