Diaporthe phaseolorum var. caulivora, a Causal Agent for Both Stem Canker and Seed Decay on Soybean

Abstract

Plant Genomics and Breeding Institute, Seoul National University, Seoul 151-921, Korea (Received on October 17, 2011; Revised on November 29, 2011; Accepted on November 30, 2011)Northern stem canker caused by Diaporthe phaseolorumvar. caulivora (Dpc) has become a serious disease insoybean. The objectives of this study were to survey theexistence of Dpc on soybean in Korea, and to examinethe potential pathogenicity of Dpc in seed decay. Onesuch isolate, SSLP-4, isolated from a field-grown plantof the Korean soybean cultivar Danbaekkong, wasidentified as Dpc, based on its morphological and mole-cular characteristics by sequences of internal transcribedspacer (ITS), translation elongation factor (TEF) 1-αand β-tubulin regions, as well as pathogenic analyses.Moreover, morphological and molecular analyses re-vealed that isolate SSLP-4 was nearly identical to Dpcstrains from the United States. Pathogenicity tests onhypocotyls of soybean seedlings and detached leavesresulted in typical symptoms of soybean northern stemcanker and inoculation on plants at R5-R7 stage causedseed decay. All results suggest that the Dpc strain SSLP-4 can cause both stem canker and seed decay on soy-bean. Thus, the SSLP-4 isolate has the potential to con-tribute greatly to understanding of host plant resistancemechanisms, both at vegetative and reproductive growthstages in soybean. Keywords :Diaporthe phaseolorum var. caulivora , Diaporthe/Phomopsis complex, Phomopsis seed decay, soybeannorthern stem cankerSoybean stem canker caused by Diaporthe phaseolorumvar. caulivora (Dpc) was first found in Iowa in the 1940s(Welch and Gilman, 1948). Welch and Gilman (1948)initially identified this pathogen as D. phaseolorum (Cooke& Ellis) Sacc. var. batatatis (Harte & Field) Wehm. Until1954, this pathogen was considered a new pathogen andwas named D. phaseolorum (Cooke & Ellis) Sacc. var.caulivora (Athow & Caldwell), following with adding thedisease name forwarded with soybean stem canker (Athowand Caldwell, 1954). In the 1950s, stem canker became adestructive disease on soybean ( Glycine max (L.) Merril) inthe north-central regions of United States and Canada(Crall, 1956; Hildebrand, 1956). This epidemic was finallybrought under control by planting new resistant varieties(Athow and Caldwell, 1954). However, in 1973, soybeanstem canker reappeared in Mississippi, USA. A few yearslater, its occurrence was reported throughout the soybean-producing areas of the southern United States (Backman etal., 1985). Based on different cultural characteristics andsymptom expression of two isolates from Iowa (northern)and Mississippi (southern), soybean stem canker wasdivided into two types: Dpc causing northern stem cankerand D. phaseolorum var. meridionalis (Dpm) causingsouthern stem canker (Smith and Backman, 1988). Dpc andDpm belong to Diaporthe/Phomopsis complex which is amajor contributor to poor seed quality and causes greaterlosses than any other single fungal pathogen in soybean(Hartman et al., 1999). Dpc has been reported in many soybean-producingcountries with high adaptability in a large number of hosts.In recent years, Dpc was also observed on soybean inArgentina and Brazil (Costamilanet al., 2008; Pioli et al.,2003). However, a limited number of studies have beenreported on Dpc of soybean in Korea. The objectives of thisstudy were to identify Korean Dpc strain and examine thepotential pathogenicity of Korean Dpc strain in both stemcanker and seed decay. Symptoms of soybean northern stem canker were survey-ed in 2008 from three different locations where soybeanwas cropped in successive years: Suwon, Daegu, andMilyang in Korea. However, plants with early symptoms ofsoybean stem canker, reddish-brown and slightly sunkenstems, were observed only in Suwon on the soybean culti-var Danbaekkong at the Seoul National University Experi-mental Farm (Fig. 1A). Collected stem tissue were surface-sterilized and plated on potato dextrose agar (PDA, DifcoLaboratories, Seoul, Korea) acidified with lactic acid (pH