First report of zonate leaf spot caused by Grovesinia moricola on adzuki bean in Korea

Adzuki bean [Vigna angularis (Willd.) Ohwi & H. Ohashi] is cultivated in more than 30 countries worldwide, and especially in East Asia. Seeds of adzuki bean are widely used as a food by at least a billion people (Xu et al. 2008). Following a prolonged period of cool and moist weather in September 2012, previously unknown symptoms of zonated leaf spot with severe defoliation were observed on a local variety of adzuki bean in Goseong, Korea (38°19′51″N; 128°28′01″E). Initial symptoms included grayish-green to brownish-gray and circular to irregular shaped leaf spots without borderlines. As the spots enlarged, they became creamy colored with characteristic target-shaped rings. Finally, individual spots coalesced to form larger lesions, leading to leaf blight and premature defoliation. Two specimens were deposited in the Korea University Herbarium (KUS-F26871 and KUS-F28950). Fruiting structures on the leaf lesions were dominantly hypophyllous, single, hyaline, erect, easily detachable, and as long as 720 µm. The upper portion of the sporophores consisted of a broadly oval to ellipsoidal head measuring 410 to 550 × 70 to 100 µm. The central stipe was hyaline and tapered to an acute or somewhat rounded apex. Globose cells were 6 to 25 µm in diameter and di- or trichotomously branched. The mycelial mats on potato dextrose agar plates, when incubated at 15°C for 3 days under 12 h light/dark conditions, were velvety white and then grayish. Sclerotia were formed on the surfaces of the colonies after 3 weeks, but conidia were not observed while in culture. These characteristics were consistent with those of Grovesinia moricola (I. Hino) Redhead [= Cristulariella moricola (I. Hino) Redhead] (Narumi-Saito et al. 2006). To confirm the identification, the internal transcribed spacer (ITS) regions were amplified with primers ITS1/ITS4 and sequenced. The obtained 452 bp sequence was deposited in GenBank (accession no. KX906373). A BLAST search revealed that the sequence showed 100% similarity with G. moricola (KC460209). To determine the pathogenicity of the fungus, sporophores with the pyramidal head were carefully detached from a lesion of a naturally infected leaflet using fine needles and transferred individually onto detached healthy leaflets which were placed in humid chambers (100% relative humidity) and incubated at 16 to 20°C. Disease symptoms developed 2 or 3 days after inoculation of the leaves. Sporophores and sclerotia were formed on the abaxial leaf surfaces 4 weeks after inoculation. No symptoms were observed on the control leaves. Several leguminous hosts including Arachis, Desmodium, Glycine, Lespedeza, Phaseolus, Pueraria, and Vigna have been associated with G. moricola (Farr and Rossman 2017). Although Lee and Kim (2002) reported Vigna vexillata var. tsusimensis infected with C. moricola, there has been no previous record from V. angularis. To our knowledge, this is the first report of C. moricola on V. angularis in Korea and anywhere in the world. This disease could be a potential threat to the production of adzuki bean, and the pathogen could also expand to other leguminous hosts under favorable conditions.