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Tar spot of maize in the Americas is caused by a complex of closely related Phyllachora species which vary in their host and geographic range
  • +16
  • Kirk Broders,
  • Gloria Iriarte,
  • Gary Bergstrom,
  • Emmanuel Byamukama,
  • Martin Chilvers,
  • Christian Cruz,
  • Felipe Dalla Lana,
  • Dean Malvick,
  • Darren Mueller,
  • Pierce Paul,
  • Diana Plewa ,
  • Richard Raid,
  • Alison Robertson,
  • Catalina Salgado,
  • Damon Smith,
  • Darcy Telenko,
  • Nathan Kleczewski,
  • Katherine VanEtten,
  • Zachary Duray
Kirk Broders
USDA-ARS

Corresponding Author:kirk.broders@usda.gov

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Gloria Iriarte
Independent Data Analyst
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Gary Bergstrom
Cornell University
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Emmanuel Byamukama
South Dakota State University
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Martin Chilvers
Michigan State University
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Christian Cruz
Purdue University
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Felipe Dalla Lana
Pennsylvania State University
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Dean Malvick
University of Minnesota Twin Cities
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Darren Mueller
Iowa State University
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Pierce Paul
The Ohio State University
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Diana Plewa
University of Illinois at Urbana-Champaign
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Richard Raid
University of Florida
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Alison Robertson
Iowa State University
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Catalina Salgado
USDA-ARS
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Damon Smith
University of Wisconsin-Madison
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Darcy Telenko
Purdue University
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Nathan Kleczewski
University of Illinois at Urbana-Champaign
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Katherine VanEtten
University of Illinois at Urbana-Champaign
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Zachary Duray
University of Illinois at Urbana-Champaign
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Abstract

The genus Phyllachora contains numerous obligate fungal parasites that produce raised, melanized structures called stromata on their plant hosts. Most members of this genus are not of significant economic concern, with the exception of P. maydis, the causal agent of tar spot of maize (Zea mays). Tar spot of maize has emerged as a major threat to maize production throughout the Americas and continues to spread throughout North America. To date, species designations for Phyllachora have been based on host associations and morphology, and the origin and diversity of the pathogen that causes tar spot is unknown. We assessed the sequence diversity of 186 single stroma isolates collected from 16 hosts representing 15 countries by amplification of the ITS and LSU gene regions. 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 data indicates there may be significant synonymy in the Phyllachora genus and additional work on species delineation and host specificity should be considered.