ABSTRACTPenetration and colonization of orchardgrass byRhynchosporium orthosporumand production of conidia by the fungus were investigated by light, fluorescent, and scanning electron microscopy. Conidia ofR. orthosporumgerminated to produce an appressorium, and the fungus penetrated directly through adaxial or abaxial leaf surfaces. After penetration, mycelium proliferated in the intercellular subcuticular region. Hyphae became intracellular after disintegration and death of the mesophyll cells. At this stage, symptoms were evident on leaf blades as irregular lenticular lesions. Conidia were produced on leaf surfaces from subcuticular mycelium on short conidiophores that extruded through stomata and cuticular holes. During early stages of tissue colonization, masses of conidia developed and matured on and above the intact cuticle. Subcuticular mycelium remained protected by the cuticle for most of the period of spore production.