A group of obligate plant parasites which grow on the surfaces of green parts or petals of higher plants. Most are hyaline, but some species, especially those growing on petals, are coloured.
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.
The main confusion is likely to be with Powdery Mildews which, under a x20 hand lens, show a 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 a x20 lens. If conidial production is very abundant, the conidia may aggregate into larger masses and the colony appear granular.
By contrast the Downy Mildew colony has no well-marked features under the hand lens, although sporangia may be visible (and not necessarily white).
White Rusts tend to form discrete white patches and contort the host growth. The identity of the host is also a useful guide.
Look for white or pale (often greyish or lavendar) coatings on green surfaces, especially the leaf surfaces (upper and under) and petals. The colonies are often very localised but sometimes cover whole leaves. Usually the pale coating is obvious, but sometimes there can be just a suggestion of cloudiness.
Also check for discolouration on petals or just a general tatty appearance.
Finally check any plants that are chlorotic (yellow) or stunted.
Examine under the compound microscope.
Mount in lactophenol. For superficial growth, it is often easiest to lay the leaf in a drop of lactophenol on the slide and scrape off the sporangiophores with a mounted needle.
Cotton blue stains the sporangia but not the sporangiophores and is little help, but they're relatively thick-walled so staining is rarely necessary.