Archivio Journal of Plant Pathology

Abstract:

In May 2014, severe foliage infection on Bidens pilosa was observed in Zhanjiang city, Guangdong Province, China. Symptoms first appeared as irregularly-shaped, dark brown spots with an average diameter of 4 to 5 mm, sometimes surrounded by a chlorotic halo. In severe infections, lesions enlarged and coalesced, resulting in leaf blighting. A fungus was consistently isolated that produced a cottony and greyish- green mycelium. The conidiophore was 25-85 x 5 μm in size, olivaceous brown, simple or 1-2 geniculate. Conidia were solitary or occasionally in chains of two, dark olivaceous brown, straight or curved, obclavate, with 5-11 transverse and 1-several longitudinal or oblique septa. The conidia measured 160-410 x 15-25 μm in size including a filamentous beak (200-300 x 5 μm), beaks often once branched, 5-7 μm thick at the base, tapering to 1-3μm at the apex. These morphological features were typical of Alternaria dauci (Simmons, 1995). Pathogenicity of the isolate was confirmed by placing seven-day-old mycelial plugs (5 mm) grown on PCA on Bidens pilosa leaves, which were wrapped in polyethylene bags and incubated in a moist chamber at 25 ± 2°C and 80-90% relative humidity. Five days after inoculation, leaf spots developed on the inoculated plants from which A. dauci was re-isolated, whereas the control plants remained symptomless. Molecular identification was carried out by PCR using the internal transcribed spacer (ITS) region primers ITS1/ITS4 and glyceraldehyde-3-phosphate dehydrogenase (gpd) gene primers gpd1/ gpd2 (Berbee et al., 1999). These amplified sequences shared 98% to 99% nucleotide similarity with those of A. dauci (GenBank Accession Nos JQ936188 and HE796759). Sequences obtained from the pathogen were deposited in GenBank under accessions KP120981 and KP123843. This is the first report of A. dauci on Bidens pilosa in China and worldwide.