First Report of Post-Harvest Gray Mold on Hyacinth Bean (Lablab purpureus) Caused by Botrytis cinerea in Korea.

Abstract

Hyacinth bean (Lablab purpureus L.) is a highly proteineous legume under the family Fabaceae. It is native to Africa, cultivated throughout the world, and recently introduced vegetable in Korea. In April 2020, approximately 10 to 15% of the total harvested pods showed gray mold rot symptoms after 3-5 days of storage at 4 °C in Jeonju, Jeonbuk province, Korea. The symptoms observed were irregular, water-soaked spots become brown or gray with white hyphae were appeared on the infected pods. Diseased tissue was excised, and surface sterilized by immersing in 1% sodium hypochlorite (NaOCl) for 1 min, rinsed three times with sterilized distilled water, placed on potato dextrose agar (PDA) plates, and incubated at 20 ± 2°C for 7 days. A total of five morphologically similar fungal isolates (HBGM001 to HBGM005) were obtained from diseased samples; isolate HBGM002 and HBGM005 were selected for identification. The fungus produced initially white colonies, after 7 days it changes to gray to dark colonies with dark mycelium that sporulated abundantly on PDA at 20ºC. The conidia (n = 50) were single-celled, ellipsoid or ovoid in shape, and 6.11 to 13.9 × 4.8 to 9.4 μm in size for HBGM001 isolate and 5.81 to 14.1× 4.5 to 9.6 μm in size for HBGM005. Conidiophores (n = 15) arose solitary or in groups, straight or flexuous, septate, with an inflated basal cell brown to light brown, and measured 103 to 420× 7 to 25 μm for HBGM001 isolate and 101 to 415 × 5 to 23 μm for HBGM005 isolate. After two weeks, the fungus formed several black sclerotia (n = 20) ranging from 0.5 to 4.2 × 0.5 to 3.4 mm for HBGM001 isolate and 0.4 to 4.4 × 0.3 to 3.3 mm for HBGM005 isolate near the edge of the Petri dish. Morphological characters were consistent with those of Botrytis cinerea Pers.: Fr. (Ellis 1971). As for molecular identification, the internal transcribed spacer (ITS) and three nuclear protein-coding genes (glyceraldehydes-3-phosphate dehydrogenase gene [G3PDH], heat-shock protein 60 gene [HSP60], and DNA-dependent RNA polymerase subunit gene [RPB2]) were amplified using primer pairs ITS1/ITS4 (White et al. 1990), G3PDH-F/G3PDH-R, HSP60-F/HSP60-R, and RPB2-F/RPB2-R (Staats et al. 2005), respectively. The ITS, G3PDH, HSP60, and RPB2 sequences of HBGM002 and HBGM005 isolates (GenBank accession number MT439648 and MT968495 for ITS; MT439649 and MT968496 for G3PDH; MT439650 and MT968497 for HSP60; MT439651 and MT968498 for RPB2 respectively) were 99% to 100% identical to those of B. cinerea (KY364366, KF015583, KJ018758, and KJ018756, respectively). To determine pathogenicity, five disinfected pods were pinpricked (3 sites per pod) with sterile needles and 50 µl of conidial suspension (1 × 105 conidia/ml) was inoculated by pipetting into the wounds. An analogous five pods, serving as controls, were inoculated with sterile distilled water. All the pods were placed in a growth chamber and maintained a temperature of 20±2ºC and a relative humidity >80%. After 5 days, gray mold symptoms developed on the inoculated pods, whereas no symptoms appeared on control pods. The pathogen was re-isolated from the inoculated pods, fulfilling Koch's postulates. B. cinerea has been reported causing gray mold in Hyacinth bean in China, Taiwan and India (Farr and Rossman 2021). To our knowledge, this is the first report of B. cinerea causing post-harvest gray mold on hyacinth bean in Korea. The disease could represent a threat for hyacinth bean post-harvest and storage and management strategies should be investigated and applied.