Studies of selection in vitro for novel resistance to phytotoxic products of Leptosphaeria maculans (Desm.) Ces. & De Not. in secondary embryogenic lines of Brassica napus ssp. oleifera (Metzg.) Sinsk., winter oilseed rape

SUMMARY Culture filtrates of Leptosphaeria maculans (Desm.) Ces. & De Not. inhibited the germination of seeds of Brassica napus ssp. oleifera (Metzg.) Sinsk., winter oilseed rape. The toxicity of filtrates was broadly correlated with the pathogen icily of the isolates of L. maculans from which they were obtained, and the sensitivity of seeds of different cultivars was broadly correlated with the known resistances ot those cultivars to the fungus. Culture filtrates of B. maculans , when incorporated into culture media, were also toxic to secondary embryoids of B. napus ssp. oleifera , as also was Sirodesmin P.L., a putative pathotoxin of L. maculans. Variation for the ability to grow in culture in the presence of toxic levels of culture filtrate and Sirodesmin P. L. was detected among populations of secondary embryoids, and the proportion of resistant embryoids present in the population was increased by successive rounds of re‐selection on toxic, selective media. However, when embryoids were removed to non‐selective media, resistance to toxic culture filtrates and to Sirodesmin P.L. was rapidly lost. Plants regenerated from secondary embryogenic lines selected for resistance to toxic culture filtrate or Sirodesmin P.L. were no more resistant to infection by L. maculans then were plants regenerated from unselected, control secondary embryoids, and were consistently more susceptible than were plants grown from seed. It was concluded, firstly, that the rate of stable, heritable somatic variation may be insufficiently high in the secondary embryogenic lines to he of value in attempts to select in vitro for novel resistance to L. maculans secondly, that although the seed germination tests indicated that toxic culture filtrates may have potential as screening agents for detecting novel resistance to L. maculans in vitro , no evidence in support of this view was obtained in the experiments with secondary embryoids; and, finally, that because a relatively high level of resistance to L. maculans already exists in cultivars of H. napus ssp. oleifera , any novel variation m disease reaction that does occur m secondary embryogenic lines is likely to be towards greater susceptibility rather than increased resistance.