Definition of Gymnosporangium
: a genus of heteroecious rusts (family Pucciniaceae) that have mostly 2-celled teliospores whose pedicels and walls form a gelatinous mass when wet, that in the telial stage produce galls on cedars and other conifers of the genera Juniperus and Libocedrus, and that in the aecial stage cause rust spots on the leaves and fruit of apples, pears, and other plants of the family Rosaceae — see apple rust
