Cytology and ultrastructure of the infection of wheat spikes by Fusarium culmorum

The infection of wheat spikes by Fusarium culmorum, one of the agents responsible for wheat head blight, was examined by light and electron microscopy. Macroconidia of the pathogen germinated 6-12 h after inoculation on all host surfaces they contacted. Developing germ tubes did not infect host tissues immediately, but gave rise to hyphae that grew and branched on host surfaces. Hyphal networks were usually formed 2 d after inoculation (dpi) on the inner surfaces of lemma, glume and palea, but not on the outer surfaces of lemma, glume and rachis. Hyphae on the outer surfaces of lemma and glume often grew over their edges to reach their inner surfaces. Penetration of host tissues occurred by infection hyphae on the inner surfaces of lemma, glume and palea, and on the upper part of the ovary. Occasionally, the pathogen invaded the host tissues through stomatal openings on the inner surface. Thereafter, the pathogen spread downwards to rachilla and rachis node by inter- and intracellular growth from above infected tissues. From the rachis node, hyphae extended downward to rachis and upward to peduncle through vascular bundles and cortical parenchyma tissue. When the pathogen arrived at the rachis 4-5 dpi, hyphae grew upwards and downwards inter- and intracellularly in vascular bundles and cortical parenchyma tissue of the rachis. During colonisation of the wheat spike, a series of alterations occurred in host tissues, including degeneration of host cytoplasm and organelles, collapse of parenchyma cells, disintegration or digestion of host cell walls and appearance of electron-dense coating materials on vessel walls.