A novel finding of Podosphaera causing powdery mildew on Cuphea rosea

Cuphea rosea is an evergreen shrub in the family Lythraceae (loosestrife). Native to Mexico, this low-growing perennial favours sunny and warm climates. It is, however grown in the UK for its abundant small pink flowers throughout the summer. Other members of the loosestrife family are used for medicinal purposes or timber. In early spring 2007, a white powdery appearance was seen on upper and lower leaf surfaces of young leaves of C. rosea in northern England. Approximately 40% of an area containing 500 plants was affected. No leaf chlorosis or necrosis was immediately evident. As the disease progressed, however, the affected leaves became chlorotic with subsequent necrosis. Stems remained unaffected. Microscopic examination revealed morphological structures typical for a powdery mildew.

Superficial hyaline hyphae were 2-5 µm wide, usually widening at a point two thirds of the way along the length of the cells where they bore erect, straight conidiophores with cylindrical foot cells 32 - 90 µm × 14 - 21 µm (sometimes somewhat swollen) and 2 - 5 other cells which were shorter (7 - 24 µm × 7 - 12 µm). No evidence of pigmentation was observed in the hyphae. Catenate conidia measuring 22 - 40 µm × 14 - 25 µm were mainly ellipsoid, the secondary conidia often being slightly doliiform (Fig. 1). Fibrosin bodies were evident (Fig. 2). The teleomorph (chasmothecium) was not present.

There is a previous record of Erysiphe lythri on species of Cuphea, but morphological features observed in this study rule out the genus Erysiphe (Braun, 1987). The observed characteristics are typical of Oidium subgenus Fibroidium, the anamorph of Podosphaera (syn. Sphaerotheca) (Braun et al. 2002). Further evidence of this genus was the smooth outer conidial wall (Fig. 1) and the whorled pattern on end walls seen under the scanning electron microscope (Figs. 3 - 4) (Cook et al. 1997).

The plants originated from Central America. However, it is not possible to ascertain whether the powdery mildew originated from this region, or it is similar to other instances of powdery mildews suddenly expanding their host ranges on plants growing outside their countries of origin as discussed by Vagi et al. (2007). This is the first record of a species of Podosphaera affecting a member of the Lythraceae.