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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.