The genus Phyllachora contains numerous obligate fungal parasites that
produce raised, melanized structures called stromata on their plant
hosts referred to as tar spot. Members of this genus are known to infect
many grass species but generally do not cause significant damage or
defoliation, with the exception of P. maydis which has emerged as an
important pathogen of maize throughout the Americas, but the origin of
this pathogen remains unknown. To date, species designations for
Phyllachora have been based on host associations and morphology, and
most species are assumed to be host specific. We assessed the sequence
diversity of 186 single stroma isolates collected from 16 hosts
representing 15 countries. Samples included both herbarium and
contemporary strains that covered a temporal range from 1905-2019. These
186 isolates were grouped into 5 distinct species with strong bootstrap
support. We found three closely related, but genetically distinct groups
of Phyllachora are capable of infecting maize in the United States, we
refer to these as the P. maydis species complex. Based on herbarium
species, we hypothesize that these three groups in the P. maydis species
complex originated from Central America, Mexico and the Caribbean.
Although two of these groups were only found on maize, the third and
largest group contained contemporary strains found on maize and other
grass hosts, as well as herbarium specimens from maize and other grasses
that include 10 species of Phyllachora. The herbarium specimens were
identified based on morphology and host association, but our sequence
data indicates some Phyllachora species are capable of infecting a broad
range of host species and there may be significant synonymy in the
Phyllachora genus and additional work on species delineation and host
specificity should be considered.