Characterization and pathogenicity of isolates of Leptosphaeria maculans from Aguascalientes and Zacatecas, Mexico

Blackleg of crucifers caused by Leptosphaeria maculans is the most importan t disease of broccoli and cauliflower in central Mexico, causing up to 70% yield loss in cauliflower crops. The pathogen was isolated from broccoli cr ops near Aguascalientes, Ags., and Zacatecas, Zac., and culture characteris tics and pathogenicity on cauliflower were examined. Colony appearance and mycelial growth on V8-juice agar, germ tube growth on water agar, and the a bsence of pigment production in Czapek-Dox medium were characteristic of is olates belonging to the aggressive strain. No pseudothecia formed when the Mexican isolates were paired nor when they were crossed with a tester isola te for mating type +, suggesting that all of the Mexican isolates were of m ating type +. This was indirectly confirmed by the absence of the MAT-2 gen e, which is associated with mating type - Reliable inoculation techniques, based on inoculation of wounded plants with pycnidiospores, were developed for cauliflower. In seedlings whose cotyledons were wound-inoculated, the f ungus progressively destroyed the cotyledons, then the petioles, the cotyle donary node, and eventually the stem, resulting in dieback and a wire stem. In stem-inoculated plants, the fungus caused wire stem and leaf wilt. The Mexican isolates were aggressive to oilseed rape and belonged to pathogenic ity group 2.