With approximately 800 different species of powdery mildew present on earth today, grouped into 13 genera (according to Braun & Takamatsu, 2000) of 5 tribes, combinations of the aforementioned features, as well as many less superficially obvious, are numerous. While many species may have similar characteristics it is often their geographical location and, more importantly, their host which finalise the identity of a fungal pathogen.
For example certain powdery mildew species will speciate further to become ‘formae speciales’ (ff. sp.) (literally translated as ‘special forms’ (singular forma specialis (f. sp.))). This name implies a specificity to a single host species and is most famous within the causal agent of powdery mildew on cereals, Blumeria graminis, which is classified into eight distinct formae speciales (fig. 1). In such cases a simple identification of host will often give a strong indication of the pathogen’s identity. Interestingly, such a concept has been recently examined by a team of pathologists in Ghent, Belgium, and the John Innes Centre of Norwich, England (Troch et al., 2014).
Figure 1: Detached leaves of Hordeum vulgare (common barley) infected with the hugely specialised and specific Blumeria graminis f.sp. hordei. (Photo by Oliver Ellingham)
Anyway, my point is that such a diversity of forms, like all life on earth, must have radiated from a single point or species. In respect to my life for the next 40 months (and beyond) this species comes putatively in the form of Parauncinula septata, the basal (most ancient) powdery mildew in numerous phylogenetic studies. This particular fungi is found exclusively on Quercus (Oak trees) in Japan and China and has features remarkably similar to those of early drawings by brothers Tulasne and Tulasne (1861) (fig. 2).
Figure 2: Drawings of powdery mildews illustrated by Tulasne and Tulasne (1861) similar to the putative ancestral feature of the powdery mildews. a Teleomorph. b Anamorph
This putative early form includes, in the sexual stage (teleomorph), relatively large chasmothecia, containing many eight-spored asci, with many uncinate-circinate apiced appendages. The asexual stage (anamorph) produced conidia in chains (Euoidium-type) and was ectoparasitic.
References:
Troch, V., Audenaert, K., Wyand, R. A., Haesaert, G., Höfte, M. and Brown, J. K. M. 2014, Formae speciales of cereal powdery mildew: close or distant relatives?. Molecular Plant Pathology.
Tulasne LR, Tulasne CC. 1861, Selecta fungorum carpologia 1 (in French). Paris
