First report of blue mold caused by Penicillium oxalicum in sweetpotato (Ipomoea batatas)in Korea.
- Plant Disease, 102(6), p.1176-1176, 2018 .
Many fungal diseases can affect sweetpotato (Ipomoea batatas [L.]Lam), a dicotyledonous plant in the Convolvulaceae family, at various stages from cultivation through storage (Clark et al. 2013). We observed blue mold disease on sweetpotato storage roots in the store house at the Bioenergy Crop Research Institute, Muan, South Korea, in 2016. The storage temperature and relative humidity were 13°C and 90 per cent, respectively. Infected storage roots showed distinct rot symptoms, and bluish green sporulation was abundantly visible on the surface. Notably, disease was observed on two different local sweetpotato varieties, namely, Yulmi and Jinhongmi. Storage root samples showing rot symptoms were collected and used to isolate the causal organism. Fungi were isolated by the single spore isolation method onto potato dextrose agar (PDA). Repeated culturing of hyphal growth from the infected roots resulted in the isolation of two fungal isolates (SPL15027 and SPL15029). These were examined for morphological and molecular characteristics. Fungal growth on malt extract agar (MEA)at 25°C after 7 days appeared as a dull green center with pale green to white margins and lacked visible secreted pigmentation. The colony diameter on MEA was 37.9 ± 0.19 mm. Conidiophores were 57.3 to 205.2 (140)?m long, 2 to 4 metulae, 11.0 to 22.0 (17.4)?m, and phialides 7.2 to 20.0 (12.5)?m long in diameter. Conidia were globose to subglobose and were borne in columns. Conidial dimensions were 3.5 to 5.5 (4.2)× 2.5 to 4.0 (2.9)?m (n = 25). Morphologically, the pathogen was identified as Penicillium oxalicum Currie Thom (Kozakiewicz 1992). Polymerase chain reaction (PCR)analysis was used for further molecular identification. Genomic DNA of the isolates was extracted using the Solg Genomic DNA Prep Kit (Solgent, Daejeon, South Korea), followed by PCR analysis using primers specific to the internal transcribed spacer (ITS5/ITS4)and ?-tubulin locus (BT2a/BT2b)genes (Jing et al. 2015). Both strands of the purified PCR products were sequenced. These sequences were deposited in GenBank (ITS-KY853419/KY853420 and BT-KY853421/KY853422). BLAST analysis of the ITS (LT797553, KX515788, and HM469410)and BT (KF225064, AB849501, and KU600425)sequences showed 100 per cent homology to P. oxalicum. This confirmed the identity of the isolated organism. Koch's postulates were fulfilled by using the isolated culture for sweetpotato storage roots inoculations. PDA plugs (5 mm)and 50 ?l of 2 × 104/ml conidial suspension were used to infect different sweetpotato varieties including Chuhanmi, Yulmi, Jinhongmi, Yonhwangmi, and Dahomi. PDA plugs from blank plates and sterilized distilled water were used as respective noninoculated controls. Fungal-inoculated storage roots developed typical blue mold symptoms after 10 days of incubation at 25°C and under high humidity. In contrast, no symptoms were observed on noninoculated control storage roots. P. oxalicum was reisolated from the artificially inoculated roots, completing Koch's postulates. To the best of our knowledge, this is the first report of P. oxalicum causing postharvest blue mold on sweetpotato in Korea.