The jackfruit tree (Artocarpus heterophyllus) is native to South and South-east Asia including Bangladesh. It is a commercially important tropical tree species that produces fruit, food, fodder, and high-quality wood (Gupta et al. 2022). During surveys in February 2022, soft rot on immature fruit at approximately 70% incidence was observed in several plantations and homesteads in the Sylhet district of Bangladesh. Infected fruit had black patches surrounded by wide bands of white, powdery masses. The patches enlarged with fruit maturation, and in some cases, covered the entire fruit. Symptomatic fruit were collected, surface sterilized with 70% ethanol for 1 min, and washed 3 times with sterile distilled water. Fen air-dried, and small pieces from the margins of lesions were transferred to potato dextrose agar (PDA). The plates were incubated at 25°C in the dark. Two-day-old colonies had diffuse, gray cottony mycelia that were hyaline and aseptate under the microscope. Sporangiophores measuring 0.6-2.5mm in length and 18 to 23μm in diameter had rhizoids and stolons at their bases. Sporangia were almost spherical and were 125μm (±65μm, n=50) in diameter. Sporangiospores were ellipsoid to ovoid and measured 3.5 to 9.32μm × 2.82 to 5.86μm (x̄= 5.86×4.1μm, n=50). Based on these morphological features, the isolates were identified preliminarily as Rhizopus stolonifer (García-Estrada et al. 2019; Lin et al. 2017). To identify the pathogen molecularly, genomic DNA was extracted using the FavorPrep Fungi/Yeast Genomic DNA extraction Mini Kit (Taiwan). Polymerase chain reaction (PCR) amplification of ITS1-5.8S-ITS2 rDNA was done using primers ITS4 and ITS5 (White et al. 1990) following the procedure of Khan and Bhadauria (2019). The PCR product was then sequenced by Macrogen, Korea. A BLAST search in GenBank revealed that isolate JR02 (GenBank accession OP692731) was 100% identical to R. stolonifer (GenBank accession MT256940). In pathogenicity tests,10 healthy young fruit at a similar maturity stage as the ones found diseased were collected from a orchard where the disease was not observed. Fruit were surface sterilized with 70% ethylalcohol and washed with sterile distilled water. Wounded (using a sterilized needle) and non-wounded fruits were inoculated with 20µl of a spore suspension (1×106/ml). Sterile distilled water was used for the controls. Inoculated fruit were covered with sterile cloth, transferred to perforated plastic bags with moistened blotting paper, and incubated at 25°C in the dark. Symptoms were first observed after 2 days on wounded fruit, but no symptoms developed on controls and non-wounded fruit. Rhizopus stolonifer was re-isolated from infected fruit, thus fulfilling Koch's postulates. Rhizopus rot is a devastating disease causing premature fruit drop, reduced crop yield, and post-harvest rot of jackfruit and other fruits and vegetables (Sabtu et al. 2019). Three Rhizopus species namely R. stolonifer, R. artocarpi and R. oryzae have been reported causing fruit rot of jackfruit in the tropics including Mexico, India and Hawaii (García-Estrada et al., 2019; Babu et al., 2018; Nelson, 2005). Appropriate management strategies are needed to be developed to prevent premature rot of jackfruit. To our knowledge, this is the first report of R. stolonifer causing premature soft rot of jackfruit in Bangladesh.
Keywords: Causal Agent; Forest pathology; Fungi; Mycology; Tree.