Aspergillus niger causing fruit rot disease on rose apple in Malaysia

Abstract

The rose apple (Syzygium samarangense) is a tropical fruit native to Southeast Asia. Rose apple cultivation occupies c. 3,300 hectares of land in Malaysia with an annual production value exceeding US$9 million (Al-Obaidi et al., 2018). However, fruit rot disease results in the loss of approximately one-third of annual production, causing significant economic hardship. Between 2017 to 2020, rose apples in many farms located in Johor, Malaysia displayed symptoms of fruit rot, characterised by dark, sunken circular lesions on ripening fruits (Figure 1). To identify the causal agent, infected fruit tissues (c. 5 × 5 mm2) were surface sterilised using 1.5% sodium hypochlorite for one minute followed by 70% ethanol for another minute. The tissue was then rinsed with sterile distilled water and placed onto potato dextrose agar (PDA). Plates were incubated at 25°C for seven days and colonies grew that initially appeared white, then turned dark green (Figure 2). Based on the morphological characteristics, particularly the range of conidia sizes (3.0 to 5.4 μm), shape (ellipsoidal) and the colour, the fungus was identified as Aspergillus niger (Silva et al., 2011; Li et al., 2020). For molecular confirmation, the internal transcribed spacer (ITS) region was amplified using ITS1 and ITS4 primers (Santos et al., 2016). The PCR amplicon was sequenced (GenBank Accession No. MF362179) and shared 100% identity with A. niger (MG228418.1, MW188561.1 and KY378943.1). To confirm the pathogenicity of the A. niger isolate (ABI-JJ2), healthy, detached rose apple fruits were surface sterilised by immersing them in a 6% sodium hypochlorite solution for five minutes, followed by washing with 70% ethanol for one minute. Wounded fruits (pin-pricked) were then inoculated with agar plugs containing mycelium of A. niger. Control fruits were inoculated with sterile PDA plugs. Four fruits were used for each treatment. The fruits were placed in plastic boxes and incubated at 25°C with a relative humidity of 80% for seven days. Three days after inoculation, all the inoculated fruits exhibited typical Aspergillus fruit rot symptoms, resembling those seen in the field. The control fruits were unaffected and showed no symptoms. Aspergillus niger was reisolated from the lesions on the inoculated fruit, fulfilling Koch's postulates. Aspergillus niger is responsible for widespread diseases in many plants and is frequently found as a contaminant in food (Lima et al., 2019). This study demonstrates the virulence of A. niger on rose apple, resulting in reduced fruit quality and production. To our knowledge, this is the first record of A. niger causing fruit rot on rose apples in Malaysia, and globally. More research is needed into the pathogenicity and epidemiology of A. niger on rose apples, to aid the development of targeted solutions and limit the effects of this disease. The authors would like to thank the National Institutes of Biotechnology Malaysia and Universiti Pendidikan Sultan Idris for their assistance during this study.