Abstract

Ascospores of the wheat pathogens Phaeosphaeria nodorum (MÜller) Hedjaroude and Mycosphaerella graminicola (Fuckel) Schroeter were trapped above infected wheat straw in 1990 in an area isolated from other possible sources. Release of ascospores by P. nodorum and M. graminicola, and by the oat pathogen P. avenaria f. sp. avenaria (Weber) Eriksson was monitored in the field between April and September 1991 to determine the availability of ascospores as a source of primary inoculum. The major period of ascospore release of P. nodorum and P. avenaria f. sp. avenaria was from mid-June to late August, which corresponded with the seedling to early flag emergence stage of crop development. The number of ascospores trapped was high, exceeding 4000 ascospores/m³ air/day for P. nodorum and 1000 ascospores/m³ air/day for P. avenaria f. sp. avenaria. The major release of ascospores of M. graminicola was from mid-May to late July, corresponding with the early seedling stage of crop development. The numbers trapped, however, were low and did not exceed 100 ascospores/m³ air/day. P. nodorum was recognised as the predominant wheat leaf pathogen at the trapping site and this was reflected in larger numbers of ascospores of P. nodorum compared with those of M. graminicola.