FHB is a potentially very damaging wheat disease, present in most wheat-growing areas and caused by a species complex from which F. graminearum, F. culmorum, F. poae, M. majus and M. nivale are the most common. These fungi may infect wheat spikes at flowering but infections at late stages of kernel development are poorly understood. Moreover, disease surveys have shown that co-infection of the same spike by two or three species is frequent and could modify their development in spikes. To investigate these questions, several isolates of the species cited above were characterised for their life traits and aggressiveness on spikes. We then investigated the development of Fusarium isolates inoculated on wheat spikes 3 days before or 0, 8, 18 and 28 days post anthers extrusion. The highest disease and toxin levels were found around anthesis. The inoculation date appeared to condition the disease severity and to influence the isolates development. Early or late contaminations were however possible with aggressive isolates, which suggest the possibility of such infections in field. In a second experiment, we studied the competition between F. graminearum and the other species (F. culmorum, F. poae, M. majus and M. nivale). The responses were variable; nevertheless, aggressive isolates were most of the time unchanged in their development or even promoted whereas less aggressive isolates were reduced against more aggressive ones. The toxin levels were generally unchanged or they decreased in co-inoculations compared to single inoculations. In a last experiment, we investigated the isolate competition and the movement of fungi in the spikes. Alone, isolates progressed in the whole spike whereas the presence of another fungus impaired their development, suggesting a competitive interaction. This study contributes to our understanding of these pathogens epidemiology and provides new elements for a better understanding of the contamination of wheat kernels at the field level.
