Erysiphe flexuosa (Peck) U.Braun & S.Takam.    
   (= Uncinula flexuosa Peck)   

Kingdom: Fungi
Phylum: Ascomycota
Class: Leotiomycetes
Order: Erysiphales

British distribution: An introduction, necessarily confined to localities where the host is planted, evidently increasing but still local in England and Wales; below might be the first Scottish record.
World distribution: North America, widely introduced in Europe as an introduction (Braun & Cook, 2012).

This is a powdery mildew, parasitic on species and hybrids of horse-chestnut (Aesculus) (family Sapindaceae), in N. Amercica also on soapberries (Sapindus). Originally it will have been on native American species of Aesculus but it is now widespread especially on the Common Horse-chestnut (A. hippocastanum), a native of SE Europe (Balkans) that is widely and commonly planted as an ornamental tree (and the source of "conkers"). In Britain, the first records are from Surrey in 2001 (FRDBI) but it has spread considerably in the few years since.

Erysiphe flexuosa
Erysiphe flexuosa
Erysiphe flexuosa: mycelial patches on lower leaves of Horse-chestnut (Aesculus hippocastanum), Paisley, Renfrewshire, September 2014.
Note that the orange-brown lesions on the leaves are nothing to with the powdery mildew, but are likely the early stages of infection by another
host-specific, phytopathogenic fungus, Guignardia aesculi.
Erysiphe flexuosa, cleistothecia
Erysiphe flexuosa, cleistothecia
Erysiphe flexuosa: developing and ± mature cleistothecia (chasmothecia); source as above.
Erysiphe flexuosa, cleistothecium
Erysiphe flexuosa, cleistothecium
Erysiphe flexuosa, cleistothecium
Erysiphe flexuosa: cleistothecium; cleistothecial appendages; source as above.

This is more or less the only powdery mildew to be expected on a horse-chestnut in Britain, including Common Horse-chestnut, much planted as a street tree and an ornamental, and Red Horse-chestnut, A. x carnea, also frequently planted as an ornamental. The fungus produces white patches on the surfaces of leaves, but it appears to be relatively innocuous and does not cause significant foliar damage. It will not spread to any other unrelated tree or other plant species. Clearance and burning of fallen leaves in the autumn may reduce infection for the following year, as is the case with Guignardia infections, but generally no action appears necessary.

Sawadaea bicornis, common on Sycamore and Maple (Acer) species (also family Sapindaceae), may occur as an "accidental" on Aesculus (Braun & Cook, 2012), but evidently this happens little in Britain – only one such record in the FRDBI as of April 2015. Phyllactinia aesculi occurs on Aesculus in south-western N.America. Erysiphe alphitoides, best known on oaks (Quercus) is reported on Aesculus, but apparently not in Britain. It follows that while field records are likely to be correct, microscopic confirmation of identifications is recommended.

E. flexuosa is notable for its distinctive cleistothecial appendages, which are stiff and radiating, with coiled (circinate) tips, and a diagnostic, sinuous and somewhat twisted zone below these tips. The upper part of the cleistothecium also bears much shorter, straight, bristle-like appendages (see lowest photograph). The cleistothecia contain numerous asci, each generally 8-spored. The conidia of the anamorph (asexual state) are hyaline, elongated-ellipsoid, 25–40 × 9–17 µm (Braun & Cook, 2012), produced singly at the ends of conidiophores. They lack fibrosin bodies. Detailed descriptions and diagrams of the sexual (teleomorphic) and asexual (anamorphic) stages are provided by Uwe Braun (1987) (as Uncinula flexuosa) and by Braun & Cook (2012).

For an explanatory overview of the powdery mildews and the terminology used here, go to the profile of the Erysiphales.

•   Braun, U., (1987). A monograph of the Erysiphales (powdery mildews). Beihefte zur Nova Hedwigia 89: 1–700.
•   Braun, U., & Cook, R.T.A., (2012). Taxonomic manual of the Erysiphales (Powdery Mildews), CBS-GNAW Fungal Biodiversity Centre, Utrecht.

© A.J. Silverside
Page uploaded April 2015.
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