M. Grabowski, UMN Extension
A new form of an old disease has been identified in Minnesota. The fungal pathogen Phomopsis sp. has long been known to cause shoot blight on spruce trees in nurseries and tree farms. The same pathogen has now been found causing cankers, needle loss, branch death and in severe cases tree death of mature spruce trees in the landscape. This disease has been named Phomopsis spruce decline. The first case of Phomopsis spruce decline in Minnesota was identified by the University of Minnesota Plant Diagnostic Clinic this spring (May 2014).
In nurseries and on tree farms, Phomopsis shoot blight causes young needles and shoots to turn brown and curl downward. In some cases dark resinous cankers formed on young stems but damage does not commonly extend beyond that years new growth. Although this disease causes damage in nurseries and on tree farms, it has not been a common problem in landscape trees.
In recent years, Dr. Fulbright of Michigan State University has been researching decline of mature spruce trees in landscapes and on tree farms. Declining trees had discolored needles that would fall off prematurely. The buds at the tip of the branch would remain alive for awhile but eventually the branch would die. These symptoms would often progress from the bottom of the tree up into the canopy. In severe cases the tree died.
Dr. Fulbright's research led to the discovery of a new type of disease caused by an old pathogen. When bark was peeled off branches of declining spruce trees dark brown staining of the wood could be seen. These infections, called cankers, were caused by Phomopsis sp. the same fungal pathogen capable of causing shoot blight in nursery spruce trees. Since then Dr. Fulbright's research group have found Phomopsis infections associated with decline of mature spruce trees in landscapes all throughout Michigan's lower peninsula.
Many questions remain to be answered about Phomopsis spruce decline. It is unknown how or why this pathogen began infecting mature landscape trees. It is known that in nurseries the disease is significantly more severe on stressed trees. It is therefore possible that stress from environmental conditions, other pests or pathogens or other factors have played a role in the development of Phomopsis spruce decline. Several other fungal pathogens infect spruce trees in both Michigan and Minnesota. These pathogens cause needle discoloration and drop, as well as branch cankers. How all of these known pathogens interact with Phomopsis sp. is unclear. At this time there are no management recommendations for trees suffering from Phomopsis spruce decline.
While research on the biology and management of this disease continues, gardeners should do their best to reduce stress on spruce trees as well as all landscape trees. Water trees during times of drought. Mulch the area around the tree with 2-4 inches of wood chips or other organic mulch. Protect trees from damage from weed whips, lawn mowers and other equipment.
For diagnosis of spruce diseases, contact the UMN Plant Diagnostic Clinic at firstname.lastname@example.org or 612-625-1275.