Phylogenetic studies on Phyllachora species infecting maize and other Poaceae
Tar spot of maize (Zea mays) caused by Phyllachora maydis (EPPO Alert List) was first reported in the USA in 2015. It then spread across several maize-producing areas of the USA and Canada, causing significant economic losses. Tar spot of maize is an emerging disease in the USA and Canada, but has been present in Mexico, several Caribbean islands and Central America for more than 70 years causing little damage. Studies have recently been carried out to better understand the genetic diversity of Phyllachora species causing tar spot using symptomatic samples of maize and wild grasses (Poaceae) collected in US maize fields and their vicinity, as well as herbarium specimens of Phyllachora spp. mainly associated with maize and Poaceae from the USA and other areas (Mexico, Central and South America, the Caribbean, Asia, and Europe). DNA sequences from 186 isolates collected from 16 host plants (Zea mays, other Poaceae and 2 dicotyledons) from 15 countries (Bolivia, Colombia, Costa Rica, Cuba, Dominican Republic, Germany, Guatemala, India, Mexico, Nicaragua, Peru, Philippines, Puerto Rico, Trinidad and Tobago, USA) were studied.
Results showed that these 186 isolates could be grouped into 5 distinct species.
- Phyllachora sp. 1 and Phyllachora sp. 2 were found only on maize (field and herbarium samples), and from a limited geographical range (Colombia, Mexico, Puerto Rico, and the USA).
- Phyllachora sp. 3 included field isolates collected from maize in the USA, but also a large number of herbarium isolates described under other Phyllachora species names (P. chaetochloae, P. diplocarpa, P. epicampsis, P. euphorbiaceae, P. graminis, P. heraclei, P. junci, P. rottboelliae, P. sylvatica, P. vulgata), in association with a broad range of hosts (Poaceae and two dicotyledons) and from various regions of the world (Asia, Europe, the Caribbean, Central America, Mexico, South America, and the USA). It is noted that the specimen of P. maydis collected in Mexico in 1904 and the P. maydis isolate used in the first report of tar spot in the United States in 2015 were both gouped under Phyllachora sp. 3 and isolates of this species have since been recorded in Illinois, Indiana, Iowa, Michigan, Minnesota, and Wisconsin.
- Phyllachora sp. 4 was found on maize from Guatemala and Venezuela (herbarium samples only), as well as on other Poaceae collected from the USA (field samples).
- Phyllachora sp. 5 was not found on maize but was recovered from a broad range of Poaceae collected in the USA (field samples).
These results show that the taxonomy of Phyllachora needs to be further clarified, as species which had previously been described only on their morphological characteristics and host range may be synonymous. They also strongly suggest that tar spot of maize in the USA is caused by a complex of several closely related Phyllachora species. The authors recommended that Phyllachora sp. 1, Phyllachora sp. 2, and Phyllachora sp. 3 should be referred to as ‘Phyllachora maydis species complex’ until further studies can properly delineate them.
Sources
Broders K, Iriarte‐Broders G, Bergstrom GC, Byamukama E, Chilvers M, Cruz C, Dalla‐Lana F, Duray Z, Malvick D, Mueller D, Paul P, Raid R, Robertson AE, Salgado-Salazar C, Smith D, Telenko D, VanEtten K, Kleczewski NM (2022) Phyllachora species infecting maize and other grass species in the Americas represents a complex of closely related species. Ecology and Evolution 12(4), e8832. https://onlinelibrary.wiley.com/doi/full/10.1002/ece3.8832