Occurrence of powdery mildew (Erysiphesp.) onEcheveriaspp.,Crassulaspp.,CotyledonandDudleyain the UK

Abstract

The Crassulaceae is a large family including 1400 species in 33 genera. They are popular horticultural plants grown for their spectacular flower heads and unique leaves, which vary in shape, size and texture. In the genus Cotyledon , plants are also used medicinally for the treatment of warts and abscesses. In February 2007, an outbreak of powdery mildew occurred on a large collection of Crassulaceae in the glasshouse at Wisley gardens. The plants affected included Echeveria gibbiflora , E. gibbiflora ‘Carunculata’, E. ‘Chantilly’, E. ‘Afterglow’, E. ‘Secunda’, E. ‘Ballerina’, Crassula capitella ‘Flame’, C. lactea , C. multicava , Dudleya palmeri and Cotyledon orbiculata . The mycelium was amphigenous causing corky lesions on both stems and leaves. Morphological characteristics of the fungi isolated from all plants were similar. The conidia were cylindrical, lacking fibrosin bodies, length 27–47 μ m (mean 34 μ m), width 10–18 μ m (mean 14 μ m). Conidiophores were erect consisting of a straight foot-cell 15–38 μ m (mean 28 μ m) × 7–11 μ m (mean 9 μ m) followed by two distal cells. Conidia were formed singly. Appressoria on the mycelium were lobed to multilobed. No chasmothecia were present but the above characteristics are consistent with Oidium subgenus Pseudoidium , the anamorph of Erysiphe , and with the powdery mildew found on Sedum alboroseum in Hungary (Jankovics & Szentivanyi, 2006). The ITS regions of isolates from E. gibbiflora , E. gibbiflora ‘Carunculata, E . ‘Chantilly’, C. capitella ‘Flame’, D. palmeri , Cotyledon orbiculata and the Hungarian isolate on Sedum were amplified and sequenced using the primers PMITS1 and PMITS2 (Denton & Henricot, 2007). All isolates had identical ITS sequences (GenBank Acc. Nos EF434394, EU185636 to EU185641) and did not match fully any powdery mildew species published in GenBank. Records of Podosphaera (syn. Sphaerotheca macularis ) on Crassula (Farr et al ., 2007), Golovinomyces (syn. Erysiphe ) orontii on Crassula , Echeveria and Cotyledon (Braun, 1987) and Neoerysiphe galeopsidis (Kiss, 1999) on Crassula have been found. The DNA and the morphology rule out these species but confirm the UK species is the same as the one affecting Sedum in Hungary. This is the first record of this powdery mildew species on Crassul a, Echeveria , Cotyledon and Dudleya in the UK.