First Report of Alternaria alternata Causing a Leaf Spot on Philodendron 'con-go' in China

Philodendron 'con-go' is widely cultivated indoors in China as an evergreen potted plant. In October 2013, a leaf spot on Philodendron 'con-go' was observed in the residential district of Luoyang (112.46° E, 34.62° N), Henan Province, China. The disease was characterized by oval-shaped, 10 to 20 × 25 to 55 mm, yellow to brown lesions with darker brown borders. Fifty potted plants were surveyed, and less than 2% of the leaves were infected. Lesions appeared mostly in old leaves. The symptomatic leaves affected on the plants' ornamental value, but had little impact on their health. Some lesions merged to form a large irregular lesion that could cover a whole leaf. Two infected leaves from one plant were selected randomly for the isolation of the pathogen. Lesions were cut into 1 cm² pieces, soaked in 70% ethanol for 30 s, sterilized with 1% sodium hypochlorite for 5 min, then washed three times in sterilized distilled water. The pieces were incubated at 25°C on potato dextrose agar (PDA) for 4 to 5 days. A fungus was consistently isolated. Colonies of the fungus were deep green with white mycelium borders. Conidiophores were light brown with 2 to 4 septa. Conidia were obclavate, 14.6 to 49.1 × 8.3 to 16.4 μm, with a short beak, and with 1 to 5 transverse septa and 0 to 3 longitudinal septa, light brown to olive-brown. Based on morphology, the pathogen was identified as Alternaria alternata. Three isolates were selected randomly for further identification. To confirm pathogenicity, eight leaves of potted Philodendron 'con-go' plants were wounded with a sterile pin after wiping each leaf surface with 70% ethanol and washing each leaf with sterilized distilled water three times. The isolates were grown on PDA for 7 days and suspended in sterile distilled water to produce a final concentration of 2 × 10⁵ spores/ml. A 5-μl drop of spore suspension was placed on each pin-wounded leaf. Each of three fungal isolates was inoculated on two leaves, and the control treatment (water inoculated) was done similarly on two leaves. The plants were placed in a growth chamber at 28°C with 80% relative humidity, 50 to 60 klx/m² light intensity, and a 10-h photoperiod. After 7 days, lesions appeared on inoculated leaves, but the control leaves remained symptomless. Pathogenicity tests were repeated three times. Similar disease symptoms and re-isolation of A. alternata fulfilled Koch's postulates. To confirm the fungal identification, the rDNA of the internal transcribed spacer region in three isolates were amplified using primers ITS1 and ITS4 (1) and sequenced. The nucleotide sequence of the ITS region was submitted to GenBank under accession KJ829535 and showed 100% sequence identity with the strain A. alternata LPSC 1187 (KF753947.1). To our knowledge, this is the first report of a leaf spot of Philodendron 'con-go' by A. alternate in China. Reference: (1) T. J. White et al. PCR Protocols: A Guide to Methods and Applications. M. A. Innis et al., eds. Academic Press, San Diego, CA, 1990.