First report of powdery mildew on potted plants of Calluna vulgaris in Scotland

In September 2000, 9-month-old potted plants of Calluna vulgaris (cv. Bonfire Brilliance) were collected from a local grower and brought into an unheated net-sided polythene tunnel at the Scottish Agricultural College, Auchincruive. By mid-November, patches of white mycelium were visible on leaves (Fig. 1). After approximately 10-12 weeks, it had colonised whole plants causing extensive foliar browning followed by defoliation. A preliminary microscopic examination revealed superficial hyphae bearing conidia typical of a powdery mildew. In Europe Erysiphe orontii (syn. Erysiphe polyphaga (anamorph: Oidium violae) and Oidium ericinum have been previously recorded on Calluna and Erica species (Blumer, 1967; Braun, 1987, 1995; Watling 1985). However, they have yet to be reported on C. vulgaris in the UK.

To determine the morphological characteristics of the powdery mildew fungus on C. vulgaris, surface mycelium was removed with small strips of clear adhesive tape and were mounted in lactophenol/aniline blue for examination by light microscopy. Conidia and conidiophores were measured with an eye-piece graticule. To augment these observations, fresh material was also examined in a scanning electron microscope (SEM) using cryoscopy. Only the anamorph of the powdery mildew was found. Conidia matured one at a time (not in chains), and were ellipsoid-cylindric ranging from 25-61 x 8-22 m m. They did not have fibrosin bodies, germinated terminally, and appressoria were simple to lobed, often paired. SEM revealed that the conidia also showed a fibrillar end wall pattern and a scaly outer wall pattern often becoming longitudinally wrinkled. The conidiophore foot cells were 15-79 x 5-10 m m, with a high proportion being kinked or twisted at the base (Fig. 2). On the basis of its conidial and conidiophore features, the powdery mildew observed on C. vulgaris could be placed into the revised anamorph taxon, Oidium subgenus Pseudoidium, (teleomorph: Erysiphe emend Braun), (Cook et al., 1997, Braun et al., in press).

To determine whether the powdery mildew fungus caused all of the disease symptoms, fresh conidia from C. vulgaris were applied with a fine camel-hair brush onto the leaves of ten 6-month-old potted C. vulgaris plants previously raised in isolation. Inoculated leaves were identified with small tie-on labels. Ten uninoculated plants served as controls. The plants were placed in an unheated net-sided polythene tunnel and enclosed in polythene bags for 24 h. Four weeks later, white mycelium appeared on the inoculated leaves and, after 6 weeks, the fungus had spread to the uninoculated leaves. Sporulation of the fungus was observed on all infected leaves. Extensive foliar browning occurred on inoculated plants after 10 weeks followed by defoliation. No symptoms were observed on control plants. Conidia that developed on the inoculated plants were used in repeated inoculation tests on cultivars Bonfire Brilliance and Flamingo. The same results were obtained, confirming that the powdery mildew fungus, Oidium subgenus Pseudoidium, was entirely responsible for the symptoms originally seen on C. vulgaris.

This is believed to be the first report of powdery mildew on C. vulgaris in Scotland and, indeed, the UK. Having the potential to cause plant losses and reduce plant quality, the disease poses a threat to the commercial production of potted C. vulgaris plants. Further research is required to investigate the distribution and economic importance of the disease and to detect the teleomorph (chasmothecium, Braun et al., in press) in order to identify the species of Erysiphe involved.