MORPHOLOGY AND ULTRASTRUCTURE OF W-PATHOTYPE AND R-PATHOTYPE OF PSEUDOCERCOSPORELLA-HERPOTRICHOIDES ON WHEAT SEEDLINGS

The early infection of wheat seedlings by W- and R-pathotypes of the cereal eyespot fungus Pseudocerosporella herpotrichoides was studied by light and electron microscopy. The two pathotypes showed different patterns of coleoptile infection. Following spore adhesion and germination, W-type isolates produced orientated germ tubes and penetrated mainly through anticlinal cell walls, whereas R-type isolates invaded coleoptile tissues randomly. Plates of mycelium developing within the coleoptile produced spores at the surface and within tissue interstices - indicating the possibility of a secondary infection cycle. Rapidly growing runner hyphae were also formed from mycelial plates, and these initiated colonisation of the underlying leaf sheath. After extension growth predominantly along grooves overlying vascular traces, tip cells of runner hyphae differentiated to form discrete clusters of parenchyma-like cells termed infection plaques. Developing plaques of W- and R-pathotypes could be readily distinguished by their differing morphologies. Multiple penetrations took place via infection hyphae formed the base of central plaque cells; the rapidity and extent of leaf sheath penetration from these infection structures may be a pathogenicity determinant, enabling the fungus to outgrow host defence responses.