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  • Author or Editor: Jawwad Qureshi x
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Control of Asian citrus psyllid Diaphorina citri Kuwayama and citrus leafminer Phyllocnistis citrella Stainton is important to reduce the spread and severity of huanglongbing (HLB) (citrus greening) and citrus canker diseases, respectively. Insecticides are critical for the management of these pests. We therefore conducted two replicated experiments using spray treatments containing single or multiple modes of action (MoA) insecticides to reduce the incidence of these two pests in bearing citrus. Tank mixing in 47 L·ha−1 (5 gal/acre) of water with synthetic plant terpenes (Requiem 25 EC, Unknown MoA) or adjuvant petroleum oil (PureSpray Green, Unknown MoA) did not improve the effectiveness of the pyrethroid zeta-cypermethrin (Mustang Max 0.15 EC, MoA 3A) against D. citri. Its control with flupyradifurone (Sivanto 200 SL MoA 4D) and PureSpray Green in 935 L·ha−1 (100 gal/acre) water was similar to Mustang Max 0.15 EC and Requiem 25 EC, but mixtures did not provide better control than Mustang Max 0.15 EC alone. Phyllocnistis citrella was controlled only with Sivanto 200 SL and PureSpray Green and Requiem 25 EC alone. The addition of cyantraniliprole (group 28 MoA in A16971 premixed with thiamethoxam MoA 4A), pymetrozine (Fulfill 50 WDG, MoA 9B), or abamectin (Agri-Mek SC, MoA 6) did not improve and in many cases reduced the performance of thiamethoxam (Actara 25 WG, MoA 4A) against D. citri and P. citrella. These results demonstrated no advantage to single applications of multiple MoAs over the most effective active ingredients when applied alone for control of D. citri or P. citrella. Therefore, rotations of these active ingredients would be preferable to mixtures to avoid selection for resistance against multiple MoAs by any one application.

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The citrus industry in Florida faces a destructive endemic disease, known as huanglongbing (HLB), associated with Candidatus Liberibacter asiaticus (CLas), a phloem-limited bacterium, and transmitted by the Asian citrus psyllid (ACP). Rootstocks are regarded as critical to keep citrus production commercially viable and help trees cope with the disease. Although most scions are susceptible, some rootstocks are HLB-tolerant and may influence ACP infestation and CLas colonization and therefore the grafted tree tolerance. This study aimed to elucidate the relative influence of rootstock and scion on insect vector infestation and CLas colonization under natural HLB-endemic conditions. Seven commercial rootstock cultivars with different genetic backgrounds were grafted with ‘Valencia’ sweet orange (Citrus sinensis) or were self-grafted (non-‘Valencia’) and planted in an open field where ACP and CLas were abundant. ACP infestation was determined weekly during periods of leaf flushing, and leaves and roots were analyzed every 3 months to determine CLas titers. Trees with ‘Valencia’ scion were more attractive to the psyllids than non-‘Valencia’ scions. This was also associated with a higher number of bacteria and a larger abundance of foliar HLB symptoms. The influence of the rootstock on the psyllid attraction of grafted ‘Valencia’ scion was less evident, and leaf CLas titers were similar regardless of the rootstock. Among the non-‘Valencia’ scions, Carrizo had the lowest and US-942 the highest leaf CLas titers. Root CLas titers also varied among cultivars, and standard sour orange roots harbored more bacteria than some trifoliate orange hybrid rootstocks such as US-942. In some trees, CLas was detected first in the roots 4 months after planting, but root CLas titers remained low throughout the study. In contrast, leaf CLas titers increased over time and were considerably higher than root titers from 7 months until the end of the study, 15 months after planting. Overall, the results of this study demonstrate a greater relative influence of the scion than the rootstock on ACP infestation and CLas colonization during the early stages of infection. This suggests that other cultivar-specific traits, such as the ability to tolerate other stresses and to absorb water and nutrients more efficiently, along with influences on the scion phenology, may play a larger role in the rootstock influence on the grafted tree tolerance during the later stages of HLB progression.

Open Access