Fusarium species infecting soybean roots: Frequency, aggressiveness, yield impact and interaction with the soybean cyst nematode

Diaz Arias, Maria
Major Professor
Gary P Munkvold
Leonor F Leandro
Committee Member
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Plant Pathology and Microbiology

Fusarium root rot is a widespread and common disease in soybean. Several Fusarium species have been reported to infect soybean roots but their occurrence, aggressiveness and impact on yield have not been quantified. To gain a better understanding of the importance of root-infecting Fusarium species in soybean productivity in Iowa, a three year root survey, and greenhouse and field experiments were conducted in order to accomplish the following objectives, 1) characterize the frequency of Fusarium species associated with soybean roots from Iowa soybean fields, 2) determine aggressiveness of predominant Fusarium species towards soybean, 3) estimate the impact of Fusarium species on growth and yield of soybean plants and 4) determine whether there is an interaction between SCN infestation and Fusarium root rot species in soybean roots. Using morphological and molecular techniques, 15 Fusarium species were identified in association with soybean root. Fusarium oxysporum, F. solani, F. acuminatum, and F. graminearum were most frequently isolated and most prevalent at the county and field level. Aggressiveness of isolates representing the most frequent species was tested under greenhouse and field conditions. In the greenhouse, aggressiveness differed between species and among isolates at V3 soybean growth stage; F. graminearum caused the most severe root rot and detrimental effects on root system morphology, followed by F. virguliforme and F. proliferatum. Significant variation in aggressiveness was observed among F. oxysporum isolates, some of which caused severe damping off. In the field, low root rot severity was observed. Mean yield was not significantly reduced but significant linear relationships were found for some isolates between yield and root health measures. To determine whether SCN infestation enhances root rot disease in soybean, greenhouse and field trials were conducted using cultivars that differ in genetic resistance to SCN. Under greenhouse conditions, seedlings of SCN-susceptible and SCN-resistant cultivars were grown in soil infested with Fusarium alone and in combination with SCN. Two isolates from each of 8 Fusarium species were tested. There were significant interactions between Fusarium isolates and SCN, with co-inoculation with both pathogens consistently causing more detrimental effects on root development than either pathogen alone. This effect appeared to be additive except for three isolates (FG2, FS2, and FSP1) with evidence for synergistic effects. In the field, Fusarium root rot severity was correlated with SCN population density, but SCN-resistant and SCN-susceptible cultivars did not consistently differ in root rot severity, suggesting that the use of SCN resistance is not likely to reduce Fusarium root rot severity. Overall, this study has identified the most prevalent and most aggressive Fusarium species causing root rot in Iowa, and shown that some species are capable of causing soybean yield losses. Our results also indicate that SCN population densities, soil pH and Fusarium root rot severity are related and they act together to affect soybean yield, although the mechanism of these interactions is unknown. These findings will guide future research on Fusarium root rot, including studies of the genetic diversity within species, epidemiological and ecological features of the disease, and host-pathogen interactions, and ultimately help to development management practices for the disease.