Production of appressoria at the tips of hyphae of Pyricularia oryzae.

Abstract

Appressoria-like structures (ALS) were formed at the tips of hyphae of the blast fungus in Petri dishes containing PDA or water agar. Some of them germinated in PDA media and produced again a relatively small appressorium at the tip of hypha germinated from the primary appressorium. The ALS were pigmented brown to olive, granular, smooth and thick walled. They were variable in size and shape, but mostly globose, ovoid or oblong, and 4-11μm (mean 10.1μm) in diameter. ALS were formed in all isolates used. More ALS were formed in plastic Fetri dishes than in glass ones. On the detached leaf sheaths of rice plants ALS adhered firmly to the host surface, from which the fungus penetrated into the host by piercing the outer walls of epidermal cells with a penetration peg. Susceptible blast lesions developed on the leaf blades of rice plants inoculated with mycelial agar blocks. Susceptible lesions also developed on the leaf sheaths of rice plants inoculated by injecting mycelial suspension. Micrographs of ultrathin sections revealed that ALS were covered with a mucilaginous substance and their walls were relatively thick and more electron-opaque than those of hyphae. From these observations of morphological and functional features, we concluded that ALS of hyphae are appressoria identical to the ones formed by germ tubes from conidia. This is the first report confirming that appressoria form at the hyphal tips of the blast fungus.