Infection pathways and penetration modes of Fusarium langsethiae

Fusarium langsethiae is one of the common Fusarium species infecting small grain cereals in the Nordic region and the UK. It is usually described as a weak pathogen, and with a strong preference for oats, although no studies have yet addressed the explanations for this at the microscopic level. Using microscope techniques we have studied the early steps of colonization of oat and wheat grain by F. langsethiae particularly addressing the role of pollen in the infection process and the fungal ability to penetrate plant cell wall. The aim was to better understand its non-aggressive colonization picture and why oat is preferred over wheat as a host. Spray inoculated oat and wheat plants were scored for fungal progression at 3, 6, 10 and 14days post inoculation (dpi) using light microscopy and scanning electron microscopy (SEM). Fungal hyphae entered the grain at the apex, or along the sides in the overlapping zone between palea wings and lemma, then spread basipetally and laterally, with a clear directional growth towards the caryopsis. Hyphal growth was clearly aided by the presence of pollen. On oat proliferating hyphae developed a variety of penetration structures on all internal surfaces. F. langsethiae infection on wheat progressed along the same routes, however slower and overall with less hyphal mass. Interestingly, hyphae closely associated to the wheat caryopsis seemed to undergo degradation, and profuse conidiation was observed at 14 dpi. Explanations for the differences in F. langsethiae colonization of oat versus wheat are suggested in light of the results.