Fusicoccum arbuti sp. nov. causing cankers on pacific madrone in western North America with notes on Fusicoccum dimidiatum, the correct name for Scytalidium dimidiatum and Nattrassia mangiferae.

Pacific madrone (Arbutus menziesii) is a broadleaf evergreen tree native to western North America that has been in decline for the past 30 years. A fungus has been isolated and was verified as the cause of cankers on dying trees. It was determined to belong in the genus Fusicoccum, an asexual state of Botryosphaeria. This genus in both its sexual and asexual states commonly causes canker diseases of deciduous woody plants. Using morphological and molecular data the fungus causing cankers on Pacific madrone is characterized, described and illustrated as a new species of Fusicoccum, F. arbuti D.F. Farr & M. Elliott sp. nov. No sexual state is known for F. arbuti. Evidence from the literature, cultures and specimens suggests that F. arbuti, often mistakenly identified as Nattrassia mangiferae, has been causing madrone canker since at least 1968. Authentic isolates of Nattrassia mangiferae as the synanamorph Scytalidium dimidiatum were sequenced and determined to be different from Fusicoccum arbuti and to belong in Botryosphaeria/Fusicoccum. In addition to molecular sequence data, the morphology of the pycnidial and arthric conidial states of Nattrassia mangiferae/ Scytalidium dimidiatum resembles that of Fusicoccum. Therefore the correct name for Nattrassia mangiferae and its numerous synonyms (Dothiorella mangiferae, Torula dimidata, Scytilidium dimidiatum, Fusicoccum eucalypti, Hendersonula toruloidea, H. cypria, Exosporina fawcetii, H. agathidia, and S. lignicola) is Fusicoccum dimidiatum (Penz.) D.F. Farr, comb. nov.