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SESSION 3: PATHOGENESIS – EPIDEMIOLOGY AND POPULATION
GENETICS
Characterization of the Fusarium root rot complex in
soybean
G. P. Munkvold, M. M. Diaz Arias, M. L. Ellis, D. R. Cruz Jimenez, L. F.
Leandro
Iowa State University, Dept. of Plant Pathology and Microbiology, Ames, IA 50011 USA
E-mail: munkvold@iastate.edu
Fusarium root rot is a widespread disease of soybean in the United States and
elsewhere in the world. Affecting seedlings as well as adult plants, it can be
caused by numerous Fusarium species, and its severity is highly variable. A multistate
effort is underway in order to better understand the diversity of Fusarium
species in the complex, their interactions with other pathogens, and management
of the disease through genetic resistance or seed treatment. Twelve pathogenic
Fusarium species were recovered from soybean roots during a 3-year survey in
Iowa. Fusarium oxysporum was the most common species, followed by F. solani,
F. graminearum, and F. acuminatum. Representative isolates of these species
caused seedling blight, root rot symptoms and detrimental effects on root system
growth and development. F. graminearum isolates were consistently aggressive
pathogens on soybean roots. Several species were identified (F. armeniacum, F.
commune, F. proliferatum) which had not previously been known to cause
soybean root rot. Interactions between Fusarium isolates and soybean cyst
nematode (SCN) were assessed in the field and greenhouse. Root feeding by
SCN enhanced the root rot symptoms caused by some, but not all, Fusarium
isolates. Isolates in the F. oxysporum complex are genotypically and
phenotypically diverse, including some that cause severe seedling blight or root
rot, and others that are nonpathogenic. These isolates segregated into 4 clades
according to a phylogenetic analysis using the TEF and MtSSU genes. Clade 2
contained many weakly pathogenic or nonpathogenic isolates. There were
significant cultivar x isolate interactions for seedling disease caused by F.
oxysporum, though no highly resistant cultivars are known. Work is continuing to
characterize the influences of environmental conditions on the disease symptoms
caused by a range of Fusarium isolates, and the sensitivity of pathogenic isolates
to seed treatment fungicides.
Keywords: genetic diversity, pathogenicity, soybean
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