Deposition patterns of Fusarium graminearum ascospores and conidia within a wheat canopy

Fusarium graminearum is the most important species in the fungal complex causing Fusarium head blight of small grain cereals. The fungus produces two types of spores on crop residues (ascospores and conidia), which are dispersed to ears by air currents and rain splashes, respectively. The distribution patterns of ascospores and conidia within a wheat canopy between booting and grain maturity were assessed by using leaflike spore traps placed at 10, 30, and 60 cm height, and ear-like spore traps at 90 cm height. Maize residues were the inoculum source for both ascospores and conidia within the wheat plot. Of the total spores counted, 93 % were ascospores and 7 % were conidia. Approximately 41, 22, 19, and 18 % of the ascospores, and 77, 10, 8, and 5 % of the conidia were sampled at 10, 30, 60, and 90 cm height, respectively. Ascospore numbers did not significantly differ between those sampled on the upper or the lower sides of the leaf-like traps or among the four orientations (north, south, east, or west) of the ear-like traps. According to the index of dispersion (D), the spatial distribution of trapped ascospores was largely random (i.e., D≤1) rather than aggregated (i.e., D>1). The collective results (averaged across all traps and sampling periods) showed that the random distribution of the ascospores within the wheat canopy and at the ear level was associated with a clear vertical distribution pattern indicating an upward movement of ascospores from the maize residues on the ground.