FIRST REPORT OF PODOSPHAERA XANTHII CAUSING POWDERY MILDEW ON AGERATUM CONYZOIDES IN INDIA

Abstract

During March and April 2015, powdery mildew symptoms were observed on A. conyzoides, a very common weed, along roadsides and in some agricultural fields of Satara District (M.S., India). Symptoms appeared as thin white patches on the stem and both leaf surfaces. Infected leaves were deformed and showed curled margins. Disease incidence was estimated to be 100%. Microscopical observations revealed nipple-shaped, solitary hyphal appressoria, erect conidiophores with cylindrical foot cells, 30-65×10-13 µm in size, sometimes slightly constricted at the basal septum or slightly swollen at the very base, followed by 1–4 shorter cells, forming long chains of ellipsoid-ovoid to doliiform conidia, 35-55 × 14-22 µm in size, bearing distinct fibrosin bodies. Germ tubes were short, lateral, simple to forked. Chasmothecia were absent. Based on these morphological characters, the pathogen was tentatively identified as Podosphaera xanthii (Braun and Cook 2012). A reference specimen (HAL-2924F) was deposited in the Geobotany Herbarium of Martin Luther University, Halle, Germany. To confirm the identity of the fungus, genomic DNA was extracted and the internal transcribed spacer (ITS) region was amplified by PCR and sequenced using the primers ITS5 and ITS4 (White et al., 1990). The resulting 257 bp sequence was deposited in GenBank (accession No. KX369237). BLAST analysis showed 99% identity with comparable sequences of P. xanthii from Verbena brasiliensis from Korea (KJ472787) and Xanthium strumarium from India (KX369236). To our knowledge, this is the first report of P. xanthii on A. conyzoides in India. This weed may be acting as a source of inoculum for powdery mildew disease on different crops and plants of ornamental and horticultural importance.