%0 Journal Article
%T Sequential breakdown of the Cf-9 leaf mould resistance locus in tomato by Fulvia fulva.
%A de la Rosa S
%A Schol CR
%A Ramos Peregrina Á
%A Winter DJ
%A Hilgers AM
%A Maeda K
%A Iida Y
%A Tarallo M
%A Jia R
%A Beenen HG
%A Rocafort M
%A de Wit PJGM
%A Bowen JK
%A Bradshaw RE
%A Joosten MHAJ
%A Bai Y
%A Mesarich CH
%J New Phytol
%V 243
%N 4
%D 2024 Aug 24
%M 38922927
%F 10.323
%R 10.1111/nph.19925
%X Leaf mould, caused by Fulvia fulva, is a devastating disease of tomato plants. In many commercial tomato cultivars, resistance to this disease is governed by the Cf-9 locus, which encodes five paralogous receptor-like proteins. Two of these proteins confer resistance: Cf-9C recognises the previously identified F. fulva effector Avr9 and provides resistance during all plant growth stages, while Cf-9B recognises the yet-unidentified F. fulva effector Avr9B and provides mature plant resistance only. In recent years, F. fulva strains have emerged that can overcome the Cf-9 locus, with Cf-9C circumvented through Avr9 deletion. To understand how Cf-9B is circumvented, we set out to identify Avr9B. Comparative genomics, transient expression assays and gene complementation experiments were used to identify Avr9B, while gene sequencing was used to assess Avr9B allelic variation across a world-wide strain collection. A strict correlation between Avr9 deletion and resistance-breaking mutations in Avr9B was observed in strains recently collected from Cf-9 cultivars, whereas Avr9 deletion but no mutations in Avr9B were observed in older strains. This research showcases how F. fulva has evolved to sequentially break down the Cf-9 locus and stresses the urgent need for commercial tomato cultivars that carry novel, stacked resistance genes active against this pathogen.