First report of hairy root disease, caused by rhizogenic Agrobacterium biovar 1, in hydroponic bell pepper crop in South Korea

Rhizogenic agrobacteria cause excessive root proliferation known as hairy root disease (HRD) on hydroponically grown tomato, cucumber, and eggplant, resulting in reduced yield (up to 10%) and severe economic losses worldwide (Bosmans et al. 2017). In 2018 and 2019, typical symptoms of HRD were observed in hydroponically grown bell pepper, specifically, Capsicum annuum cvs. Nagano (2018 and 2019), Lunik (2019), and Atlante (2018) in a commercial greenhouse located in the county Geumgok-myeon (province Gyeongsangnam-do, South Korea). Plants hydroponically grown on coco substrate showed excessive root formation, and this occurred on at least 50% of the plants in the greenhouse. Bacteria were isolated from symptomatic roots by plating a 10-fold dilution series on the semiselective 1A-Te medium (Bosmans et al. 2015, Brisbane and Kerr 1983). Six characteristic Agrobacterium colonies displaying a black color with a metallic shine were selected. Amplification of marker genes rolB and virD2 confirmed the presence of a root-inducing plasmid (pRi) (Bosmans et al. 2015). Partial 16S rRNA sequencing of the six colonies revealed the presence of only one unique isolate (strain PME19.1.1), which was identified as Agrobacterium (>99% sequence identity on a total of 564 bp). To enable a more precise classification, a multilocus sequence analysis based on four housekeeping genes (16S rRNA, recA, rpoB, and trpE loci) was performed as described previously (Bosmans et al. 2015). The resulting phylogenetic tree showed that PME19.1.1 can be classified as Agrobacterium genomospecies 9. Pathogenicity of isolate PME19.1.1 was confirmed in a bioassay using bell pepper cultivar Madison S&G (S. Rodriguez, personal communication). To confirm Koch’s postulates, three seedlings were inoculated and compared with a noninoculated plant. In addition to strain PME19.1.1, rhizogenic Agrobacterium strains ST15.13/097 and ST15.13/012, both isolated from tomato roots showing HRD symptoms in greenhouses in Belgium, were also included in the bioassay. After 9 weeks, plants were harvested to evaluate pathogenicity. Visual inspection for excessive root formation, wet weight, dry weight, and photographs of the roots were taken from each plant. A Kruskal–Wallis test using SPSS was conducted on each of the parameters measured. Visual inspection clearly showed that bell pepper inoculated with the three Agrobacterium strains resulted in excessive root proliferation. This was confirmed by significantly higher root fresh weight compared with the noninoculated control (P = 0.061). After the experiment, a specific quantitative PCR, targeting the ORF13 gene present in the pRi plasmid, was conducted and confirmed the presence of the rhizogenic agrobacterial DNA in the root samples of the inoculated plants but not in the control plants (protocol adjusted from Bosmans et al. 2016). Furthermore, bacteria were reisolated from the infected plants by plating root samples on 1A-Te medium. Sequence analysis of the rolB of the obtained clones demonstrated that it concerned the original isolate used for inoculation. To our knowledge, this is the first report of rhizogenic agrobacteria causing HRD in hydroponically grown bell pepper. Additionally, we demonstrated that rhizogenic Agrobacterium strains isolated from infected tomato plants are also able to infect bell pepper. Considering the current threat of HRD on tomato, this note highlights the potential of HRD to become an emerging disease in bell pepper cultivation as well.

Jaar van publicatie:2020

Toegankelijkheid:Open