Ultrastructural and cytochemical studies on infection of wheat spikes by Microdochium nivale

The infection process of Microdochium nivale on wheat spikes and the alteration of cell wall components in the infected host tissue were examined by means of electron microscopy and cytochemical labelling techniques. After spray inoculation at GS 65 (mid of flowering), macroconidia of the pathogen germinated with one to several germ tubes 6-12 h after inoculation (ai) on the host surfaces. Dense mycelial networks were usually formed on the interior surfaces of the glume, lemma, palea and on the ovary 24-36 hai, while hyphal growth on the exterior surfaces of the spike was scanty. Penetration of the host tissue occurred 36 hai by infection hyphae exclusively on the adaxial surfaces of the glume, lemma, palea and on upper parts of the ovary. The pathogen then extended first between the cuticle and epidermal cell walls as well as intercellularly. About 72 hai hyphae spread rapidly in host tissues both inter- and intracellularly. The hyphae reached the rachis from the infected glume, lemma, palea and ovary 7-8 dai. During colonization of the wheat spike, marked alterations in the host tissues were detected, including degeneration of cytoplasm, cell organelles, collapse of host cells, and disintegration of host cell walls. The enzyme- and immunogold-labelling investigations showed degradation of cellulose, xylan and pectin in the host cell walls of the infected tissues. These alterations of host cell walls indirectly demonstrated that M. nivale may secrete corresponding cell wall-degrading enzymes such as cellulase, xylanase and pectinase during infection and spreading in the wheat spike. These studies revealed that infection processes and spreading of M. nivale in wheat spike tissues closely resembled findings obtained with F. culmorum, F. graminearum and F. avenaceum.