Relative Influence of Rootstock and Scion on Asian Citrus Psyllid Infestation and Candidatus Liberibacter asiaticus Colonization

Abstract

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.

Response of Citrus Germplasm Seedlings to Candidatus Liberibacter Asiaticus Infection under Controlled Greenhouse Conditions

“…These results are in accordance with the results of greenhouse studies by Bodaghi et al (2022aBodaghi et al ( , 2022b, who reported higher CLas titers in leaves compared with roots in young graftinoculated Valencia trees with varying rootstocks. A field study by Tardivo et al (2023) also observed that CLas titers remained lower in the roots than in the leaves, regardless of the rootstock. However, the progression of HLB in the plant after inoculation is still not fully understood, and there are conflicting findings regarding root colonization by CLas.…”

Seasonal Patterns in the Frequency of Candidatus Liberibacter Asiaticus in Populations of Diaphorina citri (Hemiptera: Psyllidae) in Florida

“…We also do not know the probability of a successful infection given that only 1, 2,…, n bacterial cells move into a healthy host plant or vector. A successful infection probably requires several C las cells [ 126 ], and the number of available cells could depend on scion and rootstock variation in C Las titer [ 35 , 127 , 128 ] though differences are not always observed [ 129 ]. Finally, there is a behavioral component where adults on mature leaves are xylem feeders, not phloem feeders [ 118 ], and C Las is a phloem-limited organism.…”

“…Thus, for a given scion-rootstock, there is a weather effect where warmer and drier areas reduce C Las titers, at least for ‘Valencia’ or ‘Natal’ sweet orange on ‘Swingle’ rootstock [ 33 ]. There is some effect of rootstock [ 34 , 35 ] and scion [ 35 , 36 ] on C Las titers in the leaves. Seasonal cycles in citrus hosts could result in seasonal cycles in the vector if the observed variation is also present in the flush where the nymphs develop.…”

Effect of tree height and spraying methods on Diaphorina citri kuwayama endosymbionts in the context of Huanglongbing disease management in citrus orchards

“…Young trees, in their initial years of growth, may exhibit different susceptibility and response patterns to HLB compared to mature trees. This notion is grounded in the fact that young trees, while establishing themselves, undergo dynamic physiological and metabolic changes that could influence their interaction with the HLB pathogen 11,12 . Conversely, mature citrus trees, with well‐developed canopies and root systems, present distinct microenvironments that may either facilitate or hinder HLB establishment and progression.…”