| Summary: | Plant pathogens present a major challenge to crop production, leading to decreased yield and quality during growth and storage. During long-term storage, healthy onions can develop diseases from latent pathogen infections. This poses a challenge for onion growers because infected bulbs without visible symptoms can lead to significant crop losses during the growing season. In this study, we aimed to isolate and identify <i>Fusarium</i> species from yellow onion bulbs (<i>Allium cepa</i> L.) that developed disease symptoms during storage. The aggressiveness of these strains against onion bulbs and seedlings was also evaluated. The isolated strains were further subjected to morphological and molecular differentiation. The results revealed that all 16 isolated strains belonged to the <i>Fusarium</i> complex species <i>incarnatum-equiseti</i> and <i>Fusarium fujikuroi</i>, namely, <i>F. proliferatum</i> (98%), <i>F. oxysporum</i> (1%), and <i>Fusarium</i> sp. (1%). Koch’s postulate analysis of isolated strains revealed varying aggressiveness on onion bulbs and plants depending on fungal species. Disease symptoms developed more slowly on plants than on onion bulb plants according to Koch’s postulates. Moreover, the results revealed that <i>Fusarium</i> strains that can infect onion plants were less pathogenic to onion bulbs and vice versa. In addition, three isolates were found to be non-pathogenic to onions. Furthermore, the in vitro control of <i>Fusarium</i> species through <i>Bacillus velezensis</i> KS04-AU and <i>Streptomyces albidoflavus</i> MGMM6 showed high potential for controlling the growth of these pathogenic fungi. These results may contribute to the development of environmentally friendly approaches for controlling onion spoilage caused by pathogens during storage.
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