Potential soil transmission of a novel Candidatus Liberibacter strain detected in citrus seedlings grown in soil from a huanglongbing infested citrus grove

Abstract

17 Candidatus Liberibacter spp. are Alphaproteobacteria associated with plants and psyllid vectors. Most 18 cause plant diseases, including Ca Liberibacter asiaticus (Las) associated with citrus huanglongbing 19 (HLB). Replacing HLB-infected by Las-free citrus trees results in fast re-infection despite psyllid control. 20To check if HLB could be soil-borne, we performed an insect-free greenhouse-experiment with 130 21 mandarin seedlings in two citrus-grove soils (A and B), non-autoclaved or autoclaved. Liberibacter-22 specific 16S-rDNA PCR primers to detect Las were used to search for Ca. Liberibacter spp. in mandarin 23 leaves. Seven plants grown in non-autoclaved soil B showed HLB-like symptoms and tested positive after 24 2.5 and 8.5 months using three different primer systems: two based on the 16S-rDNA gene (primers 25HLBas/HLBr and OI2c/OI1) and one based on the rplA/rplJ gene (primers LAA2/LAJ5). DNA segments 26 from these plants amplified by primers OI2c/OI1 were cloned and sequenced; they were 95.9 % similar to 27Las and 94.8% to Ca. Liberibacter africanus (Laf). The DNA product from Liberibacter-group specific 28 PCR primers for the rplA/rplJ gene was 87.6% similar to that of Las and 78.2% of Laf. As the strain 29 obtained originated from soil and was different from existing Ca. Liberibacter species, this strain may be 30 a new species. 31 32

Defeating Huanglongbing Pathogen Candidatus Liberibacter asiaticus With Indigenous Citrus Endophyte Bacillus subtilis L1-21

“…Citrus-infected plants with HLB are not always but confused easily with symptoms of nutrient deficiency and without professional training (Tian et al, 2014) because the pathogen distribution is always highly patchy, as leaves and stems contain most of the bacterial titers (Li et al, 2006). In the beginning, the titer may be the highest in the roots (Johnson et al, 2014;da Rocha et al, 2019). The symptoms are clearer and more apparent in the cooler season compared to warmer months.…”