Ultrastructure of teliospores and promycelium and basidiospore formation in the two-spored form of Gymnoconia nitens, one of the causes of orange rust of Rubus

Abstract

Gymnoconia nitens (Schwein.) Kern & H. W. Thurston consists of two forms that cause orange rust of Rubus . The teliospores of both of these forms are morphologically identical to aeciospores, but upon germination form promycelia that give rise to either two or four basidiospores. In this study, we examined the teliospores and the sequence of events leading to basidiospore development in the two-spored form of G. nitens. The teliospores of this species are formed in chains from uninucleate sporogenous cells that line the base of each sorus on infected Rubus argutus Link. (blackberry) leaves. Fluorescence microscopy revealed that approximately 90% of the mature teliospores were uninucleate. During germination, the teliospore nucleus migrated into the developing promycelium and underwent a single mitotic division. A transverse septum formed between the two daughter nuclei creating two uninucleate cells. A tapered sterigma arose from each cell and gave rise to a basidiospore. These findings indicate that the basidiospores of the two-spored form of G. nitens were formed in an asexual fashion.