|Systematic Monograph of the Gnomoniaceae - Introduction|
The ascomycete order Diaporthales includes a number of plant pathogenic fungi the most notorious of which is the chestnut blight fungus (Cryphonectria parasitica) that altered the landscape of eastern North America (Anagnostakis 1987). Additional forest and tree diseases are caused by other members of the Diaporthales particularly in the Gnomoniaceae including oak dieback (Apiognomonia quercina), cherry leaf scorch (A. erythrostoma), sycamore canker (A. veneta) and ash anthracnose (Gnomoniella fraxinii, anamorph Discula fraxinea). Dogwood anthracnose, an introduced disease that is killing dogwood trees on both the east and west coasts of North America, is caused by Discula destructiva, an asexually reproducing species in the Diaporthales for which no sexual state is known despite considerable work (Castlebury et al. 2002; Zhang & Blackwell 2001). Recently it was discovered that the cause of butternut canker (Sirococcus clavigignenti-juglandacearum), a fungus that may cause the complete annihilation of another North American tree species (Juglans cinerea), has evolved from within the Gnomoniaceae (Castlebury, unpublished data). With molecular tools it is now possible to determine the closest relatives of these fungi that reproduce only asexually.
The Diaporthales are a group of microfungi that consist of about 94 genera with 500 species (Kirk et al, 2001) of which about 10-12 genera and 80-110 species belong in the Gnomoniaceae, excluding those known only as asexually reproducing species. Despite their economic importance these microfungi are relatively unstudied especially using molecular approaches. Generic concepts in the Diaporthales have not been reevaluated since they were established and are based primarily on the antiquated Saccardoan system dating from the late 1800.s that placed high value on ascospore characters. The few molecular studies in fungi in the Diaporthales suggest that the currently accepted generic concepts must be reevaluated (Castlebury et al. 2002, 2003; Zhang & Blackwell 2001). Accurate generic concepts are essential not only for grouping related species but also for determining the placement of mitotic fungi within an ascomycete phylogeny.
The majority of plant-associated microfungi reproduce asexually and lack any known sexual state although most are derived from ascomycetes (Rossman 1993). These mitotic microfungi include many serious emerging and invasive plant pathogens, thus knowledge of their taxonomic affinities is crucial both for control of the diseases they cause and determining their origin. Because they are often encountered only as asexually reproducing species, they have been subject to a separate classification system from sexually reproducing taxa, which sometimes results in names for both asexual (anamorph or mitotic) and sexual (teleomorph) states of the same fungus. The cause of dogwood anthracnose, Discula destructiva, is an example of such an introduced mitotic fungus, that first attacked dogwood trees on both the east and west coast of North America in the late 1970.s (Daughtry et al. 1996). Although its affinities to the Gnomoniaceae in the Diaporthales were hypothesized by Redlin (1991), Zhang & Blackwell (2001) were unable to infer the sexual state of Discula destructiva except that it belonged in the Gnomoniaceae. Part of this PEET project will include a study of mitotic fungi PD-2 that are derived from within the Gnomoniaceae and their associations and relationships to sexually reproducing genera.
Molecular data have supported the Diaporthales as a distinct order within the Sordariomycetes, the class including ascomycetous fungi that generally produce their asci in perithecial fruiting bodies (Castlebury et al. 2002, Farr et al 2001, Zhang & Blackwell 2001). The monophyletic Diaporthales circumscribed by molecular data correlate with morphological characteristics that define the Diaporthales. These include brown to black perithecial fruiting bodies immersed in stromata or host substrata, lack of true paraphyses at maturity, and unitunicate asci that often float free within the centrum at maturity and have a refractive ring in the apex (Barr 1978; Samuels & Blackwell 2001). The known asexual states of members of the Diaporthales are generally coelomycetous bearing their phialidic, often annellidic, conidiogenous cells and conidia in acervuli or pycnidia with or without a well-developed stroma.
Within the Diaporthales eight families have been recognized by various authors over the past 25 years although there has been no agreement on their circumscription. These familial classifications of the Diaporthales were summarized by Zhang & Blackwell (2001) based on Barr (1978, 1990), Kirk et al (2001), and Wehmeyer (1975). In the most comprehensive molecular study to date, Castlebury et al. (2002) analyzed nrLSU sequence data and determined that there were six major lineages in the Diaporthales. One of these, the Gnomoniaceae, included ten major teleomorph genera as well as a number of mitotic fungi for which no sexual state is known.