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Pathogen Biology

SDS is caused by soil-borne fungi within a group (clade 2) of the Fusarium solani species complex (Aoki et al., 2003; Aoki et al., 2005). In North America, Fusarium virguliforme (Aoki et al., 2003), formerly Fusarium solani f. sp. glycines (Roy, 1997), is the causal agent. In South America, F. brasiliense, F. cuneirostrum, F. tucumaniae, and F. virguliforme cause SDS symptoms. In greenhouse inoculation experiments, F. cuneirostrum and F. phaseoli, causal agents of dry bean root rot, are also able to cause SDS symptoms on soybean (Aoki et al., 2005). Fungi of this taxonomic complex grow slowly in culture and are difficult to isolate from diseased plants. Once a pure culture is obtained, blue spores and other cultural characteristics distinguish the SDS pathogens from other Fusarium species that can infect soybean roots. In North America, the SDS-pathogen is considered clonal and has been considered asexual, but recent crossing experiments revealed the existence of a teleomorph (Covert et al. 2007). One of the South American SDS pathogens, F. tucumaniae, formed fertile perithecia (sexual reproductive structures) in specific crosses under laboratory conditions. Based on the Vienna Code of the International Code of Botanical Nomenclature (Article 59.7) the anamorph name has been retained for this fungus (Covert et al., 2007). Fusarium virguliforme, the North America SDS-pathogen, did not form fertile crosses with itself or with F. tucumaniae; only infertile sexual structures were produced in some of the latter crosses (Covert et al., 2007). These findings support the hypothesis that the North America SDS pathogen is clonal and that distinct species cause SDS in North and South America, respectively.

Presumably, the SDS pathogen survives between soybean crops as chlamydospores in crop residue or freely in the soil (Figure 3a). The thick-walled chlamydospores develop in the soil and on soybean roots during disease development and thereafter. Chlamydospores can withstand wide fluctuations in soil temperature, including freezing, and resist desiccation. As soil warms in the spring, chlamydospores near soybean roots are stimulated to germinate, and then infect soybean roots.

Although the pathogen may produce spores (macroconidia, Figure 3b) on the surface of the taproot during the summer, these spores spread only short distances within a growing season. Over a period of years, flowing water and cultivation practices that move soil can move spores over longer distances within or between fields.

Figure 3a

Figure 3b

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