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First Report of Zonate Leaf Spot of Glycine max Caused by Cristulariella moricola in Korea

Soybean (Glycine max (L.) Merr.) is native to East Asia including Korea and is widely grown and consumed as an edible seed. In August 2011, following a prolonged period of cool and moist weather, zonate leaf spots were observed in local soybean (cultivar unknown) planted in a mountainous area of Goseong, central Korea. A voucher specimen was collected and entered at the Korea University herbarium (KUS-F26049). Initial symptoms included grayish green-to-grayish brown spots without border lines. As the lesions enlarged, they coalesced, leading to leaf blight and premature defoliation. Sporophores on the leaf lesions were dominantly hypophyllous, rarely epiphyllous, solitary, erect, easily detachable, and as long as 750 μm. The upper portion of the sporophores consisted of a pyramidal head that was ventricose, 275 to 500 μm long, and 80 to 160 μm wide. The fungus was isolated from leaf lesions and maintained on potato dextrose agar (PDA). Sclerotia were produced on PDA after 4 to 5 weeks at 18°C without light, but conidia were not observed in culture. The morphological and cultural characteristics were consistent with those of Cristulariella moricola (Hino) Redhead (2,3). An isolate was preserved in the Korean Agricultural Culture Collection (KACC46401). Genomic DNA was extracted with the DNeasy Plant Mini DNA Extraction Kit (Qiagen Inc., Valencia, CA). The complete internal transcribed spacer (ITS) region of rDNA was amplified with the primers ITS1/ITS4 and sequenced. The resulting sequence of 453 bp was deposited in GenBank (Accession No. JQ036182). A BLAST search in GenBank revealed that the sequence showed an exact match with that of C. moricola from Acer negundo (JQ036181) and >99% similarity with that of Grovesinia pyramidalis, teleomorph of C. moricola from Juglans sp. (Z81433). To determine the pathogenicity of the fungus, sporophores with the pyramidal head were carefully detached from a lesion on the naturally infected leaflet with fine needles. Each sporophore was transferred individually onto four places of six detached healthy soybean leaflets. The leaflets were placed in humid chambers at 100% relative humidity and incubated at 16 to 20°C (4). Symptoms were observed after 2 days on all inoculated leaflets (one to four lesions/leaflet). The lesions enlarged rapidly and reached ~20 mm diameter in a week. A number of sporulating structures and immature sclerotia were formed on the abaxial surface of the leaf 2 weeks after inoculation. The pathogen was reisolated from lesions on the inoculated leaflets, confirming Koch's postulates. No symptoms were observed on the control leaflets kept in humid chambers for 2 weeks. C. moricola was known to cause zonate leaf spots and defoliation on a wide range of woody and annual plants (1), but not on G. max. To our knowledge, this is the first report of Cristulariella infection in cultivated soybeans. Since the infections may be limited to the mountainous area with low night temperature and high humidity, economic losses seem to be negligible. However, the disease could be a potential threat to the safe production of soybeans in areas with prolonged periods of cool and moist weather.

References: (1) D. F. Farr and A. Y. Rossman. Fungal Databases. Systematic Mycology and Microbiology Laboratory, ARS, USDA. Retrieved from http://nt.ars-grin.gov/fungaldatabases/, January 7, 2012. (2) H. B. Lee and C. J. Kim. Plant Dis. 86:440, 2002. (3) S. A. Redhead. Can. J. Bot. 53:700, 1975. (4) H. J. Su and S. C. Leu. Plant Dis. 67:915, 1983.