First Report of Leaf Spot Disease on Coconut Caused by Colletrotrichum gloeosporioides in Hainan, China

Coconut (Cocos nucifera L.), one of the most commonly grown oil plants, is distributed widely in South China. In January 2014, a new leaf spot disease was found on coconuts in Qionghai, Hainan Province. The lesions were surrounded by yellow haloes and became dry and gray as the disease progressed. Some dark grayish mildew often appeared in the middle of the lesion. Then, the infected leaves turned gray, leading to defoliation. The pathogen was consistently isolated from the lesions and pure single-spore cultures were obtained. Twenty pieces of diseased leaf samples were plated and five pieces yielded fungal colonies after 2 to 3 days of incubation at 26°C. Colonies on potato dextrose agar (PDA) were white and later became dark gray. Conidia were oval, transparent, and measured 7.4 to 19.1 × 2.1 to 6.2 μm (n = 100). The mycelium was branched, septate. All characteristics described above were identical to the Colletotrichum gloeosporioides on Pitahaya (Palmateer et al. 2007). Pathogenicity tests were conducted. Three leaves of a 2-year-old coconut were wound-inoculated with a 2.7 × 105 spores/ml suspension of conidia and three leaves were wound-inoculated with mycelial plugs (5 mm). Plants were kept in a growth chamber at 26°C. Leaves treated with PDA plugs and sterilized water were used as controls. The experiment was repeated three times. Two weeks after inoculation, all the inoculated plants showed typical leaf spot symptoms, a dark-gray mildew appeared at the points of inoculation, whereas the control showed no symptoms. The fungus was reisolated from the infected tissues, fulfilling Koch’s postulates. For molecular identification, the internal transcribed spacer (ITS) region was amplified from genomic DNA with primers ITS1 and ITS4 and the 545-bp PCR product was sequenced (GenBank Accession No. KF697685). BLASTn analysis demonstrated 99% sequence identity with C. gloeosporioides (GenBank Accession No. HG938365.1). Comparison of the sequences available in GenBank revealed that the current ITS sequence differed by four base pairs from two C. gloeosporioides isolates (HG938365.1 and HQ264182.1). C. gloeosporioides is one of the most important pathogens of fruits, vegetables, and other crops (Vida et al. 2006; Maharaj et al. 2012), but was not previously reported causing disease on coconut. To our knowledge, this is the first report of leaf spot of coconut caused by C. gloeosporioides. © 2016, American Phytopathological Society. All rights reserved.