Since 2006 there has been a decline in Colorado blue spruce (CBS; Picea pungens) planted as landscape trees and for Christmas tree production throughout the Lower Peninsula of Michigan, United States. This decline is characterized by a gradual loss of needles in the lower portion of the tree starting at branch tips, followed by entire branch dieback which gradually progresses upwards over several years. This dieback is linked to shallow branch cankers visible in the phloem when the bark layer is removed. Isolates in the fungal genus Diaporthe have been consistently isolated from lesion margins on symptomatic branches. To determine the species of Diaporthe linked to the decline of CBS in Michigan, seven gene regions were sequenced from a collection of Diaporthe isolates collected in 2011-2018 from CBS and other woody hosts. Subsequent phylogenetic analyses indicated that Diaporthe eres and a novel Diaporthe clade were present on symptomatic CBS in Michigan. The new species Diaporthe vadumcancri is described. Koch’s postulates were confirmed for D. vadumcancri and D. eres and D. vadumcancri is significantly more virulent than D. eres on CBS. Due to sampling only of the terminal ends of branches and the inconspicuous nature of cankers, it is likely that the etiology of spruce decline has commonly been attributed to a range of needle cast diseases, instead of the canker causing Diaporthe spp. Further research focused on better understanding the epidemiology of Diaporthe spp. is essential to adequately manage for this disease issue.