How does the wheat stem rust fungus harm its host plant

This is your answer The stem rust fungus produces several different structures during its life cycle. The most obvious of these on wheat are the reddish-colored urediniospores that form on leaves, stems, and heads during the growing season. These spores are spread by wind and infect wheat or barley plants.

there are so many examples for that in different areas, like biology experiment carried out in our lab recently.

here's one link: