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First Report of Soybean Sudden Death Syndrome Caused by Fusarium virguliforme in South Africa

Soybean (Glycine max (L.) Merr.) is an important crop in many countries and production is currently increasing (from 311,450 ha in 2010 to 516,500 ha in 2013) in South Africa. On 27 February 2013 in the Lydenburg/Badfontein area, Mpumalanga Province, on a no-till commercial farm planted to soybean cultivar PAN 737 (Roundup Ready, maturity group 7) under irrigation for a second consecutive season, leaf symptoms typical of soybean sudden death syndrome were observed and reported by a farmer (3). The symptoms developed at the R6 growth stage (near physiological maturity) of the soybean plants. Leaf symptoms were interveinal chlorotic blotches that became necrotic while the veins remained green. These symptoms appeared throughout the plant but were most severe on the top leaves. Some of the severely affected leaflets dropped off with the petioles remaining attached to the plant. The vascular tissue in the upper taproot and lower stem turned gray-brown, but the pith remained white. Roots of the affected plants had decayed lateral roots. Surface disinfested root pieces with rot symptoms and spores directly from blue sporodochia on the rotten root were plated on potato dextrose agar amended with novostreptomycin 0.04 g/L (PDA⁺). Slow growing Fusarium isolates with blue to purple masses of sporodochia were consistently obtained from diseased plants. Cultures were single-spored and plated on PDA⁺. Growth rate of cultures on PDA⁺ was on average 6 to 9 mm after 5 days at 20°C. The morphology of the isolates fit the description of Fusarium virguliforme in Aoki et al. (1). Sequence analyses of the nuclear ribosomal internal transcribed spacer (ITS) and partial translation elongation factor (EF-1a) gene of the recovered eight isolates revealed that these isolates matched 99.6% with F. virguliforme O'Donnell & T. Aoki (Accession Nos. KF648835 to KF648850), one of the soybean sudden death syndrome causing species found in North and South America (1). All isolates are identical in each loci except that three isolates had one nucleotide deletion and two insertions at the EF-1a loci. The isolates are deposited at the national culture collection in Pretoria (PPRI13434 to PPRI13441). A glasshouse bioassay was conducted to test the pathogenicity of eight single-spored isolates by inoculating pasteurized planting medium (1:1:1 ratio of sand, perlite, and soil) with a layer of infested sand-bran medium (2) to each pot (13 cm in diameter) and covered with 2 cm of planting medium (4) after planting 20 seeds of soybean cultivar PAN 737. There were three pots per isolate randomized in a complete block design trial. All the South African F. virguliforme isolates tested induced leaf and root rot symptoms of sudden death syndrome on the soybean seedlings under glasshouse conditions after 4 weeks of inoculation. The fungus was re-isolated on PDA⁺ from diseased roots of the soybean seedlings to fulfill Koch's postulates. This is the first record of F. virguliforme in South Africa, and as an important component of soilborne diseases of soybean it may pose a major threat to the South African soybean industry.

References: (1) T. Aoki et al. Mycoscience 46:162, 2005. (2) S. C. Lamprecht et al. Plant Dis. 95:1153, 2011. (3) J. C. Rupe and G. L. Hartman. Compendium of Soybean Diseases, 4th ed. G. L. Hartman et al., eds. American Phytopathological Society, St. Paul, MN, 1999. (4) M. M. Scandiani et al. Trop. Plant Pathol. 36:133, 2011.