Alongside the identification of the host plant, certain morphological features (i.e. the appearance) are extremely useful when attempting to narrow down the potential identity of powdery mildew specimens. These features have been used to help identify samples sent in to the Powdery Mildew Citizen Science Scheme. Coupled with data of their hosts and DNA from specific regions a final identification is possible in approximately 90% of cases.
Powdery mildews are most often found in their asexual stage and can generally be found from spring to autumn. PMs like many other organisms can reproduce without sex, in this particular case by the production special spores.
Important microscopic features include:
Type of conidiogenesis
Fig. 1: the ‘false-spore’ Pseudoidium-type is characterised by the production of just one single maturing spore, a conidium, at a time, which will detach from the conidiophore before production of the next conidium. This is seen in the Erysiphe, Leveillula, Phllactinia, and Pleochaeta genera.
The ‘true-spore’ Euoidium-type is characterised by the production of multiple maturing spores attached to the conidiophore at once. This is seen in the Arthrocladiella, Blumeria, Cystotheca, Golovinomyces, Neroerysiphe, Sawadaea, and Podosphaera genera.
These appear as glass-shard-like structures within individual spores of some species (Fig. 2). Most commonly, fibrosin bodies are not present. However they can be seen in the genera Podosphaera, Cystotheca, and Sawadaea (all of tribe Cystotheceae), which is helpful in identification.
This can also be important in delimiting the potential genera and species in powdery mildew ID. The varying forms can often be difficult to differentiate between and range from ellipsoid, ovoid, cylindrical, doliiform (barrel-shaped), lanecolate, ovoid-lanceolate, spathulate, clavate, dumbell-like, macro- and micro-conidia (of Sawadaea).
The sexual stage of the powdery mildew is more rare; it can be found throughout the year although is most common from late-autumn to early-spring; winter perennation. The chasmothecium is a protective, shell-like structure which fungi use for overwintering containing sexual ‘ascospores’. Important microscopic features include:
Fig. 4: Appendages of the chasmothecia (fruiting bodies) enable the structure to adhere to various plant surfaces such as leaves, stems, and bark. Viewing these beautiful structures can help to identify a powdery mildew to genus.
Dichotomously-branched appendages can be characteristic of the Podosphaera genus or Eryipshe sect. Microsphaera, needle-like appendages are characteristic of the Phyllactinia genus, branched, uncinate-circinate appendages are characteristic of the Sawadaea genus; uncinate-circinate appendages can also be characteristic of the genus Podosphaera as well as Erysiphe sect. Uncinula; simple-mycelioid appendages characteristic of the Golovinomyces and Leveillua genera and Erysiphe sect. Erysiphe.
Number of asci within each chasmothecium, and the number of ascospores found within each ascus
Fig. 5 – these features are also useful features for further species delimitation. Asci within a single chasmothecium can range from one (characteristic of the genus Podosphaera) to many, while the each ascus can contain anything from 4-8 ascospores.
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