ULTRASTRUCTURE OF TELIA AND TELIOSPORES OF THE RUST FUNGUS CRONARTIUM-QUERCUUM F-SP FUSIFORME

A combination of scanning and transmission electron microscopy was use d to examine telia and teliospores of C. quercuum f. sp. fusiforme, th e cause of fusiform rust disease of southern pines. Telial columns con sisting of chains of thick-walled teliospores whose walls were fused l aterally developed either singly or in clusters on the abaxial surface s of oak leaves that had been inoculated with urediniospores. Many but not all columns emerged from old uredinia. Teliospores developed from a layer of sporogenous cells that arose subepidermally near spongy me sophyll cells. Teliospores were initially binucleate, but became uninu cleate as a result of karyogamy. Meiosis began soon after karyogamy as evidenced by the appearance of synaptonemal complexes in the teliospo re nucleus. These structures then disappeared and meiosis was arrested . A dormancy period was not required prior to teliospore germination. The first evidence of germination was the degradation of a small regio n of the spore wall to form a germination site. A metabasidium then em erged from the spore at the germination site. Once exposed on the surf ace of the telial column, the metabasidium ceased to elongate and its tip became filled with a plug-like mass of fibrillar material.