Potential Role of Rhizobacteria Isolated from Citrus Rhizosphere for Biological Control of Citrus Dry Root Rot

Abstract

Citrus trees face threats from several diseases that affect its production, in particular dry root rot (DRR). DRR is a multifactorial disease mainly attributed to Neocosmospora (Fusarium) solani and other several species of Neocosmospora and Fusarium spp. Nowadays, biological control holds a promising control strategy that showed its great potential as a reliable eco-friendly method for managing DRR disease. In the present study, antagonist rhizobacteria isolates were screened based on in vitro dual culture bioassay with N. solani. Out of 210 bacterial isolates collected from citrus rhizosphere, twenty isolates were selected and identified to the species level based on the 16S rRNA gene. Molecular identification based on 16S rRNA gene revealed nine species belonging to Bacillus, Stenotrophomonas, and Sphingobacterium genus. In addition, their possible mechanisms involved in biocontrol and plant growth promoting traits were also investigated. Results showed that pectinase, cellulose, and chitinase were produced by eighteen, sixteen, and eight bacterial isolates, respectively. All twenty isolates were able to produce amylase and protease, only four isolates produced hydrogen cyanide, fourteen isolates have solubilized tricalcium phosphate, and ten had the ability to produce indole-3-acetic acid (IAA). Surprisingly, antagonist bacteria differed substantially in their ability to produce antimicrobial substances such as bacillomycin (five isolates), iturin (ten isolates), fengycin (six isolates), surfactin (fourteen isolates), and bacteriocin (subtilosin A (six isolates)). Regarding the PGPR capabilities, an increase in the growth of the bacterial treated canola plants, used as a model plant, was observed. Interestingly, both bacterial isolates Bacillus subtilis K4-4 and GH3-8 appear to be more promising as biocontrol agents, since they completely suppressed the disease in greenhouse trials. Moreover, these antagonist bacteria could be used as bio-fertilizer for sustainable agriculture.

Effect of Zinc Oxide Nanoparticles on the Growth of Malus hupehensis Rehd. Seedlings

“…Some diseases are not caused by a single bacterium. For example, citrus dry root rot is a multifactorial disease mainly attributed to Neocosmospora solani as well as other species of Neocosmospora and Fusarium spp (Ezrari et al, 2021). Riaz et al (2020) identified Neocosmospora as a new pathogen of potato stem rot.…”

Biological Control of Plant Pathogens: A Global Perspective

“…Mechanisms of action determining the success of biocontrol are complex, which, to some extent, may explain the limited effectiveness of biocontrol against plant diseases in field crops. In this case, the screening of these BCAs can be achieved by phenotype-based screenings by evaluating pathogen growth by dual culture assay or by volatile antifungal compounds (VOCs) or by marker-based screening: secretion of antimicrobial metabolites (antibiosis via bacterial supernatant), the secretion of cell-wall-degrading or screening bacteria with specific inhibitory mechanisms [ 76 , 131 ].…”

Rhizosphere microorganisms of Crocus sativus as antagonists against pathogenic Fusarium oxysporum

“…T h e pu r e -c u l t u r e m e t h o d wa s u s e d t o i s o l a t e microorganisms from rhizosphere soil (Jogaiah et al, 2013;Ezrari et al, 2021). The rhizosphere soil collected in Section 2.1 was used to isolate rhizosphere microorganisms.…”

“…In a field experiment, inoculation with two antagonistic bacteria (Pseudomonas and Bacillus) in rhizospheric soil of tomato could stimulate its defense against R. solani (Kwak et al, 2018). Two rhizobacterial isolates Bacillus subtilis K4-4 and GH3-8 could completely suppress citrus dry root rot disease caused by Neocosmospora solani in greenhouse trials (Ezrari et al, 2021). The relationship between C. sativus rhizosphere microorganisms and phenology has been revealed by highthroughput sequencing (Ambardar et al, 2016); however, the relationship between the rhizosphere microbiome and disease resistance is unclear.…”