First Report of Lasiodiplodia theobromae Associated with Stem Canker of Cassia fistula in Guangxi, South China.

Abstract

Cassia fistula, a member of the Fabaceae, known as the golden shower tree, is native to South Asia. It is now distributed worldwide and is popular as an ornamental plant as well as being used in herbal medicine. In October 2013, symptoms of stem canker were observed on C. fistula in a nursery (108°38' E, 22°87' N) in Nanning, Guangxi, China. The symptoms began as small brown lesions, which enlarged over several months to long, striped, slightly sunken lesions, 1 to 9 cm in width and 16 to 135 cm in length. The conspicuous cankers had vertical cracks outlining the canker and evenly spaced horizontal cracks, eventually resulting in whole plants dying back. The cankers were found on 90% of six-year-old plants in this nursery and were also observed in other plantings. On potato dextrose agar (PDA), isolates with similar morphological characteristics were consistently recovered from symptomatic plant tissues after surface sterilization in 75% ethanol for 30 sec and then in 0.1% mercuric chloride for 2 min. Over 100 conidia were examined from three isolates and were found to be elliptical and hyaline when immature, becoming dark brown, one-septate, and longitudinally striate when mature and ranging from 20 to 31 × 11 to 16 μm (average 25.5 × 13.6 μm). The rDNA internal transcribed spacer (ITS) region of isolate LC-1 was sequenced (GenBank Accession No. KM387285), and it showed 100% identity to Lasiodiplodia theobromae (Pat.) Griffon & Maubl. (GenBank KC964548), confirming the morphological identification (2) as L. theobromae (also known as Botryosphaeria rhodina (Cooke) Arx). A culture of this isolate has been preserved in the Guangxi Academy of Agricultural Sciences fungal collection. The pathogenicity of the isolate was tested on healthy twigs and branches of C. fistula trees in a field setting at Guangxi Agricultural Vocational-Technical College, Nanning, Guangxi, in June and August 2014. For each treatment, five green twigs and five 2-year-old branches were used. Five adjacent needle punctures were made on each branch with a sterilized needle. A mycelial plug was then placed on the wound of each branch and wrapped with Parafilm. Control twigs were treated with sterile PDA plugs. One week later, typical lesions were observed on the inoculated branches, with symptoms becoming more extensive after two weeks, but no symptoms were seen on the controls. Koch's postulates were fulfilled by re-isolation of L. theobromae from diseased branches. L. theobromae is recognized as an important wood pathogen and has been reported to cause cankers, dieback, and fruit and root rots in over 500 different hosts, including perennial fruit and nut trees, vegetable crops, and ornamental plants (2). The fungus has been reported on C. fistula in India since the 1970s (1); however, to our knowledge, this is the first report of L. theobromae infecting C. fistula in China.