Spruce decline: Phomopsis spp. may be the main pathogen in the Lower Peninsula of Michigan
C. K. MCTAVISH (1), D. W. Fulbright (2), A. M. Jarosz (2). (1) Michigan State University, Rochester, MN, U.S.A.; (2) Michigan State University, East Lansing, MI, U.S.A.
In the last decade, spruce trees throughout Michigan have experienced a sharp increase in the incidence and severity of branch death, tip blight, and needle drop symptoms. While there are known pathogens of spruce that cause a few of these symptoms, no known pathogen causes the full range of symptoms that characterizes the current epidemic on both juvenile and mature trees found in the landscape, on tree farms, and in naturalized areas. There are no visual symptoms other than needle loss and eventual branch death. However, cankers can be found by scraping away the bark of dying stems. A survey of these cankers was performed at 24 sites across the lower peninsula of Michigan in 2013. Eight trees per site were sampled by taking one single branch per tree, and isolates were obtained from two cankers per branch for a total of more than 400 cankers. <i>Phomopsis</i> spp. were most commonly isolated from cankers (48% of isolates that are potentially pathogenic), followed by <i>Diplodia </i>sp. (26%). In contrast, <i>Cytospora kunzei</i>, a pathogen known to kill branches on spruce trees, was isolated from only 2% of the cankers. Future work includes defining the species of <i>Phomopsis</i> and <i>Diploidia</i> involved in spruce decline, as well as disease screening trials to determine host susceptibility to this emerging disease.
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