Spruce Decline and Diaporthe: Incidence, Taxonomy, Virulence, and Tree Susceptibility in Michigan.

In the early 2000s, spruce trees in Michigan began displaying basal needle drop and branch death that slowly progressed upward, symptoms of what we call spruce decline. A survey in 2013 revealed that spruce decline was common throughout Michigan's Lower Peninsula, and Diaporthe was the most likely pathogen causing the cankers associated with these symptoms. Greenhouse inoculation studies completed Koch's postulates, confirming that Diaporthe could cause cankers that cause needle loss and branch death. The five different Diaporthe haplotypes isolated from symptomatic branches during the survey differed in virulence. Haplotypes 2, 4, and 5 were more virulent, and differed from each other by only one or two base pairs using the internal transcribed spacer (ITS) region and did not differ using the β-tubulin (TUB) gene. These haplotypes were unresolved phylogenetically. Haplotypes 1 and 3 were weakly virulent to avirulent on multiple spruce taxa, and fell into a resolved Diaporthe eres clade. Spruce taxa varied in susceptibility, with Colorado blue spruce (Picea pungens) the most susceptible, followed by Norway (P. abies), then white spruce (P. glauca). Spruce taxa that were much less susceptible were Black Hills (P. glauca var. densata), Serbian (P. omorika), and Meyer spruce (P. meyeri). We demonstrate that one or more Diaporthe species is causing cankers on declining spruce in Michigan, and these cankers elicit symptoms similar to the branch death expressed by declining spruce in the landscape. Future work will focus on further characterizing Diaporthe to species, and determining biotic and abiotic stressors that may predispose spruce trees to express decline symptoms.