Mycosphaerella leaf spot diseases of bananas - CBS

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Mycosphaerella leaf spot diseases of bananas: present status and outlook • Section Caterva M.E. Barr is characterized by cylindrical asci and irregularly arranged, thin-walled, often medium-sized ascospores that are rarely constricted at the septum and inequilateral, with more or less pointed ends. Asteroma and Asteromella spermatial forms are typical. Representative species: the common polyphagous M. subradians (Fr. : Fr.) Schroeter. • Section Longispora M.E. Barr is characterized by cylindrical asci with aggregated, thin-walled, long and slender ascospores that are rarely constricted at the septum and mostly equilateral, long but slender ascospores, characteristically with rounded upper and pointed lower ends. Anamorphs: Phloeospora or Septoria sensu lato. Representative species: M. millegrana (Cooke) Schroeter (with short spores), M. latebrosa (Cooke) Schroeter (with longer spores) and M. populi (Auersw.) J. Schröt. (with the longest spores in the genus). The genus Sphaerulina Sacc., which differs only by additional septa, may be synonymous. • Section Fusispora M.E. Barr is characterized by pyriform asci and irregularly arranged, thin-walled ascospores that are rarely constricted at the septum and mostly equilateral, fusiform, pointed ascospores. Anamorphs have not been proved. • Section Plaga M.E. Barr (including Section Macula M.E. Barr) includes endophytic species sporulating on leaf spots, many of which are described as plant pathogens. This section is characterized by obovoid to ellipsoidal or cylindrical asci, small to medium sized ascopores, fusiform to obovoid with rounded ends. Many species have been described in these groups, and the majority originate from warm-temperate and tropical areas. Anamorphs include Colletogloeopsis, Mycovellosiella, Phaeophleospora, Pseudocercospora, Pseudocercosporella, Sonderhenia, Stenella Syd., Dissoconium, Uwebraunia and possibly others. Representative species: listed by Crous (1998) on Eucalyptus. How do we separate species in this complex based on morphology? Mycosphaerella encompasses species with a saprobic, plant pathogenic as well as a hyperparasitic habit. In general, however, most species are found to be associated with leaf spots. Some species have been isolated as endophytes (Crous, 1998) but this is not the norm. Importantly, up to four taxa have been reported from the same lesion on leaves of Eucalyptus (Crous, 1998) suggesting that some act as primary and others as secondary or weak pathogens. In the past, the taxonomy of Mycosphaerella relied mostly on aspects such as host, symptom type, and teleomorph morphology (pseudothecium, ascus and ascospore morphology). Very few studies focused on cultures, therefore ascospore germination patterns and cultural characteristics are unknown for most species. Furthermore, no ex-type cultures are available for molecular studies. Most links to anamorphs have also been based on association. Given the concept of sympatric colonisation of host tissue discussed above, this has led to numerous erroneous reports of anamorph/teleomorph links. Anamorph 46
Session 1 P.W. Crous et al. morphology has focused strongly on aspects such as conidiomatal structure, and mode of conidiogenesis (von Arx, 1983; Sutton and Hennebert, 1994). An important overlap has been observed between different conidiomatal types (Nag Raj, 1993; Braun 1995), hence species have been transferred from one anamorph genus to another (Sutton et al., 1996; Braun, 1998). The separation of some coelomycete and hyphomycete anamorphs of Mycosphaerella is, therefore, debatable. Subsequent to the wide taxonomic concept employed for the cercosporoid fungi by Chupp (1954), Deighton (1973, 1976, 1987, 1990) recognised the value of pigmentation, conidiogenesis and conidium release. Different types of dehiscence scars (David, 1993) have subsequently been recognised to separate genera such as Cladosporium, Cercospora and Stenella. Conidiogenesis is variable in this complex (Verkley, 1997), as are dehiscence scars (Crous, 1998), but it still remains a useful feature to separate species (Braun, 1995). Integrating morphological and molecular data sets The issue of integrating morphological and molecular data sets in Mycosphaerella is beset with numerous problems. Several hypotheses have been proposed to divide the genus into separate parts, groups, sections or genera. The molecular work to date, however, mainly supported one major clade of Mycosphaerella (Table 1), as well as two minor clades, typified by Dissoconium and Cladosporium (Crous et al., 2000, 2001). That anamorph concepts have not always correlated with different groups in Mycosphaerella, but have been shown to have evolved more than once within the genus, has caused confusion. Sometimes, however, integration has simplified or reduced the genera. For example, species of Mycovellosiella (superficial mycelium; conidia in chains), Phaeoramularia (no superficial mycelium, conidia in chains), and Passalora (no superficial mycelium, conidia solitary) consistently cluster together, leading to synonymy of the genera under the older name Passalora (Crous et al., 2001). Pseudocercospora is another confusing example. Published work (Crous et al., 2001) indicates that several other genera are also clustered here, including Pseudophaeoramularia U. Braun, and Stigmina, while the position of Stenella was still not clearly resolved. As more taxa have been added to our analyses (unpublished data), it has become clear that Stenella and Stigmina are in fact good genera, whereas Pseudophaeoramularia clusters in Pseudocercospora. These findings are still in agreement with those of Crous et al. (2001), namely that conidial catenulation is not a good feature at the generic level, and that the degree of thickening of spore scars is important, rather than just presence or absence. Molecular data will also influence the way we define the teleomorph. Preliminary data suggest that ascospore septation, and the presence of pseudoparenchymatal stromatic tissue around the ostiole is of lesser taxonomic value at the generic level (unpublished data). This will result in many older names having to be re-evaluated, and may even eventually require the conservation of Mycosphaerella, to safely preserve the name for future use over older, but lesser known names. 47
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Mycosphaerella leaf spot diseases of bananas - CBS

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