Alemu Tesfaye (PhD)Chebte Abreham2018-06-142023-11-182018-06-142023-11-182015-10http://etd.aau.edu.et/handle/12345678/987In Ethiopia onion has an immense economic, nutritional and medicinal value among all vegetables. The crop is also proved to be income generating for smallholder farmers of east Shewa. However, basal rot disease has put a major hindrance for the productivity of onion in the area. Therefore, This study was conducted to isolate and characterizes Fusarium isolates of onion and to evaluate the antagonism effects of Bacillus isolates to control the disease. For isolation of Fusarium species a total of 43 diseased plant samples were collected randomly from three onion growing districts of east Shewa Zone. Biocontrol agents were isolated from the rhizosphere soil of onion. According to morphological characters, all isolates were identified as Fusarium oxysporum, Fusarium solani and Fusarium proliferatum. Pathogenicity of the isolates were tested under in vitro and glasshouse conditions. Rot lengths measured on cross section of onion bulbs caused by Fusrium isolates ranged from 0.23 cm (AAUFI2 and AAUFI6 each) to 2.1 cm (AAUFI16). The shortest root length (22.6 mm) was produced by isolates AAUFI16 while the shortest shoot length (138 mm) and the smallest dry weight (0.5 g) were produced by isolate AAUFI2. Percentage incidence of the isolates ranged 58.3% to 100%. Bacillus isolates were effective against Fusarium isolates. In vitro mycelial growth inhibition percentage by Bacillus isolate ranged from 53.6% (in AAUFI20) to 72.3% (in AAUFI4). Incidence of basal rot disease was significantly reduced by up to 41.5% (average) after treated with Bacillus isolate while root length, shoot length and dry weight increased significantly compared to the positive control. Percentage of disease control by isolate AAUBI3 ranged 33.3% to 66.67%. Based on the internal transcribed spacer (ITS) region of ribosomal DNA (rDNA) restriction fragment length polymorphism (RFLP) analysis, ten Fusarium isolates were divided into two major groups with similarity coefficient ranged from 0.38 to 1.0. Effects of basal rot disease on onion crop can be reduced by using this bioagents with other control methods. Keywords: Allium cepa L., Antagonism, Bacillus, Fusarium, Genetic diversity, athogenicityenAllium cepa L; Antagonism; Bacillus; Fusarium; Genetic diversity; PathogenicityThesis