First Report of New Postharvest Rot in Ginger Rhizome by Aspergillus parvisclerotigenus in Pakistan

Ginger (Zingiber officinale) rhizome is widely used in Pakistan as a spice. During the summer of 2013, several ginger sellers in a local vegetable market of Lahore, Pakistan, reported a green powdery mass of an unidentified pathogen that rotted a considerable quantity of ginger in its packaging. As far as we know, this disease was limited to stored rhizomes and not reported in fields. A survey was conducted in August to September of 2013 in three different vegetable markets in Lahore to collect infected samples. From each of three survey points from individual markets, 20 bags (10 kg each) were selected randomly. Average incidence of decay (by weight) was found to be 45%. Initial symptoms appeared as discoloration, soft and slippery skin with abundant green sporulation. Ten samples (rhizomes) from each market were brought to the laboratory for further studies. Isolation of the causal agent was carried out on two growth media: malt extract agar (MEA) and Czapek Dox agar (CZA). Inoculation was carried out by direct transfer of visible green spores as well as transferring a small fragment of surface sterilized infected rhizome to the media. Inoculated media plates were incubated at 25°C for 3 to 4 days. Emerging fungal colonies were sub-cultured to get pure cultures. The fungal colony was powdery, green, 3.5 to 4 cm in diameter, and without zonation after 7 days of incubation. Sclerotia were brown to black and globose. Conidial heads were columnar and biseriate, occasionally unseriate. Conidiophores were 1 to 2.5 mm long. Vesicles were sub-globose to globose and 25 to 30 μm wide. Metulae were 12 to 18 μm high and phialides were 6 to 12 μm. Conidia were globose to sub-globose, green, and 4 to 5 μm in diameter. Based on morphology, the fungus was identified as Aspergillus parvisclerotigenus (1). The identity of the pathogen was confirmed by ITS sequence analysis of two different isolates. For this, ITS1-5.8S-ITS2 nucleotide sequence of ~560 bp was amplified using total fungal genomic DNA as a template and ITS1 forward (5'-TCCGTAGGTGAACCTGCGG-3') and ITS4 reverse primer (5'-TCCTCCGCTTATTGATATGC-3') (2). Sequences from both isolates were 100% similar with each other. A BLAST search showed that this sequence had 99% homology with that A. parvisclerotigenus CBS 121.62 (EF409240.1). A culture of the fungus was deposited in First Fungal Culture Bank of Pakistan (FCBP1352) and the nucleotide sequence of ITS region to GenBank (KJ445022). For completion of Koch's postulates, a spore suspension (10⁵ spores/ml) from a 1-week-old culture was prepared. Ten surface-disinfested, air-dried ginger rhizomes were placed on sterilized wet blotting papers in a glass beaker and inoculated by spore suspension using a hand sprayer. Similarly, 10 control rhizomes were sprayed with sterile distilled water. Rhizomes were incubated at 25°C for 7 days. The experiment was replicated three times. The same symptoms noticed in the vegetable markets were observed in 80% of the inoculated rhizomes while control rhizomes remained healthy. Re-isolation of the pathogen from symptomatic rhizomes fulfilled Koch's postulates. Poor hygiene is thought to be the main cause of rotting; therefore, this disease is not a threat to ginger if stored properly. To our knowledge, this is the first report of postharvest ginger rhizome rot from Pakistan caused by A. parvisclerotigenus. References: (1) J. Varga et al. Stud. Mycol. 69:57, 2011. (2) T. J. White et al. In: PCR Protocols: A Guide to Methods and Applications, page 315. Academic Press, San Diego, CA, 1990.