Michelle Grabowski, UMN Extension Educator
University of Minnesota researchers Dr. Ben Lockhart and Dimitre Mollov have identified a new disease of spirea in Minnesota. Spireas are common landscape shrubs grown for their delicate foliage and summer flowers. The new disease, known as spirea stunt, causes plants to produce abnormally small leaves, that may be discolored yellow to reddish purple. Small discolored leaves grow only on clumps of weak branches known as witches' brooms. As a result, infected plants have one to many pom-pom like clumps of leaves and branches. The whole plant may be stunted and many infected plants do not survive the winter.
Spirea stunt disease is caused by a phytoplasma, a tiny bacteria that lives within the vascular system of infected plants. Dr. Lockhart determined that the phytoplasma infecting spirea in Minnesota belongs to the X-disease group and is related to similar pathogens that cause witches' brooms and stunting in blueberry, prunus and other shrubs. Little is known about the spirea stunt phytoplasma at this point. It is likely that this pathogen is transferred from plant to plant by leafhoppers or through plant propagation.
Once a plant is infected with spirea stunt phytoplasma, the pathogen spreads throughout the entire plant. There is no way to cure infected plants. Gardeners should remove shrubs infected with the spirea stunt phytoplasma as soon as possible.