By Leonor Leandro, professor of plant pathology, entomology and microbiology, ISU

Final Report (December 2023) 

Project summary

In recent studies, my research group has found that time of SDS foliar symptom onset, i.e. the time when the first SDS foliar symptoms are observed, is strongly correlated with late season SDS severity and soybean yield. These results were obtained from single soybean genotypes planted in commercial and research fields, but suggest that management practices that delay disease onset should be most effective at protecting soybean yield. I am interested in testing if time of SDS onset can be a useful measure for early screening of soybean genotypes for resistance against SDS. I would monitor SDS progress over time in soybean genotypes with varying levels of resistance and determine the correlation between resistance and time of foliar disease onset. This work could include assessments of root rot progress in plants grown in infested soil or could focus on soybean resistance to F. virguliforme toxins if plants are grown fungal culture filtrates.

Selected for funding October 2020

Summary of project expansion

Leandro's research group had previously found that the time when the first SDS foliar symptoms are observed is strongly correlated with late season SDS severity and soybean yield. Although these studies were based on single soybean genotypes planted in commercial and research fields, they suggest that practices that delay disease onset should be effective at protecting soybean yield. The results also lead to the hypothesis that time of SDS onset may be a more reliable measure of soybean resistance to SDS than late season evaluations of disease. If SDS onset is found to be well correlated with SDS resistance, this could greatly benefit breeding programs by improving the reliability of screening methods currently used to identify the most resistant genotypes.

Leandro had obtained funding (for two years in 2020) from the Iowa Soybean Research Center to conduct field, greenhouse and lab studies comparing time of SDS onset on a set of soybean genotypes with different levels of resistance to SDS. Field experiments were established in two locations in Central Iowa in 2021. In each experiment, nine soybean varieties were planted in replicated plots. The varieties differed in SDS resistance ranking from susceptible to resistant. At one experiment (ISU Hinds Farm), the soil was artificially infested with Fusarium virguliforme, the SDS pathogen, and the plots were irrigated weekly through the 2021 growing season. The other experiment is in a farmer field that has a history of SDS and is not irrigated. The plots have been monitored regularly for SDS symptoms.

Leandro's team developed protocols and conducted three runs of a greenhouse experiment comparing SDS onset using a set of soybean genotypes with different levels of SDS resistance. In the first two runs, a collection of standard soybean genotypes that are often used as checks in SDS research were used. However, some of these checks did not express SDS disease as expected. Therefore, in the third experiment we used the same nine soybean genotypes that were planted in the field. Assessments included time of SDS onset, disease progress over time, and root health at the end of the experiment. This experiment was completed and Leandro is setting up the repetition of this experiment. Also, Leandro's team conducted a preliminary experiment using an in-vitro assay with pathogen culture filtrates. In 2021, they did not see SDS symptoms develop in either field experiment. Due to the drought conditions, it was likely that SDS would not occur or be present with low incidence this season, as SDS infection and disease development are highly dependent on high soil moisture. Leandro thought her team may not be able to obtain useful field data from the two field experiments conducted in 2021.

This project proposal requested additional funds to add more field locations in year 2 (2022) and to expand ongoing work into a third year of field research in 2023. This expansion will enhance the team's ability to collect enough reliable data to test their hypothesis. Because time of onset assessments require frequent disease data collections, the additional funding will also allow for the dedication of more labor hours to this project, cover costs of transportation to field sites across the state and increase efficiency with the greenhouse and lab experiments.

Selected for additional funding October 2021