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Asiatic huanglongbing (HLB) is a devastating disease of citrus associated in North America with the bacterium ‘Candidatus Liberibacter asiaticus’ (LAS) vectored by the Asian citrus psyllid (ACP), Diaphorina citri Kuwayama. ACP management is considered a vital component of a program aimed at reducing the incidence and spread of HLB. Considerable research has been published comparing the efficacy of different insecticides for ACP control as well as on seasonal strategies for applying ACP insecticides. However, published information was largely lacking for even the most intense insecticide programs on their effectiveness for keeping HLB out of a new citrus planting in Florida citrus. We therefore conducted two replicated (individual plots 0.2 to 0.4 ha in size) experiments on protecting young citrus from HLB using different ACP management programs. An intensive insecticide program was evaluated in each experiment: eight annual calendar applications of traditional insecticides (hereafter referred to as the “complete” program). In one experiment, citrus was either planted alone and subjected to the complete program or citrus was interplanted with orange jasmine, Murraya exotica L. (a favored ACP host plant) and subjected to a reduced insecticide program (four calendar sprays of traditional insecticides). There was one set of plots in which both jasmine and citrus were treated with insecticides and one set in which jasmine was not treated at all. In the second experiment, citrus was either subjected to the complete program or to one of two other programs: a reduced insecticide program consisting of five calendar applications of traditional insecticides or a mineral oil program (oil applications every 3 weeks plus one dormant insecticide spray). The results of the two experiments were similar. Relatively good ACP control was achieved under each ACP management program during the first year but, as the experiments progressed and trees increased in size, ACP outbreaks occurred regardless of the psyllid management program. Little HLB developed under any ACP management program during the first year, but thereafter HLB increased and large percentages of the trees in each experiment became LAS-infected in less than two to three years. The combined results of the experiments indicated that up to eight monthly pesticide treatments per year applied on a calendar schedule were ineffective for preventing young citrus from becoming diseased. Of important significance is that the orchard within which the two experiments were conducted was subjected to a minimal psyllid management program and contained many older trees known to be infected by the HLB pathogen, a challenging situation for getting young trees into production without contracting the disease. Thus, the ACP management programs we evaluated might have been more effective if ACP in the surrounding areas had been more aggressively controlled and diseased trees in the surrounding areas removed to reduce inoculum loads. Also, the programs might have been more effective for slowing the spread of HLB if our research plots had been larger.

Citrus bacterial canker [causal agent Xanthomonas axonopodis pv. citri (Xac)] is a serious threat to the citrus industry. Currently, there are no effective means to control citrus canker. Our objective was to determine the resistance of selected citrus species, citrus hybrids, and citrus relatives to Xac. Our first experiment focused on determining if differences in resistance exist among 20 C. reticulata genotypes and included three other citrus species and citrus relatives (Glycosmis pentaphylla and Clausena hardimandiana). Plants were inoculated with Xac strain A either by injection infiltration or needle-prick. Our second experiment included 10 members of the genus Citrus and Poncirus trifoliata, representing a total of 31 different selections. Plants were needle-prick inoculated with both Xac strain A and Xac strain A Wellington (AW). All inoculations were done using suspensions of Xac at a concentration of 104 cfu/mL. In both experiments, there were highly significant differences among genotypes in response to inoculation with Xac. In the first experiment, regardless of inoculation method, G. pentaphylla and C. hardimandiana were found to be highly resistant to Xac, whereas C. paradisi was least resistant. In the second experiment for both Xac strain A and AW, Chinotto sour orange, Carrizo citrange, Eustis limequat, and P. trifoliata were the most resistant. Thornless key lime × Meiwa kumquat hybrids showed a range of resistance from among the most susceptible to among the most resistant. Our results expand on previous studies on resistance of citrus and citrus relatives to Xac and indicate that there may be potential for increasing resistance by breeding using selected parents.