Leaf lesions and root rot symptoms typical of soybean sudden death syndrome (SDS) caused by Fusarium virguliforme O'Donnell & T. Aoki were observed in commercial soybeans (Glycine max (L.) Merr.) in southern Michigan. Leaf symptoms ranged from chlorotic spots to severe interveinal chlorosis and necrosis, no foliar pathogens were noted. In 2008, isolates were collected from Berrien and St. Joseph counties. In 2009, isolates were collected from Cass, St. Joseph, Van Buren, Allegan, and Monroe counties. Pieces of roots with root rot symptoms were washed prior to surface disinfestation with 70% ethanol for 30 s and 0.5% NaOCl for 1 min and incubated on water agar (WA) in petri plates amended with 50 μg/ml of chloramphenicol for the production of sporodochia. Alternatively, spores were collected directly from nondisinfested roots expressing blue sporodochia. Single-spore cultures were derived by streaking macroconidia with a bacterial loop onto 3% WA + chloramphenicol and incubated overnight. With a dissecting microscope, single germinated macroconidia were collected with a sterile 0.2-mm-diameter insect pin and transferred to potato dextrose agar (PDA). Cultures on PDA grew slowly and developed blue-to-purple masses of sporodochia typical of F. virguliforme descriptions and similar to a representative isolate, Mont-1, grown alongside (1,2). Size of macroconidia from the six representative isolates, one from each county (including isolates derived from surface-disinfested and nondisinfested roots), and Mont-1 were determined to be within the range for F. virguliforme (42 to 56 × 5 to 6 μm), with an average of four septa per macroconidia. Identity of the representative isolates was confirmed by partial DNA sequencing of both strands of the internal transcribed spacer (ITS) region of the ribosomal RNA gene, translation elongation factor 1-α, and β-tubulin loci. All six representative isolates were identical in each of the three loci and matched with 100% similarity F. virguliforme accessions in GenBank and Fusarium-ID database searches, except for the β-tubulin locus in which a single nucleotide insertion was noted (Accession Nos. HM453328--HM453330). Sequences were 98 to 99% similar to other SDS Fusarium spp. not yet recorded in the United States. Koch's postulates were performed in the greenhouse according to Malvick and Bussey (3). Infested sorghum seed (~20 g) was placed 2 cm below soybean seed of susceptible cv. Williams 82 in plastic pots. Noninfested sorghum seed was used as a negative control and sorghum infested with Mont-1 as a positive control. Chlorotic spots developed 2 weeks after establishing the trial, and 3 to 4 weeks postinoculation, severe SDS symptoms of foliar interveinal chlorosis and necrosis and severe root rot developed. Koch's postulates were completed by reisolating F. virguliforme from a subset of infected plants. In addition, an isolate of F. virguliforme collected in 2008 was used to inoculate a 2009 field trial in East Lansing, MI with no history of SDS. Typical SDS symptoms developed in the field trial and F. virguliforme was isolated from a symptomatic plant that was identified as described above. Despite being reported across the majority of soybean-producing states, to our knowledge, this is the first confirmation and distribution report for SDS in Michigan.
References: (1) T. Aoki et al. Mycoscience 46:162, 2005. (2) G. L. Hartman et al. Plant Dis. 81:515, 1997. (3) D. K. Malvick and K. E. Bussey. Can. J. Plant Pathol. 30:467, 2008.
