Occurrence of European Tar Spot (Rhytisma acerinum) on Norway Maple (Acer platanoides) Causing Severe Infections in Minnesota

Rhytisma species causing tar spot of maple (Acer spp.) have a worldwide distribution wherever maples are found. In North America, three Rhytisma species are found affecting Acer: R. americanum (Hudler & Banik), R. punctatum (Pers.) Fr., and R. acerinum (Pers.) Fr. For a number of years, R. americanum was erroneously called R. acerinum, despite early reports that there may be two distinct species (Bracher 1924). Hudler et al. (1998) described R. americanum as a separate species that occurs on red (A. rubrum) and silver (A. saccharinum) maples, whereas R. acerinum is only found on Norway maple (A. platanoides) in North America. Norway maple was introduced to the United States in about 1856 from England (Nowak and Rowntree 1990), and since then it has been planted widely as an urban ornamental tree. It is currently considered one of the most common invasive exotic tree species in the United States. R. acerinum appears native to Europe and was introduced into the United States, with the first outbreaks of severe defoliation noted in New York (Hudler et al. 1987). R. acerinum infects Norway maple, causing lesions on leaves and black masses of stromatal tissue. During severe infections, multiple lesions coalesce and remaining leaf tissue becomes necrotic, leading to defoliation. However, there are conspicuous morphological differences between the three Rhytisma species in North America. In the late summer of 2017, we observed extensive infections on Norway maple in the Minneapolis/St. Paul, MN, area, including the maple collection at the University of Minnesota Landscape Arboretum, which includes several A. platanoides cultivars, most of which showed severe infection. Several cultivars (‘Olmstead’, ‘Emerald Queen’, and ‘Crimson King’) appeared to have more leaf necrosis, which affected large portions of the crown compared with others (‘Columnare’), indicating there may be differences in susceptibility. In addition, two sugar maple (A. saccharum) cultivars (‘Sweet Shadow’ and ‘Bailsta’) showed limited infection, as well as a hybrid (A. truncatum × A. platanoides ‘Keithsform’), which to our knowledge has not yet been reported as a host. Further observations and testing are needed to identify possible resistance in Norway maple cultivars. R. acerinum DNA was extracted directly from infected leaves by carefully excising small portions of tar spots and briefly surface sterilizing with a 5% bleach solution, rinsing with 95% EtOH, and air drying, followed by a CTAB extraction protocol, which provided the template for the amplification of the internal transcribed spacer (ITS) region of rDNA (Blanchette et al. 2016). A Bayesian phylogenetic analysis using ITS data showed that R. acerinum samples separated from other Rhytisma species that formed two clades. One Minnesota sample shared a clade with the type specimen (GenBank GQ253100) originating from Germany along with tar spot samples obtained from Germany for this study with a posterior probability of 100%. The other Minnesota samples grouped in a closely related clade with samples from Massachusetts, Maine, and New York. R. americanum grouped with strong support with the type species described by Hudler et al. (1998). R. acerinum has just started to cause severe infection in Minnesota, and it is important to recognize the damaging impacts of this pathogen across the regions where Norway maple is planted in order that specific control measures may be implemented.