First report of leaf spot of Smilax china caused by Alternaria longipes in China

Smilax china is a small perennial plant belonging to the Liliaceae frequently found in the warm temperate mountainous or hilly regions across eastern Asia. It is a commonly used Chinese herbal medicine. Leaf spot of S. china has been found to be a destructive disease in Tongcheng County, Hubei, China, where this plant is extensively cultivated. Symptoms were observed on infected leaves and fruits from 2006 to 2008 in a plantation established in 2002. Up to 70% of the plants in the areas surveyed showed severe leaf spot symptoms each year.

Typical lesions observed were brown, circular or oval and 5-7 mm across (Fig. 1). These enlarged and linked together, extending until entire leaves withered. Isolations were made onto potato dextrose agar (PDA) initially forming white colonies. After seven days on PDA, the colonies turned grey or brown (Fig. 2), and by 12 days, the agar became blue-grey to dark green throughout (Fig. 3). Sporulation was induced on cut filter paper following Zhang (2003). Conidiophores were fasciculated or single, straight or knee-curved, light brown with regular septa, and 20-58 x 3.6-5.5 µm. Each conidium was obclavate, brown, 28-48 x10-17 µm, with 3-7 transverse and 0-3 longitudinal or oblique septa (Fig. 4). The tops of some conidia developed into secondary conidiophores, which were cylindrical, hazel-coloured, and 5.5-22 x 2.5-5.2 µm. The pathogen was identified as Alternaria longipes based on descriptions in Simmons (1981, 1999). Sequences of rDNA-ITS were obtained from three isolates, and comparisons with GenBank showed 100% similarity with A. longipes (AY751457).

Pathogenicity tests were performed by spraying until runoff (200 ml/plant) a conidial suspension (5x10⁵ conidia/ml) containing 0.1% Tween-20, onto upper and lower surfaces of 30 leaves of six one-year-old 20-cm tall plants. Plants were incubated with a 12 h photoperiod at 25^°C and 90% relative humidity. Twelve days after inoculation, brown spots were observed on inoculated leaves, but no symptoms were seen on water-treated control plants. Koch’s postulates were fulfilled by re-isolating A. longipes from diseased leaves. Alternaria longipes is known in China to cause tobacco brown rot, but to our knowledge, this is the first report of A. longipes infecting S. china, and furthermore, no previous reports of a disease of S. china have been found.