First report of powdery mildew on Echeveria agavoides in India

Echeveria agavoides belongs to the family of Crassulaceae. It is commonly grown as an ornamental plant in India. Diseased leaves of E. agavoides collected from Shillong, Meghalaya (North East India) were found to be heavily infected by powdery mildew during January 2008. Almost 70% of the plants surveyed were found infected in Shillong. Disease symptoms included greyish white circular patches consisting of epiphytic mycelia and conidia, mostly on the upper surface of leaves. Infected leaves later on turned necrotic. Heavy infection resulted in severe defoliation. A voucher specimen has been deposited in CABI Bioscience UK Centre (IMI No. 396393).

Microscopic observations revealed conidia were formed singly, which ruled out the possibility of Golovinomyces orontii, Neoerysiphe galeopsidis and Podosphaera macularis reported on Crassulaceae (Farr et al., 2005). Hyphae were between 4.3-5.2 mm wide with multilobed appressoria. Conidiophores were mostly erect containing a foot cell (22.4-36.7 x 12.4-15.2 µm) followed by two or three short cells, and conidia were cylindrical in shape (30.6-46.2 x 15.2-19.3 µm) (Fig. 1). Fibrosin bodies were absent. Conidia produced short germ tubes on the apical portion terminating in lobed appressoria upon germination. Based on these morphological characters the powdery mildew belongs to the genus Oidium subgenus Pseudoidium, the anamorph of Erysiphe (Cook et al., 1997). No perfect stage (chasmothecium) was found to be associated with this fungus. Pathogenicity was confirmed by dusting conidia on healthy potted plants of E. agavoides, with uninoculated plants serving as controls. Inoculated plants developed symptoms after a week whereas control plants remained healthy.

When searched in GenBank, DNA sequences of the rDNA ITS region (Acession No. AB467356) and D1/D2 domains of the 28 rDNA (AB467357) were unique. The ITS sequences (EF434394, EU185636 to EU185641) of an anamorph of Erysiphe reported onvarious members of the Crassulaceae in the UK and Hungary were found to be different from the present sequence (Henricot, 2008). However, these sequence analyses support the morphological observation that the fungus belongs to Oidium subgenus Pseudoidium.

To our knowledge, this is the first record of powdery mildew on Echeveria in India. This disease is also of regulatory significance since this part of India (North East) shares its boundaries with Nepal, China, Burma, Bhutan and Bangladesh, and this disease has not been reported from any of these countries.