Anamorph genera associated with Botryosphaeria
Anamorphs of Botryosphaeria species have been placed in Botryodiplodia (Sacc.) Sacc., Diplodia Fr., Dothiorella Sacc., Fusicoccum Corda, Lasiodiplodia Ellis & Everh., Macrophoma (Sacc.) Berl. & Vogl., Macrophomopsis Petrak. and Sphaeropsis Sacc. These genera were not clearly delimited because the morphological features that separate them were poorly defined. Considerable progress has been made towards stabilizing the species concepts. For example, Sutton (1980) considered Macrophoma to be a synonym of Sphaeropsis. Pennycook & Samuels (1985) showed that Macrophomopsis is a later synonym of Fusicoccum. Crous & Palm (1999) established that the type species of Botryodiplodia is a valsoid fungus (ascomycete) and, therefore, the name cannot be used for a coelomycete. They also showed that the type species of Dothiorella is better placed in the genus Diplodia. Phillips et al. (2005) re-examined the type species of Dothiorella and determined that it differed from Diplodia, as typified by D. mutila, in that the conidia become coloured and septate while within the pycnidial cavity and often before they are released from the conidiogenous cells. This is in contrast to Diplodia in which the conidia become dark and septate only after they are discharged from the pycnidia. For this reason Phillips et al. (2005) reinstated Dothiorella as a genus distinct from Diplodia. Although the status of Lasiodiplodia has not been critically evaluated, Denman et al. (2000) suggested that it should be included as a synonym of Diplodia. Thus, of the eight genera in which anamorphs of Botryosphaeria species have been classified, only four (Fusicoccum, Diplodia, Dothiorella and Sphaeropsis) can be considered with any degree of confidence, but even the status of Sphaeropsis is doubtful and it should probably be included as a synonym of Diplodia.
The genus Sphaeropsis has been the subject of controversy because the distinction between it and Diplodia has never been clear. As defined by Sutton (1980), percurrent proliferation in conidiogenous cells has been regarded as more typical of Sphaeropsis than of Diplodia. However, percurrent proliferation is also found in strains of Diplodia (Phillips, 2002). Septation of conidia has also been used to separate these two genera. According to Sutton’s (1980) concepts, conidia of Sphaeropsis are aseptate but they become euseptate prior to germination, while in Diplodia the conidia become euseptate as they mature. The concept of aging and maturity are not well defined, thus making the feature of septation an unreliable criterion for species separation. Age and state of maturity also affect conidium pigmentation. Mature conidia of Sphaeropsis are invariably opaque brown and this occurs while the conidia are within the pycnidial cavity, but conidia of Diplodia can remain hyaline for a long time even when mature. However, in Sutton’s (1980) account of the history of Diplodia mutila Fr. apud Mont. (the type species of the genus) he recalls that when Stevens (1933) examined slides from Montagne’s exsiccata, he found pycnidia with a mixture of hyaline and one-celled brown conidia. When Shoemaker (1964) described B. stevensii as the teleomorph of D. mutila, he stated that conidia remain hyaline even after discharge from the pycnidium, and that brown, one-septate conidia are rarely found. In contrast to this, however, conidia of the specimens that Laundon (1973) considered to be the anamorph of B. stevensii were hyaline, but they gradually became dark coloured and 1-septate. Therefore, D. mutila can be regarded as having hyaline conidia that can become brown with age. In contrast to this, conidia of Sphaeropsis become coloured at an early stage of development. Taking into account the same mode of conidiogenesis in these two genera, but disregarding the stage at which conidia become septate and coloured, Denman et al. (2000) stated that it becomes difficult to distinguish Sphaeropsis from Diplodia and the two genera can be considered synonymous, with the older name of Diplodia taking preference (Denman et al., 2000). This suggestion has been generally accepted in the literature. Whether the differences in stage at which conidia become coloured and septate are taxonomically meaningful has yet to be determined.
A similar type of problem is found in Lasiodiplodia. This genus has long been considered as separate from Diplodia and the two genera are separated on morphological details of the conidia. Conidia of Lasiodiplodia species are initially hyaline and remain so for a long time before they become coloured and one-septate. Melanin is deposited in regular stripes on the inner surface of the conidium wall giving the appearance of longitudinal striations. This feature, together with paraphyses in pycnidia of Lasiodiplodia species is the main one that separates the genera. Whether striate conidia and paraphyses are good taxonomic characters at the genus level has not been decided. Thus, some authors prefer to keep the two genera separate while others regard them as synonymous.
Denman et al. (2000) placed Botryosphaeria anamorphs into two groups based on conidium colour. Accordingly, species with hyaline conidia were placed in Fusicoccum while those with dark conidia fall within Diplodia. This idea was taken up by Zhou & Stanosz (2001a) who referred to the two anamorph genera as sections Hyala and Brunnea. However, as stated above, the type species of Diplodia (D. mutila) has hyaline conidia that can darken with age. In some collections darkening is delayed, or apparently never occurs. Thus, Diplodia cannot strictly be considered a genus with dark spores. Although Denman et al. (2000) amended the concept of Diplodia to include species with conidia of variable colour, their proposal to place hyaline-spored species in Fusicoccum would mean that some collections of Botryosphaeria stevensii (the teleomorph of D. mutila) would have Fusicoccum anamorphs. Moreover, conidia in some species of Fusicoccum become darker with age. Therefore, conidium colouration is a difficult character to apply in practice.
A major distinction between Fusicoccum and Diplodia is that in the former the conidia are thin-walled, while in the latter they are thick-walled. Furthermore, conidia of Fusicoccum spp. are usually less than 10 µm wide, while in Diplodia spp. they are normally more than 10 µm wide. Conidium width and wall thickness are more reliable features for separating Fusicoccum from Diplodia.
As more isolates of Fusicoccum and Diplodia are studied, the differences between the two genera becomes less distinct, and it may be possible to unite them under the older name of Fusicoccum, thus reaching the logical state where only a single anamorph genus is associated with the teleomorph genus Botryosphaeria. This, however, is not possible at present. Furthermore, phylogenic analysis of ITS sequence data (Jacobs and Rehner, 1998; Phillips et al., 2002) suggests that the difference between Fusicoccum and Diplodia is clear. These same studies reveal that the difference between Diplodia and Sphaeropsis is indistinct. Therefore, in the light of the current state of knowledge, the question of whether Sphaeropsis, Lasiodiplodia and Diplodia are separate genera or synonyms has not been finally resolved. However, Fusicoccum should be retained as a distinct genus.
When Phillips et al. (2005) reinstated Dothiorella they determined that the teleomorphs were unusual in Botryosphaeria in having brown, one-septate ascospores. This feature is the only one that separates Dothidotthia from Botryosphaeria (Barr, 19XX). Nevertheless, Phillips et al. (2005) showed that ITS phylogeny placed these isoltes within Botryosphaeria, which raised the question of the validity of Dothidotthia. However, ITS is not strictly suitable for determining relationships at genus level and it may be possible that the species with Dothiorella anamorphs will ultimately be transferred to Dothiorella. In the meantime, and until further studies with more suitable genes are done, these species will be treated under Botryosphaeria.
Crous PW & Palm ME. 1999. Reassessment of the Botryosphaeria anamorph genera Fusicoccum, Dothiorella and Botryodiplodia. Sydowia 51: 167–175.
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Zhou S & Stanosz GR. 2001. Relationships among Botryosphaeria species and associated anamorphic fungi inferred from the analysis of ITS and 5.8S rDNA sequences. Mycologia 93: 516–527.
Anamorph genera associated with
Botryosphaeria. Version 01 saved 25 July 2002