Aggressiveness and Phylogenetic Relationships of <i>Fusarium oxysporum</i> Associated with Crown and Root Rot in Pyrethrum Plants

In Australia, pyrethrum (<i>Tanacetum cinerariifolium</i>) cultivation provides a significant portion of the global supply of natural insecticidal pyrethrins. However, crown and root rots, along with stunted growth and plant loss during winter, are significant issues affecting certain sites and contributing to yield decline. Several isolates of the <i>Fusarium oxysporum</i> species complex (FOSC) have been identified as causal agents of crown and root rot in pyrethrum, highlighting these as key pathogens contributing to this decline. However, the genetic and pathogenic diversity of the FOSC impacting pyrethrum is unclear. This study isolated <i>F. oxysporum</i> consistently from symptomatic and asymptomatic field-grown pyrethrum plant tissues, identified through morphological and multigene phylogenetic analyses. Phylogenetic analyses of partial gene sequences of calmodulin (<i>cmdA</i>), RNA polymerase II second-largest subunit (<i>rpb2</i>), translation elongation factor 1-alpha (<i>tef1-α</i>) and β-tubulin (<i>tub2</i>) resolved the placement of these isolates within the context of different published FOSC taxonomies and revealed notable genetic diversity among the isolates. Glasshouse experiments effectively reproduced the crown and root rot symptoms observed in field conditions, demonstrating a similar level of aggressiveness among <i>F. oxysporum</i> isolates from pyrethrum plants. The results indicate the importance of soilborne disease management to reduce yield decline in pyrethrum fields and will help with the selection of aggressive isolates for resistance screening of pyrethrum varieties.