Chrysomyxa rust: morphology and ultrastructure of D-haustoria, uredinia, and telia

An electron- and light-microscopic study revealed that D-haustoria of Chrysomyxa rhododendri (DC.) De Bary and Chrysomyxa pirolata Winter were pedicellate and had a more or less tuber-shaped haustorial body. The haustorial necks were wrapped by a fold of the extrahaustorial membrane (velopedunculate haustorial type). The aecioid uredinia of Chrysomyxa rhododendri and Chrysomyxa empetri (Pers.) Schröter were covered by a one-layered peridium of the pucciniastreous type. In Chrysomyxa pirolata, young uredinia were covered by one to three layers of sterile fungal cells. In all investigated species, urediniospore mother cells were formed blastically by proliferating basal cells. Subsequent cell divisions of the urediniospore mother cells led to the formation of urediniospores and intercalary cells. In Chrysomyxa rhododendri, the intercalary cells had a pronounced proximal thickening of the radial wall. In Chrysomyxa pirolata andChrysomyxa empetri, the wall was uniformly thickened. Telial morphology of Chrysomyxa rhododendri and Chrysomyxa pirolata was very similar. In Chrysomyxa rhododendri, the probasidia formed true chains without intercalary cells, and they are probably of thallic origin. During maturation they partly separated at their septa by dissolution of the primary wall of the septa between the probasidia. No sterile elements were present in the telia. The occurrence of velopedunculate D-haustoria and a uredinial peridium of the pucciniastreous type confirm the systematic position of Chrysomyxa close to pucciniastreous rusts, Cronartium, and Coleosporium within Melampsoraceae.