Understanding soil suppressiveness towards the sudden death syndrome / soybean cyst nematode disease complex

Objectives:

• Determine the biological component(s) that lead to soil being suppressive against sudden death syndrome (SDS) establishment and soybean cyst nematode (SCN) proliferation.
• Develop an understanding of timing required for initiation and establishment of suppressive soils.
• Survey fields with SDS/SCN history and determine the level of inhibition within these fields.
• Downstream potential for discovery of biocontrol agents and cultural practices for suppression of SDS and SCN.

Impacts:

• Understanding the impacts of the SDS/SCN suppressive soil microbiome can lead to environmentally and economically sustainable methods of maintaining high yielding soybeans in at-risk fields through beneficial agronomic practices.
• Potential development of biocontrol agents will allow soybean producers to help control SDS and SCN in a prophylactic manner. There are currently very few options for control of these diseases in terms of biological or chemical treatment and those that are available have relatively limited potential in areas of high disease pressure.

Scientific Summary:

Sudden death syndrome (SDS) and soybean cyst nematode (SCN) continue to pose a persistent threat to soybean crops in southern Ontario, which continues to intensify annually. A long-standing disease nursery, maintained for over 15 years, has been a focal point for research due to its history of severe SDS and SCN symptoms. Despite historically favorable conditions for disease development, recent observations indicated a marked decline in SDS symptoms. Molecular and microscopy analysis of soil samples has confirmed the persistently high levels of SDS (Fusarium virguliforme) and SCN in the soil. However, the absence of visible SDS symptoms post-inoculation from indoor bioassays suggests that biotic factors are contributing to the observed disease suppression. This hypothesis was reinforced by the development of severe SDS symptoms in plants grown in sterilized artificially inoculated soil from the same field, underscoring the potential role of native soil biota in mitigating this disease. Conversely, fields experiencing newer SDS outbreaks, despite similar management practices, exhibit limited biotic suppression. This contrast highlights the unique characteristics of long-term soybean monoculture inducing suppressiveness in soils.

To investigate the impact of rotation versus monoculture on SDS development, split-plot trials were established at two field sites that differed in their long-term cropping history and disease suppression under controlled experiments. These sites were classified as suppressive (Essex) and conducive (Chatham). Trials were conducted from 2018 to 2023, with sections of plots separated into continuous soybeans, flanked by sections of corn/soybean (or soybean/corn) rotational plots. From this six-year rotational study, we were able to initiate suppressiveness at the conducive site. In susceptible varieties, foliar symptoms were reduced by 45%, and yield increased by nearly 50%. Similar trends, though less pronounced, were observed with the more tolerant varieties. In contrast, at the suppressive Essex site, resilience of the suppressiveness was not overly durable; an increase in SDS symptoms and a decrease in yield were seen after the second rotation into corn in 2021. These findings indicate that while suppression can be induced, it is not overly robust. Indoor bioassays are ongoing to further validate these observations under controlled conditions.

Fungal community analyses revealed that the introduction of rotation at the Essex site and continuous monoculture at the conducive Chatham site led to distinct shifts in fungal populations. A high abundance of Purpureocillium lilacinum and Orbiliales spp. was observed at the suppressive site, and their increasing abundance at the conducive site following five years of monoculture suggests a potential role in suppressing SDS. Preliminary assessments of these organisms as biocontrol agents are promising, exhibiting growth inhibition of F. virguliforme in culture. Analysis of SCN associated microbiome colonization revealed an increase of a number of known nematophagous fungi and bacteria. Soil bacterial microbiome sample analysis is ongoing.

This study highlights the potential for inducing suppressiveness in areas with severe SCN and SDS pressure as an alternative management strategy. While continuous soybean monoculture is not recommended, these initial fundamental findings highlight the potential for managing the microbial communities to mitigate damage caused by SDS and SCN.

Key findings for soybean producers:

1) Continuous soybean monoculture showed potential in heavily diseased fields. At our conducive Chatham site, we observed reduced SDS symptoms and improved yields after four to five years of continuous soybeans. Yield penalties for continuous soybeans were not observed at our sites and yields increased at the conducive site through suppression of disease. Historically a ~10% yield penalty has been seen in soybeans without rotation.

2. Rotating out of soybeans on a two-year rotation with corn reduces established disease suppression. In our known suppressive field in Essex, SDS symptoms increased after just one or two cycles of corn–soybean rotation, suggesting that suppressiveness is not durable. This likely also occurs with other rotational crops, though it has not been tested.

3. Soil microbial communities have been correlated to disease outcomes. Fungal groups such as Purpureocillium and Orbiliales were correlated with lower SDS pressure and increased yield. There may be future opportunities to assess soil microbial profiles to estimate SDS and SCN risk levels. Quantitative PCR assays were developed for Purpureocillium showing increased concentration in the soil in the suppressive areas, though this is unlikely to be the sole driver of the phenomena and additional genera would need to be included to have a comprehensive multiplex “suppressive” test.

4. Further developments are expected. This work may provide a foundation for novel tools and management strategies for supporting disease-suppressive soils and improving long-term soybean sustainability.

External Funding Partners:

This research was funded in part by Agriculture and Agri-Food Canada through the Sustainable Canadian Agricultural Partnership.

Project Related Publications:

None.