Powdery mildew on Rudbeckia hirta in India

Black-eyed Susan (Rudbeckia hirta)belongs to the family Asteraceae. Plants have yellow flowers with purplish black, raised central discs. Flowers from this genus are often called coneflowers. It is commonly used for ornamental purposes in India and grown commercially in hill states like Himachal Pradesh, Meghalaya, Sikkim and others. During July 2008, plants of R. hirta from Shillong, Meghalaya (Northeast India) and nearby areas were found to be infected with powdery mildew. Dense white circular to irregular mycelial patches were present on both surfaces of the leaves (Fig 1). Symptoms were also present on stem and flowers. Older leaves were found to be more susceptible. More than 50% of the plants selected randomly from ten localities were infected.

Powdery mildew belonging to the genera Golovinomyces, Podosphaera (=Sphaerotheca) and Phyllactinia have previously been reported on Rudbeckia from United States, Sweden, Germany, Canada, Korea, Switzerland, Italy and Japan (Farr et al., 2005). Presence of ectophytic mycelium with nipple shaped appressoria and conidia without fibrosin bodies excluded the possibility of Phyllactinia and Podosphaera but suggested Golovinomyces (Fig 2.). Conidiophores contained a cylindrical foot cell (33-68 x 8-13 µm) followed by two to three short cells. Conidia were mostly cylindrical, a few doliiform (27-36 x 11-18 µm) (Fig.3). The pathogen was identifed as the anamorph of Golovinomyces cichoracearum (Cook & Braun, 2009), which was further confirmed by the germination pattern (terminal with moderately long germtubes and swollen appressorium) (Fig. 4). A recent report by Garibaldi et al. (2008) refers to the presence of G. cichoracearum on Rudbekia spp. from India. However, subsequent analysis of conidial dimensions and germination pattern shows that the reported pathogen belongs to a different species, namely G. ambrosiae (syn. G. cichoracearum var. latisporus) (Cook & Braun, 2009).

Pathogenicity was confirmed by dusting conidia on healthy potted plants of R. hirta, non-inoculated plants serving as control. Inoculated plants developed the original powdery mildew symptoms after 8-10 days whereas control plants remained healthy. Voucher specimens (HAL 2290 F.) were deposited at Martin-Luther-Universität, Institut für Biologie, Bereich Geobotanik, Herbarium, Germany and Institute Herbarium (ICARHNEH-26) ICAR Research Complex for NEH Region, Meghalaya, India. The pathogen has not been reported from the neighboring countries such as Pakistan, Sri Lanka, Bangladesh so it may have been introduced to India by imported material. To our knowledge, this is the first record of powdery mildew of R. hirta caused by G. cichoracearum in India.