The teleomorph stage, Mycosphaerella graminicola, in epidemics of septoriatritici blotch on winter wheat in the UK

Earlier studies have shown that primary infections of winter wheat crops by septoria tritici blotch are initiated in autumn by air-borne ascospores of the teleomorph stage, Mycosphaerella graminicola, which originate from sou rces outside the crop. Subsequent disease development within the crop, and damage to the upper leaves in summer, were commonly ascribed to the anamorp h stage, Septoria tritici, which first arises from primary, ascosporic lesi ons and develops to give rise to splash-borne pycnidiospores produced in py cnidia. Trapping studies, using a Burkard volumetric trap sited adjacent to field trials of winter wheat, skewed that ascospores are released from pse udothecia throughout the year; peak release is not restricted to the autumn and early winter when the primary infections occur. As M. graminicola asco spores can be confused with those of other species, their authenticity was established by incubating ascospores on trap tapes for 24-48 h before count ing. Examination of wheat plants cv. Riband, removed from unsprayed crops j ust prior to harvest and exposed to external weather conditions over the wi nter in 1995 and 1996, showed that pseudothecia mature on residues of the u pper leaf layers between August and March. However, peak development occurr ed in December/January, when there was a corresponding decline in pycnidial formation. On regenerated wheat in set-aside (left uncultivated) fields, s ources of pseudothecia of M. graminicola were exhausted by February of the following year. Natural development of the teleomorph stage was monitored i n an unsprayed crop of winter wheat cv. Riband during the 1996-97 season, a nd was present on leaf 8 at growth stage 32 (24 April) and on leaf 3 at gro wth stage 85 (8 July), then a week later, on both leaf 2 and the flag leaf. The temporal development of M. graminicola asexual and sexual phases was a lso studied in inoculation experiments under external weather conditions on pot-grown seedlings of winter wheat cv. Longbow, using inoculum from two s ingle-ascospore cultures.