Michelle Grabowski, UMN Extension
Quince rust, caused by Gymnosporangium clavipes, can infect over 480 species of plants within the Rosaceae family. This includes serviceberry (Amelanchier spp.), hawthorn (Crataegus spp.), chokeberry (Aronia sp.), mountain ash (Sorbus spp.), apple and crabapple (Malus spp.). Quince rust frequently infects fruit and petioles, causing them to be swollen and deformed. At this time of year, the fungus produces short white cylindrical spore producing structures all over infected fruit. These spore producing structures open up to release bright orange powdery spores that give rust fungi their name. Infected fruit will die. Although odd looking, these infections are not harmful to the overall health of the plant.