Soybean cyst nematode (SCN) is the most devastating pathogen of soybean in the US, causing annual yield losses estimated at $1.5 billion. The frequently used soybean varieties with resistance from the breeding line PI 88788 have become less effective in managing this pest due to the widespread virulent SCN populations that are able to overcome host resistance. One increasingly deployed strategy to combat the rise in virulent SCN has been growing soybean seeds treated with biologicals according to two surveys conducted in 2015 and 2020 by the SCN Coalition. However, one issue with commercially available seed treatment products using bacterial biologicals is the lack of consistency in controlling SCN, mainly due to the failure of beneficial microbes to establish themselves under different soil conditions.
Our goal is to develop beneficial soil-dwelling fungi as new biologicals to effectively and consistently control SCN. Our approach is to identify isolates of different SCN-killing fungal species adapted to and established under various soil conditions in the US. In the first quarter of the project, we performed a preliminary screening of all of fungal species in our current collection and tested their antagonistic activities against SCN eggs. Soil samples from soybean fields were collected from different states, including Indiana, Iowa, Minnesota, North Dakota, Arkansas, North Carolina by working with nematologists, plant pathologist and soybean growers. The activities conducted followed the work plan in the approved proposal. We have identified 16 fungal species which are effective in killing SCN eggs or juveniles. In the second quarter of the project, we have worked on identification and purification of different isolates of SCN killing fungal species confirmed in the first quarter of the project. Until now, from the 36 soil samples collected from 7 soybean-growing states (see details in the table below), we identified different isolates of 9 fungal species. We are testing the capability of these fungal isolates in suppressing SCN egg and J2 viability. The activities conducted followed the work plan in the approved proposal. In the third and last quarter, we have tested the majority of the 13 isolates of 9 fungal species identified in the second quarter of the project for their individual efficacy of SCN suppression using a soil-plant assay. Until now, 5 fungal species showed being able to reduce SCN reproduction by 30-50% compared to the untreated control using the soil samples collected in Indiana. We have tested the efficacy of a mixture of three fungal species (Trichoderma virens, Purpureocillium lilacinum, Pochonia chlamydosporia) and found improved effectiveness in reducing SCN infection and reproduction on soybean plants (more than 60%). We also tested the mixture with the three fungal species in different soils and found that consistent and efficient efficacies were positively associated with the levels of organic matter contents in the soils. Therefore, increasing soil organic contents in the fields may help to improve the consistency and efficacy of fungal bio-control agents to suppress SCN.