Resistance to 'Candidatus Liberibacter asiaticus,' the Huanglongbing Associated Bacterium, in Sexually and/or Graft-Compatible Citrus Relatives

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 graftcompatible
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 challengeinoculation
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