First Report of Mucor piriformis Causing Postharvest Fruit Rot of Kiwifruit in Greece

Abstract

Kiwifruit (Actinidia deliciosa) is one of the most important crops in Greece. In February 2018, six cubes (each contained about 150 fruit) of kiwifruit (cv. Hayward) were collected from three warehouse cooling chambers (cold stored for 4 months at 2°C) located in Naoussa, the Prefecture of Imathia, Greece, and moved to room temperatures (15 to 20°C). Symptoms of postharvest fruit rot were observed in kiwifruit (about 12% of the fruit showed similar symptoms of infections) 7 days later. The symptoms started with water-soaked lesions, which rapidly extended and softened. Decaying fruit become very “juicy.” A profuse growth of coarse white mycelium bearing black pin-shaped spore heads may be present. Isolations were made on potato dextrose agar at 22°C. Columellate sporangia attached terminally on short and tall branched and unbranched sporangiophores were observed. Sporangiospores were ellipsoidal, subspherical, and smooth. Identification of pathogen as Mucor piriformis E. Fischer was based on cultural characteristics according to Michailides and Spotts (1990). Species-level identification was conducted by Westerdijk Fungal Biodiversity Institute, Identification Service, Utrecht (Netherlands) and was confirmed by isolating genomic DNA from one isolate. A fragment containing the partial ribosomal internal transcribed spacer (ITS) region was amplified using the forward primer V9G (5′-TTACGTCCCTGCCCTTTGTA-3′) and the reverse primer LS266 (5′-GCATTCCCAAACAACTCGACTC-3′) (Zheng et al. 2012). The large subunit region D1 and D2 (LSU) was also amplified using the primers LR0R (5′-ACCCGCTGAACTTAAGC-3′) and LR5 (5′-TCCTGAGGGAAACTTCG-3′) (Samson et al. 2010). The amplified products were sequenced and deposited in GenBank (accession nos. MK063806 and MK063807, respectively). BLASTn analysis revealed a 99% identity with M. piriformis (KT780842 and MH866514, respectively). Koch’s postulates were conducted in the laboratory by the spraying with sporangiospores of artificially wounded (removing skin 3 mm in diameter by using a flamed needle) fruits of cultivar Hayward that were first surface sterilized in 0.1% chlorine. There were 10 inoculated and 10 control fruits (similarly sprayed with sterilized water) in a randomized design. Fruits were maintained at room temperature (20 ± 2°C) for 2 days. Lesion development was recorded daily for each fruit. Koch’s postulates were fulfilled after reisolating the fungus from inoculated fruit that developed symptoms similar to those observed on fruits collected from packinghouse storage. The control fruits remained symptomless. This is the first report of the occurrence of M. piriformis as causal agent of postharvest fruit rot of kiwifruit in Greece. Previous works showed that this pathogen has been reported to cause postharvest fruit rots of kiwifruit in New Zealand (Manning et al. 2003).