Update:
Objective 1. Evaluate the effect of cover crops and green manures on SDS, Fusarium root rot, white mold and iron deficiency chlorosis in field conditions

We established seven field trials in May 2018: four SDS trials (Ames, Kanawha, and Roland, IA and Ontario, Canada), two white mold trials (Nashua and Kanawha, IA) and one IDC trials (Ames). The experimental design for all 2018 trials consisted of two cover crop treatments: i) spring oats planted as a companion crop with soybeans and terminated at V3-V4, and ii) no oats, applied to two soybean varieties: i) susceptible and ii) resistant to each disease. The trials include 6 replications of each treatment combination.
Late spring and erratic weather delayed all planting operations and created challenges with the establishment of cover crops in the 2018 season. Oats stands did not develop successfully in most trials and disease development was limited. Throughout the 2018 season we collected plant growth, disease and yield data. IN addition, root samples were collected in Ontario, Canada, and sent to ISU to test the levels of Fusarium virguliforme (Fv, the SDS pathogen) in roots.

Objective 2. Identify simple, rapid, and cost-effective bioassays to test efficacy of additional cover crop species on disease suppression in controlled environment conditions

We conducted greenhouse experiments to test the ability of different cover crop amendments (rye, oat, alfalfa, clover and corn) to suppress SDS and Fusarium root rot. In the first two runs of the experiment, there were severe infestations with fungal gnats (insects that feed on the fungus and damage soybean roots), and we could not collect reliable data. The experimental protocol was modified and a third experiment was conducted with Fv. Soil was infested with sorghum grain infested with Fv and amended with chopped leaves and roots of the different cover crops. The soil was allowed to incubate for two weeks, after which soybean seeds were planted. Soybean plants were assessed for root rot, root dry weight and foliar disease severity after four weeks. Data is currently being analyzed.

Objective 3. Identify cover crops with the most potential for inhibiting pathogen growth and sporulation in vitro and survival in soil.

We conducted lab studies on the effects of cover crop plant material on in-vitro growth and sporulation of Fv (SDS pathogen), and F. graminearum and Pythium sylvaticum (root rot pathogens). Culture media was amended with plant extracts from oats, rye, alfalfa, and clover, and soybean, corn. Media with no crop amendment was used as a control. Each of the pathogen were then cultures in the plates with the different amendments and colony growth and sporulation were quantified after two weeks. Preliminary data suggests that alfalfa stimulates growth of the two Fusarium species tested, while other crops, including oats and rye, did not stimulate growth compared to the control. Alfalfa also increased sporulation on F. virguliforme in culture. These results suggest that some cover crops species may favor pathogen growth while others do not affect the pathogens. Field and greenhouse studies will complement and validate these results.

The goal of this project was to investigate the potential use of cover crops and green manures as tools to manage important soil-associated diseases of soybean, namely: i) SDS, caused by F. virguliforme, ii) root rot caused by F. graminearum, iii) white mold, caused by S. sclerotiorum, and iv) IDC. We focused our efforts on field trials but also conducted greenhouse and lab studies to determine the effect of the green manures on disease and pathogen growth in more controlled conditions.

Objective 1. Evaluate the effect of cover crops and green manures on SDS, Fusarium root rot, white mold and iron deficiency chlorosis in field conditions

We established seven field trials in May 2018: four SDS trials (Ames, Kanawha, and Roland, IA and Ontario, Canada), two white mold trials (Nashua and Kanawha, IA) and one IDC trials (Ames). The experimental design for all 2018 trials consisted of two cover crop treatments: i) spring oats planted as a companion crop with soybeans and terminated at V3-V4, and ii) no oats, applied to two soybean varieties: i) susceptible and ii) resistant to each disease.

Late spring and erratic weather delayed all planting operations and created challenges with the establishment of cover crops in the 2018 season. Oats stands did not develop successfully in most trials and disease development was limited. We measured plant growth, disease and yield data and found no differences between treatments.

Objective 2. Identify simple, rapid, and cost-effective bioassays to test efficacy of additional cover crop species on disease suppression in controlled environment conditions

Work focused on greenhouse experiments to test the ability of different cover crop amendments (rye, oat, alfalfa, clover and corn) to suppress SDS. We developed and optimized protocols to conduct these experiments. Soil was infested with sorghum grain infested with Fv and amended with chopped leaves and roots of the different cover crops. The soil was allowed to incubate for two weeks, after which soybean seeds were planted. Soybean plants were assessed for root rot, root dry weight and foliar disease severity after four weeks. We found that green manure amendments reduced the severity of SDS compared to plants grown in soil without green manures. The experiments have to be repeated to check if these effects are repeatable.

Objective 3. Identify cover crops with the most potential for inhibiting pathogen growth and sporulation in vitro and survival in soil.

We conducted lab studies on the effects of cover crop plant material on growth and sporulation of F. virguliforme (the SDS pathogen), and F. graminearum and Pythium sylvaticum (root rot pathogens). We grew the pathogens in culture media mixed with plant extracts from oats, rye, alfalfa, and clover, and soybean, corn, and compared growth and spore production with cultures without plant extracts. Preliminary data suggests that alfalfa stimulates growth of the two Fusarium species tested, while other crops, including oats and rye, did not stimulate growth. Alfalfa also increased sporulation on F. virguliforme in culture. These results suggest that some cover crops species may favor pathogen growth while others do not affect the pathogens. Field and greenhouse studies will complement and validate these results.