Daily inoculum levels of Gibberella zeae on wheat spikes

The inoculum level of Gibberella zeae on wheat spikes was measured during 1 995 and 1996 in nine locations of Canada and the United States prone to Fus arium head blight of wheat. Spikes were exposed after exsertion and until k ernel milk or soft dough stage in fields with wheat or corn residue as a so urce of inoculum; other spikes were exposed in a location remote from any o bvious inoculum source; and in 1995 only, control plants remained in a gree nhouse. After 24 h, spikes were excised and vigorously shaken in water to r emove inoculum. Propagules were enumerated on selective medium and identifi ed as G. zeae from subcultures. Significantly more inoculum was detected fr om fields in epidemic areas than from remote sites in an epidemic and from fields in nonepidemic areas. The median inoculum level was 20 CFU of G. zea e per spike per day in fields experiencing an epidemic, 4 CFU in locations remote from epidemic fields, 2 CFU in nonepidemic fields, and 1 CFU in loca tions remote from a source of inoculum in nonepidemic areas. In an epidemic region, inoculum levels near corn stubble reached up to 587 CFU of G. zeae per spike per day, and the median inoculum level of 126 CFU was significan tly higher than the median of 13 CFU found near wheat residue. Inoculum was not detected or occurred sporadically during extended dry periods. While i noculum increased during rainy periods, timing of increased levels was vari able. Fusarium head blight epidemics were associated with multiple inoculat ion episodes and coincident wet periods.