First Report of Leaf Spot Caused by Alternaria porri on Velvet Bean (Mucuna pruriens) in China.

Mucuna pruriens is a tropical legume known as velvet bean. It has many traditional and medicinal usages in treating Parkinson's disease (1), abdominal pain, cholera, infertility, scorpion bites, diabetes (3) and is found in tropical Africa, India, the Caribbean and China. During the fall of 2011, the velvet bean plants in Longan County, Guangxi, China, were damaged by a leaf disease previously unreported in China. Field inspections revealed disease incidences as high as 80%. Symptoms consisted of large spots developing between the leaf veins that ranged in length from 1 to 3 cm. Spots were dark brown, generally rectangular, and were visible from both sides of the leaf. Lots of black mycelia and conidia were found on the backs of the lesions. Lesions typically expanded and affected the entire leaf, resulting in leaves withered and killed. An Alternaria sp. having conidia with prominent beaks and spores produced singly was consistently observed on and isolated from symptomatic leaf tissue. The conidia body was brown, ovoid, obclavate, muriform, and septate, with transverse and longitudinal septa that varied from 6 to 9 and 0 to 2, respectively, and its dimensions varied from 60 to 120 × 15 to 20 μm. The beaks were 10 to 70 × 3 to 6 μm and were filamentous, slender, and unbranched. Pure culture of the fungus was made from a single spore. DNA was extracted and used in an internal transcribed spacer (ITS) PCR used ITS1 and ITS4 primers. The PCR products were purified and sequenced. The sequences were used in BLAST searches to interrogate the GenBank for sequence similarity. High sequence similarity of 100% was obtained with Alternaria porri isolate AP-18. Based on the morphological and molecular characterization, the isolate was identified as A. porri (Ellis) Ciferri (GenBank Accession No. JX556683) (2). The pathogenicity of five isolates was investigated to demonstrate Koch's postulates. Forty 8-week-old seedlings of M. pruriens, without wounding on their leaves, were sprayed with the spore suspension (10⁵ spores per ml), prepared by using 10-day-old cultures of the isolates grown on potato dextrose agar at 28°C in the dark. Every plant was sprayed with 4 ml of spore suspension. The inoculated plants were incubated in a humid chamber for 48 h and then maintained in a greenhouse. After 5 to 7 days, leaf spots similar to those observed in the field developed on all inoculated plants. The pathogen was reisolated and identified as A. porri. Control plants sprayed with distilled water remained symptomless. The inoculation test was repeated and results were the same. Because A. porri was reported to infect plants of Allium spp and cause purple blotch, we also inoculated this isolate to small onion plants, and the symptoms of purple blotch appeared after 13 days at 25 to 28°C in a greenhouse. To our knowledge, this is the first report of leaf spot caused by A. porri on velvet bean in China. References: (1) R. Katzenschlager et al. J. Neurol. Neurosurg Psychiatry. 75:1672, 2004. (2) S. T. Koike and D. H. Henderson. Plant Dis. 82:710, 1998. (3) S. O. Majekodunmi et al. Asian Pac. J. Trop. Med. 4:632, 2011.