Thread by @C_H_Mesarich

Excited to share our preprint “Sequential breakdown of the complex Cf-9 leaf mould resistance locus in tomato by Fulvia fulva”, led by Silvia de la Rosa & Chris Schol! 🍅🔬1/8 biorxiv.org/content/10.110…

Most commercially available tomato cultivars resistant to leaf mould disease have the Cf-9 resistance locus, which carries the Cf-9B & Cf-9C resistance genes. Cf-9C provides resistance during all stages of plant growth, whereas Cf-9B provides resistance to mature plants only. 2/8

Shortly after the deployment of the Cf-9 locus in the 1970s, F. fulva strains began to emerge that could overcome Cf-9C through deletion of the cognate Avr9 effector gene. More recently, F. fulva strains have emerged that can overcome both Cf-9C & Cf-9B. 3/8

In this preprint, we report the identification of the Avr9B effector gene corresponding to Cf-9B & show that it encodes a cysteine-rich SSP. We then show nine independent Cf-9B resistance-breaking mutations in Avr9B across F. fulva strains collected from around the world. 4/8

Notably, in F. fulva strains recently collected from Cf-9 plants, we show a strict correlation between Avr9 deletion & resistance-breaking mutations in Avr9B. In contrast, in older strains, we show Avr9 absence but no mutations in Avr9B. 5/8

This suggests that the Cf-9 resistance locus has been sequentially broken down through the evolution of F. fulva strains over time, with Cf-9C circumvented prior to Cf-9B. 6/8

As F. fulva strains have overcome both Cf-9B and Cf-9C worldwide, we suggest that the Cf-9 resistance locus now has limited value for controlling leaf mould disease in commercial tomato production. 7/8

This work is the result of a fantastic collaboration between teams from @MasseyUni, @WUR (@pbr_wur/@phytoWUR) & @Setsunan_PP. 8/8