First report of brown leaf spot caused by Bipolaris australiensis on Cynodon spp. in China

Bipolaris leaf spot affects many turfgrasses. Bipolaris cynodontis and B. hawaiiensis were reported to be pathogens of Cynodon spp. in Hainan (Liu & Pu, 2004) and Guangdong Provinces (Xiang & Zhong, 1999), respectively. From the summer of 2003 to the winter of 2005, a new disease was observed on C. dactylon x C. transvaalensis cultivar ‘Tifgreen’ at the Wuhan and E-Zhou golf courses in Hubei province. The disease which caused discoloration of up to 25% of affected patches, was characterised by irregularly shaped, brown, water-soaked spots on leaves and sheaths. Severely infected leaves became straw-coloured.

A fungus was isolated on quarter strength potato dextrose agar (PDA) which was then purified by single sporing. Cultures were initially white, but after three days acquired an olive green to black cast found to contain cylindrical conidia with three pseudosepta. Sizes ranged from 13-35 µm x 8-10 µm. Conidiophores were brown, simple or branched, geniculate and sympodial. The hilum was characterised by a flattening of the basal cell, slightly protruding at the tip. Morphological characterstics matched those of B. australiensis. Sequencing of the internal transcribed spacer region of rDNA showed a 100% match with Cochliobolus australiensis (teleomorph of B. australiensis) GenBank accession AY923860.

To confirm pathogenicity of B. australiensis, 20 day-old, pot-grown ‘Tifgreen’ leaves were treated with plugs or spores. Mycelial plugs from PDA (6-mm-diameter) were placed on leaves for 48 h and then removed. Small circular brown spots from 3 to 5 mm in diameter appeared three days later. Other leaves were sprayed with a conidial suspension of 2×105 spores per ml until run off. Deep brown spots similar to those seen at the golf courses appeared after five days. Leaves with attached sterile agar plugs or sprayed with sterile water, remained healthy. We re-isolated and confirmed the presence of B. australiensis on induced symptoms.

B. australiensis has been reported previously to be pathogenic on Chloris and Pennisetum species in Australia, Kenya and India (Smith et al., 1989). To the best of our knowledge, this is the first report of brown leaf spot caused by Bipolaris australiensis on Cynodon spp. in China.