Chromosomal deletion in isolates of Phytophthora infestans correlates withvirulence on R3, R10, and R11 potato lines

In Phytophthora infestans, a cluster of three dominant avirulence genes is located on the distal part of linkage group VIII. In a mapping population f rom a cross between two Dutch field isolates, probe M5.1, derived from an a mplified fragment length polymorphism (AFLP) marker linked to the Avr3-AVr1 0-Avr11 cluster, hybridized only to DNA from the parent and Fl progeny that is avirulent on potato lines carrying the R3, R10, and R11 resistance gene . In the virulent parent and the virulent progeny, no M5.1 homologue was de tected, demonstrating a deletion on that part of linkage group VIII. P. inf estans is diploid, so the avirulent strains must be hemizygous for the regi on concerned. A similar situation was found in another mapping population f rom two Mexican strains. The deletion was also found to occur in many field isolates. In a large set of unique isolates collected in The Netherlands f rom 1980 to 1991, 37% had no M5.1 homologue and the deletion correlated str ongly with gain of virulence on potato lines carrying R3, R10, and R11. Als o, in some old isolates that belong to a single clonal lineage (US-1) and a re thus highly homogenous, deletions at the M5.1 locus were detected, indic ating that this region is unstable.