First Report of Powdery Mildew Caused by Erysiphe quercicola on Acacia auriculiformis in India

Earleaf acacia, also known as Australian acacia (Acacia auriculiformis), is planted as a source of firewood and as an ornamental tree. In India, it was introduced as a plantation tree to cover barren lands and hill slopes. During October 2014 to February 2015, severe infection of powdery mildew on earleaf acacia was observed in various localities of Satara District, Maharashtra State, India. Disease symptoms included grayish white, circular to irregular patches consisting of epiphytic mycelium and conidia on both surfaces of leaves. As disease progressed, leaves were covered by a gray powdery fungal mass, and older leaves became yellow to brown and dropped prematurely. Infections on pods were not observed. Disease incidence was estimated to be 100%. An examination of diseased tissues using light microscopy revealed amphigenous mycelium, effuse or forming dense patches, persistent, and often causing brownish discoloration of leaves; hyphae were hyaline and smooth, 2 to 6 µm wide with hyphal appressoria solitary or in opposite pairs and lobed. Conidiophores measured 25 to 37 × 7 to 10 µm, straight or curved at base with cylindrical foot-cell, followed by 1 to 2 cells, shorter, equal in length, or occasionally somewhat longer than foot-cell. Conidia were borne singly, obovoid-ellipsoid, doliiform, and measured 28 to 39 × 14 to 20 µm. Fibrosin bodies were absent. Conidia germinated terminally or subterminally. Chasmothecia were not observed. The morphological characteristics were consistent with those of the anamorphic state of Erysiphe quercicola (Braun and Cook 2012). A reference specimen (HAL-2940 F) was deposited in the Geobotany Herbarium, Institute of Botany, Martin Luther University, Halle, Germany. The internal transcribed spacer (ITS) region of the rDNA was amplified with primers ITS5/ITS4 (White et al. 1990), and sequenced directly. The resulting 523-bp sequence was deposited in the NCBI database (GenBank accession no. KX369238). The BLASTn analysis showed 100% nt identity to sequence of E. quercicola (AB237805) reported on A. auriculiformis from Thailand (Limkaisang et al. 2006). Pathogenicity was confirmed by gently dusting conidia from infected leaves onto leaves of 10 healthy earleaf acacia plants. Five noninoculated plants served as controls. Inoculated and control plants were exposed to high relative humidity (>80%) for 48 h and then maintained in a greenhouse at 20 to 30°C. All inoculated plants developed similar symptoms to the original observation after 12 days, whereas control plants remained symptomless. Microscopic examination of the fungus on inoculated plants showed that it was morphologically identical to that originally observed. Until now, only Oidium sp. and E. acaciae were reported to occur on Acacia spp. in India (Borah et al. 2012; Braun and Cook 2012). To the best of our knowledge, this is the first report of powdery mildew caused by E. quercicola on A. auriculiformis in India. The presence of powdery mildew on A. auriculiformis detracts from the esthetic value of these plants. © 2017, American Phytopathological Society. All rights reserved.