Depending on species: Some cause small yellow spots on leaves. Some completely distort and stunt the plant's growth. Many causes swelling of certain tissues that when opened are shown to be filled with brown-black spores.
Having wounds in a plant. Hot and dry weather
The spores germinate in the spring and infect young plants through wounds. The fungi multiply inside areas of plants causing them to swell. These swellings will eventually dry out and burst open releasing massive amounts of black spores that infect other hosts.