First Report of Persimmon (Diospyros kaki) Fruit Rot Caused by Mucor inaequisporus in South Korea

Abstract

Fruit rot symptoms were observed at the end of June 2018 and early August 2019 at two different persimmon orchards in South Korea (35°40′15.1″N, 128°35′51.8″E; 36°14′25.8″N, 128°03′53.1″E). The initial symptoms included dark to dark brown sepals. Severely diseased fruits were covered with light yellow mycelium and easily dropped from the tree. The estimated average disease incidence in the orchard was about 30% in both orchards. Two fungal strains (named as KNU-CD01 and KNU-GC01) were isolated from diseased fruits and sepals, collected from Cheongdo and Gimcheon in Korea. Both fungal strains, KNU-CD01 and KNU-GC01, grew rapidly on potato dextrose agar (PDA) and malt extract agar (MEA), filling the entire Petri dish (90 mm) after 3 days at 25°C in the dark. The strain KNU-CD01 after 5 days of culture on MEA was used for description of morphological characteristics. On PDA, the colonies were yellow, and the reverse was light yellow. On MEA, the colonies were light yellow with white spots, the reverse was yellow to dark yellow, and the margin was undulate. Sporangia were globose to subglobose, yellow or brown, echinulated, lightly depressed, and 54.0 to 120.9 × 51.6 to 112.4 μm (n = 30). Sporangiospores were extremely variable in size and shape, hyaline, globose to subglobose, 4.4 to 9.3 × 2.2 to 7.2 μm (n = 50), or sometimes larger, ellipsoid to globose 6.5 to 24.2 × 6.4 to 20.3 μm (n = 50), with granular contents. According to morphological structures, KNU-CD01 was similar to Mucor inaequisporus CBS 255.36 (Santiago et al. 2013). Total genomic DNA was extracted from single-spore cultures of KNU-CD01 and KNU-GC01. It was amplified using primers ITS1/ITS4 for the ITS region and NL1/NL4 for the LSU region (O’Donnell 1993; White et al. 1990). The obtained ITS region and partial 28S rRNA gene sequence (GenBank accession nos. LC469048, LC469049, MN539629, and MN539633) showed 100% similarity to M. inaequisporus JN206179 and JN206501, respectively (Walther et al. 2013). Both fungal strains, KNU-CD01 and KNU-GC01, were deposited in the Korean Collection for Type Cultures as KCTC 56404 and KCTC 56433, respectively. A pathogenicity test was conducted using the isolate KNU-CD01 and healthy persimmon fruits. A total of 30 healthy persimmon fruits were collected from the orchard, wiped with 70% ethanol, and dried. The top of the pedicel was cut using a sterilized blade for the inoculation, and then a sporangiospore suspension containing approximately 6.7 × 10⁵ sporangiospores/ml was prepared. Paper discs wetted with 20 μl of spore suspension (double-distilled water for the negative controls) were carefully placed on the top of the pedicels of 20 fruits, so that the spore suspension could flow into the fruit. All treated fruits were kept in a plastic box (>90% relative humidity) at 25°C. At 3 days after inoculation, all spore-inoculated fruits showed rotting symptoms and were covered with mycelium and sporangia (visible as small yellow dots), whereas the 10 negative control fruits had no specific symptoms. From fruits inoculated with the spore suspension, a fungal isolate was reisolated and identified as having the same cultural and morphological characteristics as KNU-CD01. Fruit rot caused by M. inaequisporus was recorded on Artocarpus glaucus, Flacourtia inermis, Musa paradisiaca, and so on (Boedjin 1958). Especially, fruit rot on unripe persimmon caused by M. inaequisporus was only reported in Japan so far (Naganishi and Hirahara 1966). This finding can improve our understanding of the ecology of M. inaequisporus and may help to reduce premature fruit drop and the consequent economic loss.