First record of Oidium anamorph of Podosphaera xanthii on Medusagyne oppositifolia

Powdery mildew first appeared on mature 3-yr-old bushes of 'jellyfish tree', Medusagyne oppositifolia Medusagynaceae), grown from seed at Eden Project, Cornwall, England, in collaborative conservation work with the Seychelles Government. Initially, young leaves showed distortion and patches of mycelium bearing conidia. Later, light brown lesions developed (Fig. 1) and badly affected plants showed extensive leaf-drop, especially destructive in seedlings (Fig. 2). This is the first report of powdery mildew on Medusagyne. Conidia are catenate, elliptical to doliiform, (19) 24 – 34 (47.5) x (13.5) 15 – 18 µm with fibrosin bodies and a sinuous wrinkling pattern. Conidiophores erect with a long cylindrical straight or twisted foot-cell, 41 – 86.5 (140) x (7) 9 –14 (16) µm, arising towards one end of its hyphal mother cell, the lower septum occasionally raised up to 10 µm, followed by a generative cell and 2 – 5 maturing conidial units (Fig. 3). Superficial hyphal cells, 25 – 105 x 4.5 – 11.5 µm, branched at right angles, bearing inconspicuous or slightly nipple-shaped appressoria. No chasmothecia were present, but characteristics are consistent with Oidium subgenus Fibroidium, the anamorph of Podosphaera. The short, broad germ tubes typical of the fibroidium type, brevitubus subtype (Fig. 4) narrowed the identification to Podosphaera section Sphaerotheca subsect. Magnicellulatae (Cook & Braun, 2009) with the morphology close to that of Podosphaera (syn. Sphaerotheca) fusca, apparently a ubiquitous species with a broad host range.

The ITS region (GenBank Accession No. GQ390796) showed 99% similarity to Podosphaera fusca and P. xanthii sequences, but did not allow differentiation. However, U. Braun (personal communication) now restricts P. fusca to the host genus Doronicum. This is in contrast to P. xanthii sensu lato that is now a complex species containing forms that may cross infect. When the disease was first observed in England, M. oppositifolia plants were growing together with cucurbits infected with powdery mildew already known on the latter in the Seychelles (Kingsland, 1983), but not reported on Medusagyne there. Thus, forms of P. xanthii adapted to new hosts may be present in Europe, but not in more isolated regions such as the Seychelles. This highlights the potential for a new host association to appear when plants are moved from one geographical area to another.