Thesis (Ph.D.) - Cairo University - Faculty of Agriculture - Department of Agricultural Microbiology

Potato is one of the major food crops belonging to family Solanaceae. Black scurf and stem canker, caused by Rhizoctonia solani are destructive diseases of potato causing serious damage in most potato-producing areas throughout the world. Chemical pesticides are widely used to fungal pathogens control. However, the application of chemical pesticides is harmful to both the environment and human health. Therefore, the use of biocontrol agents (BCAs) is considered to be potentially sustainable, cost-effective and eco-friendly ways to control R. solani. Accordingly, 84 isolates of PGPR were recovered and screened for their ability to produce antifungal compound(s), from rhizosphere of healthy potato plant collected from different areas. One strain was selected based on its ability to suppress the growth of R. solani. Phylogenetic analysis of this strain based on 16S rRNA gene sequences showed highest similarity (99%) with Bacillus sp. GRP (GenBank accession number: MK030136). The Bacillus sp. GRP culture filtrate contained protease, diffusible antibiotic, hydrogen cyanide and siderophore, it was capable of inhibiting growth of the pathogen up to 15 days compared to 7 and 10 days for other Bacillus isolates. It also produced indole acetic acid which promoted plant growth. Morphological and structural changes that took place as a result of Bacillus sp. and R. solani interaction were evaluated using light, SEM and TEM. The results showed that Bacillus sp. caused loss of structural integrity, abnormal coiling, shriveling and lysis of the R. solani hyphae, in addition to complete cytoplasm and internal organelles depletion. Among four media tested, the maximum growth and antifungal activities were found in tryptone soya broth medium under static condition at 30{u00B0}C, pH7, inoculum density of 1% and 72 h incubation

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