Erysiphaceae

Erysiphaceae (commonly known as powdery mildews) is a family of ascomycete fungi in the order Helotiales. The members of this family are obligate parasites with a superficial mycelium that forms a characteristic white, powdery coating on their host plants.

The family contains seventeen genera and around a thousand species. Combined they infect over 10,000 species of plants, many of which are commercially valuable.

Description

Powdery mildews exist in two states, the asexual anamorph state reproducing by means of conidia, and the sexual teleomorph state reproducing by means of ascospores produced in asci developing in more or less spherical chasmothecia. The ripe spores become detached and are readily dispersed by the wind, causing fresh infection. The usually conspicuous whitish mycelium and conidiophores on leaves, and sometimes on stems, fruits or even petals, are what is generally seen first. The chasmothecia are usually visible to the naked eye and turn from translucent or yellow-orange to purple or black. They are produced on the mycelium later in the season, though may be absent. Upon reaching maturity the chasmothecia may develop hair‐like appendages that may branch in characteristic ways.

The mycelium can appear in varying degrees of thickness and patchiness, on leaves, petioles, stems, twigs, fruits or flowers. On leaves, mycelium can be epiphyllous (on leaf uppersides), hypophyllous (on leaf undersides) or amphigenous (on both sides). Phyllactinia in particular is characterised by hypophyllous mycelium, although members of other genera also infect only the leaf undersides.

Erysiphaceae species are occasionally also associated with galls. For example, Podosphaera phytoptophila is one of the causers of witch's broom galls on hackberry (Celtis occidentalis). Although not fully understood, it is believed that a minute, wormlike, eriophyid mite (Aceria celtis) induces the brooms and P. phytoptophila exploits the weakened plant and favourable habitat created by deformed buds and dense branching. Cystotheca lanestris is also known as a North American gall-former, on various Quercus species.

The similarly-named but unrelated downy mildews (Peronosporaceae) can have a similar appearance to powdery mildews, however the former always grow on the underside of leaves, often causing yellowing on the upperside. The mycelium of these members of the Peronosporales is internal in the plant tissue and the sporangiophores (more or less equivalent to conidiophores) emerge through the stomata; in the Peronosporaceae the sporangiophores are elongated and branched above so that the colonies are downy or felt‐like and usually greyish in colour.

Ecology

Erysiphaceae species are obligate biotrophs, meaning they infect living plants. Specialised haustoria are able to penetrate the plant cells while keeping them alive. Most powdery mildew species are highly host-specific, infecting at most a few genera and frequently just a single species. The few species that are still rather broadly defined are considered species complexes with a narrow sensu stricto host range. Despite this, they are capable of 'jumping' host to infect a new genus. It is thought the now-common species in Europe Erysiphe alphitoides did this to first infect oaks (Quercus), facilitating its spread to the continent. Since then, it has also been reported as occurring on plants such as Wisteria, demonstrating the host-jumping ability that has led to its success.

Some Erysiphaceae species host the hyperparasite Ampelomyces quisqualis. This fungus reduces growth and may even kill the mildew, leading to its use as an active ingredient in some fungicides. Around 40 other fungal species have been known to parasitise powdery mildews: mainly ascomycetes such as Cladosporium but including basidiomycetes such as Tilletiopsis.

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