First Report of Basal Stem Rot on Oil Palm Caused by Thielaviopsis paradoxa in Hainan, China

Abstract

Oil palm (Elaeis guineensis) is one of the most widely grown and distributed oil plants in southern Asia. It is usually grown as a garden ornamental in China. A serious basal stem rot disease was observed on young oil palm (Tenera variety) during the months of October and December in 2017 in Wenchang, Hainan Provence, China, at several locations. A survey of 100 2-year-old oil palm plants revealed that the disease caused serious damage during the typhoon season of Hainan Province, with 20 to 25% incidence in plants. The leaf symptoms included wilting and necrosis, beginning with the lowest leaves in the canopy. Cross-sections revealed brown rot confined to one side of the basal stem, where the rot was spreading inward from the surface. Diseased stem tissues from six trees in two palm gardens were collected and cut into 5-mm cubes. After tissue collection, surface sterilization was performed by the immersion of the previously washed plant tissue in ethanol (75%) for 1 min and then in hypochlorite (0.1%) for 1 min; it was then rinsed with sterile distilled water and cultured on potato dextrose agar (PDA) at 25°C. A fungus was isolated from lesion margins of diseased oil palm trees. Colonies on PDA were initially white and downy, becoming black and velvety after 2 days and emitting a fruity odor. The fungus produced conidia, which were cylindrical, colorless to pale brown, and 6.1 to 13.2 × 3.4 to 5.8 μm (n = 50). Chlamydospores were terminal, in chains, obovate to oval, thick walled, brown, and 7.3 to 18.9 × 4.4 to 11.7 µm (n = 50). Conidiophores were straight, sometimes sinuous at the base, hyaline to pale brown, septated at the base, smooth, and producing a succession of conidia through the open end. The morphological characteristics and measurements of this fungal isolate matched the previous descriptions of Thielaviopsis paradoxa (De Seynes) Hohn. (Morgan-Jones 1967). For the molecular characterization, sequences of the internal transcribed spacer (ITS) regions were amplified from isolates using the primer pair ITS1/ITS4. A BLAST search of GenBank showed that the ITS (MG706124) sequences of the isolate were 99% homologous with T. paradoxa strain PG3 (GenBank accession no. KF312152). A maximum likelihood tree was generated by MEGA6.0, and isolate OPC3 clustered together with T. paradoxa strain PG3 (98% bootstrap). For a pathogenicity test, three 2-year-old healthy basal stems were wounded with a fine needle and inoculated with mycelial plugs (5-mm-diameter plugs), and another set of three basal stems were treated with pure PDA plugs as a control. All stems were placed in a growth chamber at 25°C and 90% relative humidity. The pathogenicity test was repeated three times. Two weeks after inoculation, all the inoculated rachises showed typical symptoms, and a rusty brown lesion appeared at the points of inoculation. No symptoms were observed on the controls. The fungus was reisolated from the infected tissues, fulfilling Koch’s postulates. The results obtained on morphological characteristics, pathogenicity, and molecular data corresponded with those of T. paradoxa. T. paradoxa has been reported as causing rot diseases on coconut (Yu et al. 2012), pindo palm (Tang et al. 2014), and triangle palm (Yu et al. 2016) in HaiNan. To our knowledge, this is the first report of basal stem rot of oil palm caused by T. paradoxa in China. Given its wide host range, T. paradoxa has great potential to become an economically important plant pathogen.