First Report of Leaf Spot Disease Caused by Colletotrichum siamense on Chrysalidocarpus lutescens in China

Chrysalidocarpus lutescens (areca palm) is a tropical ornamental plant grown in parks and houses worldwide. In August 2018, leaf spot symptoms were observed on C. lutescens plants in Fujian Agriculture and Forestry University (26°5′16″N, 119°14′6″E), Fuzhou, China. Approximately, 60% of C. lutescens plants were infected, with initial symptoms characterized as small dark brown to black spots of 1.5 to 20 mm in diameter with a distinct yellow halo on leaves that gradually changed from circular to elliptical lesions. As the lesion expanded or coalesced, the affected area became blight with a gray color at the center. In severe cases, the whole infected leaf turned orange yellow, dried, and eventually fell. For isolation of the pathogen, 18 infected leaf samples from 12 plants were collected and sterilized with 75% alcohol and rinsed two to three times in sterile distilled water; infected leaf areas were cut into 1-cm pieces, placed onto 2% potato dextrose agar, and incubated at 25°C for 3 days. Obtained fungal colonies were grayish white and slightly cottony. Individual colony was 80 mm in 7 days at 25°C with visible orange conidial masses, and the reverse was light gray-pink. Conidiomata were brown to dark brown having brown setae, 40 to 105 × 3 to 4 μm. Conidia were hyaline, aseptate, fusiform with both rounded ends or obtuse at one end, 7 to 21 × 3 to 5 µm (n = 50). Appressoria were brown to dark brown, ovoid to clavate, slightly irregular to irregular, and ranged from 7 to 10 × 4 to 5 μm. These morphological characteristics were consistent with the species in the Colletotrichum genus (Prihastuti et al. 2009). Genomic DNA of six selected isolates was extracted, and partial internal transcribed spacer (ITS) regions, TUB, EFl-a1, ACT, and CHS-1 genes were amplified using primers ITS4/ITS1, EF1728 F/EF1-986R, Bt2a/Bt2b, CHS-79F/CHS-354R, and ACT-512F/ACT-783R. Obtained sequences showed 99 to 100% similarity with C. siamense accessions in GenBank. Sequences (ITS, MK072975; TUB, MK208459; EFl-a1, MK208458; ACT, MK371407; and CHS-1, MK371408) of an isolate (TR17) have been deposited in GenBank. To confirm pathogenicity, four sets (five leaves per set) of disinfected healthy palm leaves were punctured three to five times, and 10 to 15 ml of conidial suspension (10⁵ conidia /ml in sterilized distilled water) was sprayed over punctured areas of one set of leaves, and mycelial discs (5 mm) from 7-day-old cultures were placed over wounded points of leaves in a second set, placed in a moist plastic box, and incubated at 25°C. For the control, two sets of leaves were inoculated with sterilized distilled water and noncolonized agar plugs onto the wounds, respectively. After 4 to10 days, symptoms were observed on all inoculated leaves (spore and mycelium), whereas control leaves remained asymptomatic. The same fungus was reisolated from the lesions, confirming Koch’s postulates. Previously, Helminthosporium sp., Bipolaris setariae, Exserohilum rostratum, and Phaeotrichoconis crotalariae have been reported as leaf spot pathogens on C. lutescens from Sri Lanka and the United States (Chase 1982; Jegathambigai et al. 2008). To our knowledge, this is the first report of C. siamense associated with leaf spot disease on C. lutescens in China. This report can play an important role in the development of appropriate strategies for this tropical plant disease management.