Assessing the Pathogenicity of <i>Berkeleyomyces rouxiae</i> and <i>Fusarium oxysporum</i> f. sp. <i>vasinfectum</i> on Cotton (<i>Gossypium hirsutum</i>) Using a Rapid and Robust Seedling Screening Method

Cotton (<i>Gossypium</i> spp.) is the most important fibre crop worldwide. Black root rot and Fusarium wilt are two major diseases of cotton caused by soil-borne <i>Berkeleyomyces rouxiae</i> and <i>Fusarium oxysporum</i> f. sp. <i>vasinfectum</i> (<i>Fov</i>), respectively. Phenotyping plant symptoms caused by soil-borne pathogens has always been a challenge. To increase the uniformity of infection, we adapted a seedling screening method that directly uses liquid cultures to inoculate the plant roots and the soil. Four isolates, each of <i>B. rouxiae</i> and <i>Fov</i>, were collected from cotton fields in Australia and were characterised for virulence on cotton under controlled plant growth conditions. While the identities of all four <i>B. rouxiae</i> isolates were confirmed by multilocus sequencing, only two of them were found to be pathogenic on cotton, suggesting variability in the ability of isolates of this species to cause disease. The four <i>Fov</i> isolates were phylogenetically clustered together with the other Australian <i>Fov</i> isolates and displayed both external and internal symptoms characteristic of Fusarium wilt on cotton plants. Furthermore, the isolates appeared to induce varied levels of plant disease severity indicating differences in their virulence on cotton. To contrast the virulence of the <i>Fov</i> isolates, four putatively non-pathogenic <i>Fusarium oxysporum</i> (<i>Fo</i>) isolates collected from cotton seedlings exhibiting atypical wilt symptoms were assessed for their ability to colonise cotton host. Despite the absence of <i>Secreted in Xylem</i> genes (<i>SIX6</i>, <i>SIX11</i>, <i>SIX13</i> and <i>SIX14</i>) characteristic of <i>Fov</i>, all four <i>Fo</i> isolates retained the ability to colonise cotton and induce wilt symptoms. This suggests that slightly virulent strains of <i>Fo</i> may contribute to the overall occurrence of Fusarium wilt in cotton fields. Findings from this study will allow better distinction to be made between plant pathogens and endophytes and allow fungal effectors underpinning pathogenicity to be explored.