What is "telium"

n. A sorus, in the form of a pustule, in the tissue of plants infected with the fungus ''rust''

Telium, plural Telia, are structures produced by rust fungi as part of the reproductive cycle. They are typically yellow or orange drying to brown or black and are exclusively a mechanism for the release of teliospores which are released by wind or water to infect the alternate host in the rust life-cycle.