First Report of Diaporthe phaseolorum var. meridionalis on Soybean Seeds from Ghana

Abstract

Fifty-nine soybean samples (Glycine max) of various varieties, grown in various agro-ecological zones of Ghana, and harvested in 1995 and 1996, were collected during May through June 1996 and sent to Denmark for analysis. Seed samples were analyzed by the blotter method: seeds were placed on three layers of moistened blotter paper in petri dishes, 10 seeds per dish, 200 or 400 seeds per sample. The seeds were incubated for 7 days at 22 ± 2°C, exposed to alternating cycles of 12 h NUV-light and 12 h darkness. Subsequently, they were analyzed microscopically for fungal infection. Phomopsis spp. were detected in 68% of the samples, at infection levels ranging from 0.25 to 21.0% (mean 3.4%). One to five isolates of Phomopsis spp. were selected at random and cultured on potato dextrose agar acidified to pH 4.5 with 90% lactic acid (APDA) for identification to species. Isolates were cultured on APDA, with and without sterile soybean stem pieces, under either NUV-light/darkness or artificial daylight/darkness cycles. Of 103 isolates, 101 isolates produced only pycnidia and were identified as P. longicolla T. W. Hobbs based on the production of aggregated pycnidia with prominent beaks (2). Seventy-seven of these isolates produced only alpha conidia whereas 24 isolates produced both alpha and beta conidia, which differs from the description by Hobbs et al. (2). Two isolates from one seed sample produced perithecia but no pycnidia. Perithecia were evenly scattered in the colony, 200 to 350 × 250 to 400 μm, and had a beak of 900 to 1500 μm. Apical beak width was 60 to 110 μm, basal beak width 70 to 120 μm. Asci were 25.0 to 37.5 × 5.0 to 7.5 μm. Ascospores were two-celled, biguttulate in each cell, and 10.0 to 12.5 × 2.5 to 4.0 μm. Based on the description by Fernández and Hanlin (1), these isolates were identified as Diaporthe phaseolorum (Cooke & Ellis) Sacc. var. meridionalis Fernández (DPM), the causal agent of southern stem canker of soybean, which has not previously been reported from any African country. Ten 14-day-old soybean seedlings were inoculated with an isolate identified as DPM by the toothpick method (3) and two seedlings inoculated with sterile toothpicks served as controls. Local expanding lesions formed after 1 week on all plants at the point of inoculation with DPM; 2 weeks after inoculation, the pathogen was reisolated from all lesions by plating stem pieces, surface sterilized in 1% NaOCl, on APDA. In control plants no lesions were seen and no mycelial growth occurred from stem pieces plated on APDA. References: (1) F. A. Fernández and R. T. Hanlin. Mycologia 88:425, 1996. (2) T. W. Hobbs et al. Mycologia 77:535, 1985. (3) B. L. Keeling. Phytopathology 72:807, 1982.