Phragmidium mucronatum Rose Rust
This photomicrograph was made by scraping a pustule on a rose leaf collected about a hundred years ago and stored in a labelled packet in the mycological herbarium at the University of Guelph.
The micrograph shows the characterisitic multi-celled teliospore (sexual resting spore) of the rust Phragmidium. The spore has thick, pigmented walls that protect it during extreme weather and from ultraviolet (solar) radiation during the long winter months.
Rose rust is a disease caused by any one of several
species of the fungus Phragmidium. Both native and cultivated varieties of
roses are susceptible to infection. Phragmidium affects Roses during hot
humid weather. At first small, yellow to orange-yellow spore pustules are produced
on the underside of the leaves. These are the asexual uredospores that are windborne
and spread the disease to other plants during the summer months. Later the pustules
become dark brown to blackish with the producion of the thick-walled, brown, sexual spores
(teliospores) that are the overwintering stage of he fungus. In the spring the
teliospores germinate to produce another spore type (basidiospore) that start a fresh
cycle of disease in the new growing season.