Pathotyping Citrus Ornamental Relatives with Xanthomonas citri pv. citri and X. citri pv. aurantifolii Refines Our Understanding of Their Susceptibility to These Pathogens

Abstract

Xanthomonas citri pv. citri (Xcc) and X. citri pv. aurantifolii (Xca) are causal agents of Citrus Bacterial Canker (CBC), a devastating disease that severely affects citrus plants. They are harmful organisms not reported in Europe or the Mediterranean Basin. Host plants are in the Rutaceae family, including the genera Citrus, Poncirus, and Fortunella, and their hybrids. In addition, other genera of ornamental interest are reported as susceptible, but results are not uniform and sometimes incongruent. We evaluated the susceptibility of 32 ornamental accessions of the Rutaceae family belonging to the genera Citrus, Fortunella, Atalantia, Clausena, Eremocitrus, Glycosmis, Microcitrus, Murraya, Casimiroa, Calodendrum, and Aegle, and three hybrids to seven strains of Xcc and Xca. Pathotyping evaluation was assessed by scoring the symptomatic reactions on detached leaves. High variability in symptoms and bacterial population was shown among the different strains in the different hosts, indicative of complex host-pathogen interactions. The results are mostly consistent with past findings, with the few discrepancies probably due to our more complete experimental approach using multiple strains of the pathogen and multiple hosts. Our work supports the need to regulate non-citrus Rutaceae plant introductions into areas, like the EU and Mediterranean, that are currently free of this economically important pathogen.

Keywords: Rutaceae; citrus bacterial canker; host–plant interaction; hyperplastic tissue; ornamental plants; pathotype.

Figure 1

Host–bacterial strain interaction assessed by an in vitro test on detached leaves of Rutaceae inoculated with bacterial strains of Xanthomonas citri pv. citri and pv. aurantifolii. (A–G): exemplificative scale of symptomatic reactions measured as: “0”, wound repair (A) or necrosis (B); “ws”, water-soaked margin surrounding the wound sites (C); “+”, pustule or blister-like lesions (D); “++”, beginning of callus formation or crystalline callus at inoculation site (E); “+++”, abundant crystalline callus (5 sites) (F); “++++”, confluent crystalline callus in a single hyperplasia (G). (H) leaves of different Rutaceae placed on the surface of Petri dishes containing soft water agar (1%) with the abaxial side up; (I), wound inoculation with a sterile syringe needle (five wounds per site); (J–L), crystalline callus visible at 20 days post-inoculation on leaves of Eremocitrus glauca (J), Citrus aurantiifolia (K), and Murraya ovatifoliolata (L).