Pathogenicity of sirodesmin-deficient mutants of Phoma lingam

Sirodesmin-deficient mutants were produced from aggressive isolates of Phoma lingam by UV-mutagenesis. The mutation rates were 7.1 x 10(-3) for mutants expressing a 100-fold reduction of sirodesmin PL in liquid cultures and 1.8 x 10(-3) for mutants with a 1000-fold reduction. The low ability of the mutants to produce sirodesmins was also apparent in planta after infection of cotyledons or the stem basis of Brassica napus. Growth rates under in vitro conditions, colony morphology, mating types and the formation of pycnidia and asci were not affected by the mutation. The mutants had the same ability as the wild types to infect cotyledons of the susceptible B. napus cv. Lirabon and to cause a typical grey-green tissue collapse. On the stem basis of the cv. Cobra, however, mutants caused significantly smaller lesions. These results indicate that phytotoxic sirodesmins produced by P. lingam have no basic role in pathogenesis and apparently are not involved in the virulence on cotyledons. However, it cannot be excluded that the compounds act as virulence factors on the stem base of B. napus.