During September of 2009, leaves of a local cultivar (yellow bulb) of onion (Allium cepa L.) in the late vegetative growth stage were heavily infected with powdery mildew in the Birjand Region (Giuq Sofla), southern Khorasan, Iran. Disease symptoms included circular to oblong, chlorotic to necrotic lesions that were 1 to 2 cm in diameter as well as white and effuse colonies on young and mature leaves. On the basis of microscopic examination, the whitish powdery colonies consisted of conidiophores and conidia. Conidiophores emerged from stomata, singly or branched in groups of two to three, and formed dimorphic conidia. The primary conidia were lanceolate with a narrowed apex (17.5 to 20 × 55.4 to 67.5 μm) and the secondary conidia were cylindrical to ellipsoid (17.5 to 22.5 × 55 to 70 μm). The teleomorph of the fungus was not observed. Leveillula taurica (Lév.) G. Arnaud (anamorph Oidiopsis sicula Scalia) was subsequently identified by the presence of endophytic mycelia emerging through stomata, branched conidiophores, and hyaline, single-celled, dimorphic conidia borne singly (1,3). To confirm pathogenicity, conidia from diseased onion leaves were brushed onto leaves of 35- to 40-day-old onion plants in triplicate. The inoculated plants were subsequently covered with plastic bags for 3 days to maintain high humidity. After 12 to 14 days, symptoms of powdery mildew appeared on the leaves of inoculated plants. No symptoms developed on the noninoculated plants. Powdery mildew caused by L. taurica on onion has previously been reported from the United States, Brazil, Israel, and Sudan (4). Although this pathogen has previously been reported on Allium spp. by Khodaparast et al. (2) in Iran, to our knowledge, this is the first record of powdery mildew of onion (A. cepa L.) from Iran. The disease outbreak has recently been noted on onion in Aryan Shahr, 70 km north of Giuq Sofla where the pathogen was originally identified. Voucher specimens have been deposited at the Fungal Collection of the Ministry of Agriculture, Tehran, Iran.
References: (1) L. J. du Toit et al. Online publication. doi:10.1094.PHP/1129-01-HN. Plant Health Progress. 2004. (2) S. A. Khodaparast et al. Rostaniha 4:108, 2003. (3) S. K. Mohan and N. D. Molenaar. Plant Dis. 89:431, 2005. (4) A. Reis et al. Hortic. Bras. 22:758, 2004.
