||Powdery mildews are obligate plant parasites, infection usually being through the stomata. The mycelium is superficial on the surface of the leaf or other green parts.
During the early stages of infection, reproduction is asexual by means of conidia, formed in chains on long erect conidiophores.
Later in the season, the sexual (perfect) state is formed, although not in all species. This consists of black cleistothecia (yellow, orange then brown when immature), anchored to the substratum by characteristic radiating hyphae.
||Look for white coatings on green surfaces. Usually this is obvious, but sometimes there can be just a suggestion of cloudiness.
||If cleistothecia are present, the group is easily recognisable - they are just visible to the naked eye and easily seen with a hand lens. The cleistothecia are usually in groups but often very localised and have to be searched for. Look for grey patches.
In the absence of cleistothecia, Powdery Mildews can be distinguished from Downy Mildews under an x20 hand lens by the network of criss-crossing fine white hyphae running closely over the leaf surface like the railway lines at Clapham Junction. Above this is often a forest of erect conidiophores with the conidia just visible under the dissecting microscope or a x20 lens. If conidial production is very abundant, the conidia may aggregate into larger masses and the colony appear granular.
Do not be confused by hyperparasitic fungi, especially Ampelomyces, which is very common.
|Characters to note in the field
||Unless it's obvious from the specimen, always note the identity of the host. It's best to identify this to species if possible, rather than just to genus.
|What to collect for identification
||For species identification collect material with cleistothecia, if they are present.
|Important features for identification
||The structure of the anchoring hyphae on the cleistothecium, the number of asci in the cleistothecium and the number of ascospores in the ascus.
Where cleistothecia are absent, the structure of the conidiophore and mode of production of conidia are used.
||Examine under the dissecting microscope to get a general idea of the structure, then examine properly under the compound microscope.
For conidial anamorphs: mount in lactophenol cotton blue.
For cleistothecia: select several ripe cleistothecia with a mounted needle (they'll stick better if the needle is first dipped in lactophenol and the point touched on paper to remove the excess.) Mount in a generous drop of lactophenol and examine without compression. Then gradually increase the compression a bit at a time by gently tapping the coverslip once or twice; re-examine each time. Count the asci as the cleistothecia burst, and the ascospores as the asci are squeezed out.