Pathogenicity and toxin production of different Fusarium oxysporum isolates infecting onion (Allium cepa L.)

Publisher Copyright: © 2021 Association of Applied Biologists. Fusarium oxysporum forma specialis cepae strains are pathogens causing Fusarium basal rot of onion, which is a major problem in onion cultivation worldwide. In this work, the pathogenicity and toxin production of different F. oxysporum isolates, originating either from mature harvested onions or from onion sets, were studied. The F. oxysporum isolates belonging to the f. sp. cepae phylogenetic clade had the full set of seven secreted in xylem (SIX) genes and the gene C5, whereas all the other F. oxysporum isolates tested were negative for these virulence-related genes. However, in the pathogenicity test on onion seedlings the separation between the f. sp. cepae isolates and the others was not clear-cut. One of the f. sp. cepae isolates that had low in planta expression levels of SIX9 showed only mild virulence on the seedlings. A different F. oxysporum isolate, which did not have the known virulence-related genes, was pathogenic on seedlings, although non-pathogenic on mature bulbs. Thus, the two different pathogenicity assays frequently used, seedling survival test and onion bulb inoculation test, measure partially different features of the fungal isolates. Coinfection with pathogenic and non-pathogenic isolates caused more severe disease in the mature onion bulbs than the pathogenic isolate alone. All the isolates tested for toxin production capacity on rice medium produced beauvericin and, in addition, the non-pathogenic isolates produced moniliformin. Small amounts of beauvericin were detected in the infected onion bulbs. The amount of beauvericin did not show significant correlation with the fungal colonisation level or symptom severity.