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.
Highlights
Citrus is an important economic crop in Morocco, covering an area of 126.600 ha with a production of 2.6 million tons of citrus fruits a year
Regarding the PGPR capabilities, an increase in the growth of the bacterial treated canola plants, used as a model plant, was observed. 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
Citrus plantations are threatened by several pathogens of which the Fusarium species, mainly F. solani, is causing citrus dry root rot (DRR) disease, which is responsible for significant economic loss [1,3,4]
Summary
Citrus is an important economic crop in Morocco, covering an area of 126.600 ha with a production of 2.6 million tons of citrus fruits a year. Citrus plantations are threatened by several pathogens of which the Fusarium species, mainly F. solani, is causing citrus dry root rot (DRR) disease, which is responsible for significant economic loss [1,3,4]. Fusarium species can cause other serious diseases on citrus plantation such as twig rot, decline dieback, twig blight, and vascular wilt, thereby are a major threat to citrus production worldwide [1,5,6]. The pathogen penetrates citrus roots and cause root rot; the roots become blackened with discoloured peels and necrosis, and when the pathogen reaches the xylem vessels it leads to the weakening of the tree. The disease worsens with biotic (Phytophthora spp., citrus tristeza virus (CTV), or attacks by pests such as nematodes (e.g., Tylenchulus semipenetrans Cobb), rodents, and insects) and/or abiotic (drought, root asphyxiation due to over-watering, high temperature, poor root aeration, and excessive fertilization) stressors [1,8,9,10]
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