During 2007 to 2009, symptomatic and asymptomatic soybean plants were collected from fields in 18 Iowa counties. Fusarium isolates were recovered from surface-sterilized root tissue on peptone PCNB agar (2). Single-spore isolates were transferred to synthetic low nutrient agar (SNA) overlain with pieces (1 × 2 cm) of sterile filter paper, and to potato dextrose agar (PDA), and placed in the dark for 10 to 14 days for morphological identification (4). Twenty-three isolates were identified as Fusarium commune K. Skovg., O'Donnell & Nirenberg, previously in the F. oxysporum species complex (4). Colonies on PDA had white, fluffy, aerial mycelium with magenta to violet pigmentation in the medium. On SNA, macroconidia, chlamydospores, and microconidia on monophialides and polyphialides were consistent with the species description (4). Identification of all 23 isolates was confirmed by DNA sequencing of the translation elongation factor (EF1-α) gene, using ef1 and ef2 primers, and the mitochondrial small subunit (mtSSU), using primers MS1 and MS2 (4) [GenBank accessions for two representative isolates: EF1-α (JX289892 and JX289893), and mtSSU (JX289894 and, JX289895)]. Pathogenicity of two representative isolates of F. commune was tested on soybean (cv. AG2403) in a greenhouse, in water baths set at 18°C, using autoclaved soil mixed with infested sand-cornmeal inoculum (3). The experiment entailed a completely randomized design (CRD) with five replications (single plant/150 ml cone) per treatment, and was conducted three times. Dry root and shoot weights, and root rot severity (visual estimate of percent root rot on the entire root system) were evaluated after 6 weeks. Mean seedling emergence in soil infested with F. commune was 47 and 40% for the two isolates; in contrast, non-inoculated control plants had 100% emergence. There were significant differences in root (P < 0.0001) and shoot (P < 0.0001) weights, and root rot severity (P < 0.0001), between inoculated and non-inoculated plants. Seedlings that emerged were severely stunted and had dark brown lesions. F. commune was reisolated from infected roots of inoculated plants, but not from non-inoculated plants. Pathogenicity of both isolates to soybean (cv. MN1805) was also tested using a petri dish assay, in which eight seeds were placed on a plate with a 4-day-old culture growing on 2% water agar (1). Plates were rated 7 days later for seed germination, seed rot, and lesion development, using an ordinal scale (1). The experiment entailed a CRD with three replicate plates/treatment, and was conducted three times. Germination of inoculated seeds ranged from 37.5 to 75.0%, and germinated seedlings had dark brown lesions on the taproots. There was a significant difference between isolates in the petri dish assay (P = 0.0030); one isolate was less aggressive, but both isolates resulted in significantly more disease than on the non-inoculated control plants, which had 100% germination and no symptoms (P < 0.0001). F. oxysporum is a known soybean pathogen (1), but isolates of F. commune may have been misidentified as F. oxysporum in previous studies. To our knowledge, this is the first report of F. commune as a pathogen on soybean in the U.S.A.
References: (1) K. E. Broders et al. Plant Dis. 91:727, 2007. (2) J. F. Leslie and B. A. Summerell. The Fusarium Laboratory Manual. Blackwell Publishing, Oxford, UK, 2006. (3) G. P. Munkvold and J. K. O'Mara. Plant Dis. 86:143, 2002. (4) K. Skovgaard et al. Mycologia. 94:630, 2003.