Abstract
Huanglongbing (HLB) is the most destructive, yet incurable disease of citrus. Finding sources of genetic resistance to HLB-associated ‘Candidatus Liberibacter asiaticus’ (Las) becomes strategic to warrant crop sustainability, but no resistant Citrus genotypes exist. Some Citrus relatives of the family Rutaceae, subfamily Aurantioideae, were described as full-resistant to Las, but they are phylogenetically far, thus incompatible with Citrus. Partial resistance was indicated for certain cross-compatible types. Moreover, other genotypes from subtribe Citrinae, sexually incompatible but graft-compatible with Citrus, may provide new rootstocks able to restrict bacterial titer in the canopy. Use of seedlings from monoembryonic species and inconsistencies in previous reports likely due to Las recalcitrance encouraged us to evaluate more accurately these Citrus relatives. We tested for Las resistance a diverse collection of graft-compatible Citrinae species using an aggressive and consistent challenge-inoculation and evaluation procedure. Most Citrinae species examined were either susceptible or partially resistant to Las. However, Eremocitrus glauca and Papua/New Guinea Microcitrus species as well as their hybrids and those with Citrus arose here for the first time as full-resistant, opening the way for using these underutilized genotypes as Las resistance sources in breeding programs or attempting using them directly as possible new Las-resistant Citrus rootstocks or interstocks.
Highlights
Huanglongbing (HLB) is the most destructive disease of citrus worldwide
Searching for resistance to HLB within Citrus and its relatives of the family Rutaceae, subfamily Aurantioideae, has been an active area of research due to the severe damages caused by the disease on tree performance, production and fruit quality
There is a need for confident and reliable sources of resistance to either the Liberibacter asiaticus’ (Las) bacterium, the insect vector D. citri or both, which could be used for introgression into the Citrus germplasm (i) to generate new Citrus-like cultivars that may be useful as rootstocks or scions, (ii) to map and identify the genes involved in the resistance trait for direct modification of wellknown elite Citrus cultivars using modern biotechnology tools, especially those which cannot be improved by sexual breeding due to their high heterozygosity, or (iii) to be used promptly as new rootstocks or interstocks to potentially alleviate HLBinduced damages
Summary
Huanglongbing (HLB) is the most destructive disease of citrus worldwide. Its occurrence is associated with the infection of trees with one of the following Gram-negative intracellular α-proteobacteria, ‘Candidatus Liberibacter asiaticus’ (Las), ‘Ca. L. americanus’ (Lam) or ‘Ca. L. africanus’ (Laf), which colonize the phloem of their host plants (Bové, 2006). Las and Lam are Resistance to ‘Candidatus Liberibacter asiaticus’. Reaches higher titers in Citrus and is more efficiently transmitted than Lam (Lopes et al, 2009, 2013), all this making the former associated with the most severe damages caused by HLB in Asia and the Americas (Gottwald et al, 2007). HLB-infected, severely affected trees produce small and irregularly shaped fruits with a thick peel that remains green. In areas where no control of the insect vector is done, the severity of symptoms and disease progression in the orchards increase rapidly (Gottwald et al, 1989, 1991)
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