Inoculum ofPyrenophora tritici-repentisin relation to epidemics of tan spot of winter wheat in Ontario

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Abstract
Inoculum, host development, weather factors, and disease were monitored in 1986-87 and 1987-88 as components of tan spot epidemics in winter wheat, caused by Pyrenophora tritici-repentis. Development of pseudothecia on wheat stubble residues exhibited a seasonal pattern in which the ascocarps enlarged during August-October, asci formed during December-March, and ascospores differentiated beginning in late February or March. Mature pseudothecia capable of releasing ascospores were found from early April to mid June, coinciding with wheat growth stages from tillering to anthesis or medium milk development. Significant linear relationships were found between incidence of pseudothecia that had matured and degree-day accumulation (base temperature 0°C). Conidia of the pathogen dispersed from diseased wheat leaves were trapped chiefly in June and July. From observations of inoculum and incidence of lesions, the epidemics were characterized by a prolonged simple-interest phase initiated by ascospores followed by a short compound-interest phase resulting from infection by conidia. Circumstantial evidence indicated that ascospores infected mainly the lower leaves (leaf 3 and below) and conidia the upper leaves. Inoculum densities equivalent to about 12 700-31 200 pseudothecia /m2 plot were required to produce moderate to severe tan spot. Implications of the data for tan spot management are discussed.