Corresponding author: Z. Wilhelm de Beer (
Academic editor: T. Lumbsch
Recent reclassification of the
de Beer ZW, Marincowitz S, Duong TA, Wingfield MJ (2017)
Oak wilt is a serious disease of many
Oak wilt is caused by a fungus in the genus
Generic boundaries within the
The fourth unresolved lineage in the study of
During the course of the six decades following the
The aim of this study was firstly to consider the appropriate generic placement of the oak wilt fungus in the
Herbarium specimens labelled as
Isolates of
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Minnesota | = |
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Iowa |
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Iowa | = |
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Iowa | = |
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†Information on other species and isolates included in this study and their GenBank accession numbers are available in
‡
§ Where DNA sequences of different isolates were identical, we only deposited one sequence representing each haplotype in GenBank. Identical sequences obtained from other isolates are indicated with ‘=’
EP = Ex-epitype.
Three gene regions, the nuclear ribosomal DNA large subunit (
Representative species of the dominant genera in the
Morphological characters of sexual and asexual structures taken from the herbarium specimens and living isolates were compared with each other and with the original descriptions (
Microscopic structures taken from herbarium specimens were mounted and studied in 10 % KOH, and those from living cultures were mounted in water, later replaced with 85 % lactic acid in which they were then studied. Up to 50 measurements were made for each characteristic structure where possible. Microscopic structures were studied with a Nikon SMZ18 stereoscope and a Nikon Eclipse Ni compound microscope. Images were captured using a Nikon DS-Ri2 camera. Measurements were made using the Nikon Imaging Software (NIS) Elements (v. 4.3).
DNA sequences obtained for the
A total of 39 isolates representing 35 species were included in the phylogenetic analyses. Alignment of the
The four
Bayesian phylogram derived from the analyses of the concatenated dataset (
The herbarium specimen of
The herbarium specimen of
Laboratory crosses between the living isolates (Table
Features of the conidiophores were almost identical between the two herbarium specimens and the living isolates (Figure
Culture characteristics of the fresh isolates were similar to those of the Bretz specimen (FP 97476), forming fluffy, thick mycelial mats containing the sexual structures (Figure
Only a few broken ascomata were removed from the Bretz specimen (FP 97476) for this study. The shape of the ascomata was similar to those described by
Morphological features of herbarium specimens and a living isolate of the oak wilt fungus.
Morphological comparisons with herbarium specimens representing
Named after Theodore W. Bretz who first discovered and described the sexual state of the type species of this genus (
The genus is distinguished from all other genera of the
The only known species in the genus causes vascular wilt on various oak species in North America.
Line drawings of the oak wilt fungus. These illustrations are based on previously published line drawings and observations of the herbarium specimens (BPI 595712, FP 97476) in the present study.
Based on the one fungus one name principles adopted in the Melbourne Code (
In the present study, we have shown that the oak wilt fungus does not belong in
In the protologue of
The lectotypes designated here for
The oak wilt fungus is an economically important pathogen in the USA, with the potential to become a serious, alien invasive if it was ever introduced into other countries having oak forests. It is listed as a quarantine organism by the European and Mediterranean Plant Protection Organization (EPPO) and the European Union (EU) (
In addition to phylogenetic data, the unusual biology of the oak wilt fungus supports the description of the new genus,
The choice of an epithet for the new species name in
Subsequent to careful morphological comparisons, two lectotypes and an epitype have bene designated for the two basionyms,
We thank the curators at BPI (U.S. National Fungus Collections, Beltsville, Maryland) and FP (Forest Service at the Center for Forest Mycology Research, Madison, Wisconsin), for making specimens available for this study. We also acknowledge the financial support of members of the Tree Protection Cooperative Programme (TPCP), the NRF/DST Centre of Excellence in Tree Health Biotechnology (CTHB), and the University of Pretoria.