First Report of Powdery Mildew Caused by Erysiphe diffusa on Cluster Bean in India

Cluster bean (Cyamopsis tetragonoloba (L.) Taub.) is an important vegetable crop cultivated widely in India. During a field survey in November 2021, about 60% of plants exhibited characteristic powdery mildew disease symptoms and signs in a 15 ha field in Northern Karnataka (Raichur), India. Initially, the symptoms and signs appeared as tan lesions, which later became small, circular and chlorotic. The abaxial surface turned yellow and was covered with white mycelial growth. As the disease progressed, white mycelia grew on the adaxial leaf surface, stems and pods as well. In severe infections, drying and premature defoliation of infected leaves were observed. Infected leaf samples with mycelia were collected (n=8) and the fungus was subjected to morphological and molecular observations. Mycelia on leaves was characterized as epiphytic, amphigenous, producing dense, white patches on the upper and lower leaf surfaces, stem and young pods. Hyphae were hyaline, thin-walled, 1.8 to 4.2 µm wide with erect conidiophores consisting of a cylindrical foot-cell, straight flexuous at the base and measured 20 to 36 × 6 to 9 μm (n=30), followed by 1 to 2 shorter cells. Ellipsoid conidia were produced singly and measured 28 to 42 × 12 to 20 μm (n=30) without fibrosin bodies. Chasmothecia were not observed. A reference specimen was deposited at the Institution of Excellence, University of Mysore Herbarium (UOM-IOE 2022_1). The morphology and other characteristics of conidia were consistent with an Erysiphe species (Braun and Cook 2012). Genomic DNA was isolated from a conidial suspension harvested from the powdery mildew affected cluster bean samples. The ITS region was amplified from three samples using powdery mildew-specific primer pair PN23/PN34 and sequenced directly (Chen et al. 2008). nBLAST analysis revealed that the ITS sequence shared 100% similarity with the reference sequence (E. diffusa vouchers HMJAU02177 - KM260363, BRIP 71013 - MW009058) of Erysiphe diffusa (Cooke & Peck) U. Braun & S. Takam. In addition to 100% match to voucher specimens of E. diffusa, there were no vouchers from other species that also had 100% match. The representative sequences were deposited in GenBank with accession numbers OM669776 - OM669778. Koch's postulates were conducted on healthy cluster bean plants grown under greenhouse conditions. Conidia were harvested from infected leaves, suspended in water and sprayed on 40 to 50-day-old cluster bean plants (28 ± 2°C and >70% relative humidity). The development of powdery mildew symptoms was recorded on 22 plants after 10-14 days of post inoculation. Control plants inoculated with sterile water remained healthy without powdery mildew symptoms. Microscopic observation of spores from inoculated plants confirmed the pathogen as E. diffusa. The genus Erysiphe is known to infect many crop plants. E. diffusa has been reported to infect Vigna radiata, Glycine max and Phaseolus mungo in Australia (Kelly et al. 2021). No reports are available at USDA's host-fungus database for cluster bean and E. diffusa (Farr and Rossman 2022). To the best of our knowledge, this is the first report of E. diffusa associated with powdery mildew of cluster bean in India. Further comprehensive investigations will shed a light on the economic impact of powdery mildew disease on the cluster bean in India.