Ultrastructure of telia and teliospores of the rust fungus Cronartium quercuum f. sp. fusiforme

Abstract

A combination of scanning and transmission electron microscopy was used to examine telia and teliospores of C. quercuum f. sp. fusiforme, the cause of fusiform rust disease of southern pines. Telial columns consisting of chains of thick-walled teliospores whose walls were fused laterally developed either singly or in clusters on the abaxial surfaces 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 mesophyll cells. Teliospores were initially binucleate, but became uninucleate as a result of karyogamy. Meiosis began soon after karyogamy as evidenced by the appearance of synaptonemal complexes in the teliospore 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 region of the spore wall to form a germination site. A metabasidium then emerged from the spore at the germination site. Once exposed on the surface of the telial column, the metabasidium ceased to elongate and its tip became filled with a plug-like mass of fibrillar material.

Key Words:

• dormancy
• germination
• karyogamy
• meiosis
• Quercus nigra
• Q. shumardii
• synaptonemal complex