Pathogenicity of Fusarium oxysporum and Fusarium proliferatum isolates from symptomless onions (Allium cepa) and onions with Fusarium basal rot

Abstract Several Fusarium species cause disease in onion ( Allium cepa ), resulting in significant yield losses. In this study, isolates of Fusarium oxysporum and Fusarium proliferatum from diseased and symptomless mature onion bulbs were tested for pathogenicity on onion seedlings and mature bulbs. For F . oxysporum isolates, the outcome of the test on mature bulbs correlated well with the symptom status of the original bulb, whereas for F . proliferatum , such a correlation was not seen, suggesting a different mechanism of symptom development. Those F . oxysporum isolates that carried the Secreted In Xylem ( SIX ) genes, CRX genes and C5 gene were significantly more aggressive on both seedlings and mature bulbs. However, the SIX‐ negative F . oxysporum isolates also significantly reduced the emergence of onion seedlings, and thus a division into pathogenic and nonpathogenic F . oxysporum isolates was not obvious. On mature bulbs, those SIX ‐negative isolates that carried the gene CRX2 were significantly more virulent than those without CRX2 . The F . proliferatum isolates were all pathogenic to onion and carried one SIX2 gene homologue, SIX2‐1 . The majority of F . proliferatum isolates also carried another homologue, SIX2‐2 . Several isolates negative for SIX2‐2 had another potential virulence gene, CRX2 , suggesting that these two genes could encode overlapping functions. The SIX2‐2 sequence was identical in all those isolates harbouring it, while SIX2‐1 showed variation that agreed with the phylogeny based on the translation elongation factor 1‐α sequence. This suggests that the two SIX2 genes of F . proliferatum might have been acquired separately.