1University of the OFS, Bloemfontein, South Africa; 2University of Zambia, Lusaka, Zambia

Background and objectives
Rusts caused by Melampsora spp. are common in temperate regions of the Northern hemisphere. However, in the past few years Melampsora spp. have also been observed on Salix, Populus, Vitis and Euphorbia spp. in Zambia. Their occurrence has not yet been reported in the literature. Furthermore, few ultrastructural studies exist on uredospore development and spore shape in Melampsora [1]. No literature on ultratructure could be found for species occurring in Africa, despite the importance of these organisms as plant pathogens. The aim of this study was to examine uredospore shape using the SEM and uredospore development using the TEM. It is important to understand the ultrastructural morphology of uredospores as an aid in the identification and classification of Melampsora species.

Results and conclusions
Isolates of Melampsora were collected from Salix, Vitis, Euphorbia and two different Populus trees. SEM studies showed two distinct spore types. The first has an elongated spore shape with echinulations covering all of the spore surface, ca 1700 nm apart (Salix isolate) or with echinulations absent from one end of the spore, ca 1400 nm apart (two Populus isolates). The echinulations appear to be smoothly incorporated in the wall. The second type is spherical with echinulations covering all of the spore surface (isolates from Euphorbia and Vitis). The echinulations of the spherical forms are prominently isolated by surrounding thickened wall regions, ca 800 nm apart. The echinulations occur on top of these thickened wall regions in the Euphorbia isolate and slightly lowered into a pit in the wall in the Vitis isolate.

Development of the echinulations was studied using the TEM. The echinulations are formed within vesicles in the spore cytoplasm. The vesicles coalesce with the plasmalemma and the echinulation material is deposited into the wall. One spore form showed no echinulations at one end of the spore. This phenomenon can be explained by the deposition and maturation of the echinulations during development. The more prominent echinulations are observed at the lower end of the spore attached to uredophore. This is an indication that the spore wall matures from the attached end towards the opposite end. The presence or absence of echinulations at one end of the spore is only an indication of the maturity of the spore, and bears little taxonomic significance. However, the occurrence of echinulations on thickened wall areas, in pitted areas or smoothly incorporated in the wall seems to be taxonomically important.

This study shows that specific mechanisms of spore wall ornamentation and its development remains unanswered. However, the differences in spore shape between isolates suggest that spore shape and spore wall ornamentation are neglected but seemingly important characteristics that can be used in the taxonomy of Melampsora. Many questions remain unanswered, and a comprehensive ultrastructural comparative study of the genus in Africa should continue.

1. Spiers AG, Hopcroft DH, 1993. Mycological Research 98, 889-903.