Blue mold caused by Penicillium oxalicum on muskmelon (Cucumis melo) in Thailand
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Fruit rot infected with blue mold was found on muskmelon growing in a polytunnel located at Songkhla province southern Thailand in 2018. The blue mold fungus isolated from the infected fruit was identified as Penicillium oxalicum based on morphological characteristics and DNA sequences of the internal transcribed spacer (ITS) and the large subunit (LSU) regions of ribosomal DNA, and the calmodulin (CaM) gene region. Pathogenicity tests showed that P. oxalicum isolate was pathogenic to healthy muskmelon and Koch’s postulates were satisfied by re-isolating the pathogen. This is a first report of muskmelon fruit rot caused by P. oxalicum in Thailand.
KeywordsPenicillium oxalicum Fruit rot Muskmelon Thailand
Cucumis melo var. reticulatus, belonging to the Cucurbitaceae family, is commonly called muskmelon or cantaloupe. The shape is round, with light green skin and a light brown reticulated rind, and with green or orange pulp. Recently, muskmelon has become popular among the consumers and economically important in Thailand. Consumers prefer this fruit for its texture, sweetness, aroma, and it is a rich source of phytonutrients such as ascorbic acid, carotene, folic acid and potassium (Menon and Ramana Rao 2012). Fungal diseases of cucurbits that reduce yield and quality at various fruit growing stages include Fusarium wilt caused by Fusarium oxysporum f. sp. melonis (Oumouloud et al. 2013), downy mildew caused by Pseudoperonospora cubensis (Savory et al. 2011), powdery mildew caused by Golovinomyces cichoracearum and Podosphaera xanthii (Aguiar et al. 2012), and gummy stem blight caused by Stagonosporopsis cucurbitacearum (syn. Didymella bryoniae) (Wolukau et al. 2007; Nuangmek et al. 2018). Furthermore, yellow leaf disease of muskmelon caused by Tomato leaf curl virus was reported in Thailand (Samretwanich et al. 2000).
Colonies on PDA were circular or elliptical, with even margins, attaining a diameter of 4.0 cm within 7 days at room temperature. Colony surfaces were flat and velvety in texture. Colony centers were blue-green fading to white towards the colony margins (Fig. 1c, d). Conidiophores were monoverticillate, or biverticillate and asymmetrical. Phialides were cylindrical (Fig. 1e, f). Conidia were consistently elliptical and smooth, measuring 2.7–3.9 × 4.1–5.6 μm as observed with the light microscope (Fig. 1g). Based on these morphological features, the isolates from muskmelon were identified as Penicillium oxalicum, in the Pencillium section Lanata Divaricata. A culture of the fungal pathogen was deposited in the Culture Collection of Pest Management Department, Faculty of Natural Resources, Prince of Songkla University (PSU), Thailand, with accession number PSU-PM-CM01.
For pathogenicity tests, detached healthy muskmelon fruit (10-day-old) were surface sterilized by immersing the fruit in 70% ethanol for 3 min and then rinsing two times in sterile distilled water and drying with sterilized tissue paper. The flowering end of the fruit were sprayed with a spore suspension of P. oxalicum (1 × 106 conidia/ml) prepared from 10-day-old colonies of the isolates. The control treatment was sprayed with sterilized distilled water. Four replicate fruit were tested per treatment and the experiment was carried out two times. All fruit were placed in moist plastic box and then kept in a polytunnel (30–32 °C, 60% relative humidity) for 2 weeks. Water-soaked lesions appeared on fruit 7 days after inoculation, and these then rotted within 14 days. The fungal pathogen was re-isolated and its morphology matched P. oxalicum.
Plant disease caused by P. oxalicum has been reported on many host plants: for example, stem and fruit rot of cucumber (Cucumber sativa) in Canada (Jarvis et al. 1990) and tomato stem and fruit rot in Mexico (Picos-Munoz et al. 2011). Blue mold fruit rot caused by P. oxalicum on melon has been reported in Korea (Kwon et al. 2002) but the variety was different (C. melo cv. gayabaegja). There have been no reports of P. oxalicum causing blue mold fruit rot on muskmelon in Thailand or anywhere else, so this is the first such report. Consequently, further research is required to find efficacious methods to manage the disease.
The authors would like to thank the Prince of Songkla University for funding and facilities, and Mr. Diregrit Plyduang for providing a photograph of blue mold rot on maturing muskmelon fruit. The copy-editing service of RDO/PSU and the helpful comments of Assoc. Prof. Dr. Seppo Karrila are gratefully acknowledged.
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