Vertical distributions of Fusarium spp. infections on mature wheat ears under natural field conditions

Wheat spikes have a vertical architecture, presenting multiple spikelets on which Fusarium spp. infectious propagules can land, initiate grain infection and possibly spread into the rachis to infect neighboring kernels. However, it is not known whether fungal infections occur randomly along the ear nor if species naturally interact within the ear. Over 4 years, five wheat fields in Belgium were surveyed annually and Fusarium spp. were isolated from kernels sampled from three positions on the ears (top, middle and bottom). This study revealed a vertical gradient of infection by Fusarium spp., with more isolates recovered from the bottom than from the top of the ears. This observation could result from infections by an inoculum from the ground, a downward movement of propagules present on the ear surface carried by rain and/or a downward spread of the fungus within the rachis. Moreover, multiple infected kernels originating from the same spike were frequent but, in most cases, the same species could be isolated from those grains and almost no relationships were observed among species present on the same spikes. Therefore, Fusarium spp. infections on wheat ears occur preferentially at the bottom of the spike and independently of the other species present.