Mexican isolates of Leptosphaeria maculans belong to the aggressive strain of the fungus1

Abstract

Black leg of crucifers, caused by Leptosphaeria maculans (Desm.) Ces. and de Not., is the most important disease of broccoli and cauliflower in central Mexico, causing up to 70% yield loss in cauliflower crops. Isolates recovered from broccoli crops at Aguascalientes and Zacatecas, central Mexico, had the morphological characteristics of the aggressive strain of the fungus. Most of the isolates were pathogenic to cauliflower and canola, but two isolates had a low aggressivity on these hosts. Electrophoretic karyotypes of the Mexican isolates were generated by contour-clamped homogeneous electric field gel electrophoresis. The karyotypes were characteristic of isolates belonging to the aggressive strain ofL. maculans, with 9–12 chromosomes distributed in three size ranges, namely 2.3–1.9, 1.6–1.2, and 0.9–0.8 (minichromosome) megabases. Overall, minimum genome size ranged from 16.1 to 23.0 megabases. We have shown that the LMR1 repetitive element, which is specific to aggressive isolates ofL. maculans, was present in all the Mexican isolates, including the two isolates with low aggressivity on cauliflower and canola, confirming that these belong to the aggressive strain of the fungus.