Appendage Morphology

Amongst the most important visual, superficial features of the powdery mildew are their appendages. These limb-like features arising from the surface of the, sexual spore containing, chasmothecia are important for latching onto the stems and leaves of their hosts. They vary greatly and can help us to distinguish between tribes and genera.

The ectoparasitic, single conidia producing tribe Erysipheae are a good indication of the diversity of forms within a single tribe: appendages are distinct at their tips (or apices) and can be grouped as uncinate-circinate, dichotomously branched or mycelioid (fig. 1).

Figure 1: Appendages of powdery mildew fruiting bodies (chasmothecia). The ‘uncinate-circinate’ appendages (left) are said to be the most basal form from which dichotomously branched appendages (middle) have evolved. The most simple, ‘mycelioid’ appendages (right), then evolved at least twice from the branched type. Such a series of adaptations shows a simple devolution of complexity in the forms. (From Takamatsu, 2013)

Significant debate has arisen as to the pathway along which these have evolved. As recently as 2012 (Braun and Cook) mycelioid apices were considered to be the most primitive of the three (understandable due to their simplicity). However molecular research, focusing on the rDNA ITS region, has shown a devolution of complexity and as such the curled tips of uncinate-circinate appendages have in fact been around longest

Figure 2: Microscope images accurately showing the characteristic curled apices of Erysipheae section Microsphaera (left) (Image: Steve Clements) and branched apices of Erysipheae section Uncinula (Image: Xerantheum). (From Flickr: The Commons)

Such varied appendages are closely connected to their hosts and overwintering strategies (Takamatsu, 2004). The complex curled and branched apices are required by spores parasitising trees and shrubs as the tougher substrates require more substantial anchorage to last the winter. In contrast the simple, mycelioid appendages are found on pathogens associated with herbaceous plants, which are often protected from harsh, windy, wintery conditions by taking refuge in fallen leaf material and  buds.

References

Braun U, Cook RTA (2012) Taxonomic manual of the Erysiphales (powdery mildews). CBS Biodiversity Series No 11. CBS, Utrecht

Takamatsu, S (2004) Phylogeny and evolution of the powdery mildew fungi (Erysiphales, Ascomycota) inferred from nuclear ribosomal DNA sequences. Mycoscience 45:147–157

Takamatsu, S (2013) Molecular phylogeny reveals phenotypic evolution of powdery mildews (Erysiphales, Ascomycota). Journal of General Plant Pathology (2013): 1-9.