Soil-borne disease issues continue to plague soybeans across Minnesota. Routinely, heavy soils and wet fields promote root disease development. Generally, an initial period of low disease pressure is expected when growing a newly introduced crop in a region. The objective of this research conducted in 2004 was to determine if specific production practices, such as rotation (number of times soybeans were grown from 1997-2004), or weed management system (Roundup Ready, conventional herbicides, and organic) contributed to disease development.

Materials and Methods: A survey was conducted during the 2004 growing season where 36 fields located in 11 NW MN counties from the Canadian border to Wilkin and Ottertail counties, were identified for specific cropping histories of interest. Producer records and observations provided crop production practice histories beginning in 1997 and extending through the survey year. In July, soil samples and a total of 25 soybean plants (growth stages: unifoliate to first trifoliate leaves) were collected. Collected plants were rated for leaf and root disease, and leaves stored for serology tests (soybean mosaic virus, alfalfa mosaic virus, and bean pod mottle virus). The percent leaf area killed from disease and root rot ratings (where 0=healthy, 1=discolored, 2=defined lesion, and 3=root rot) were assigned to each plant that was collected. Root tissue isolations were made to isolate those pathogens present. During August, soil was collected once again and analyzed for soybean cyst nematode. Plants in the R2-R5 growth stages were examined for leaf and root disease and leaves were stored for serology tests as described above.

Results and Discussion: Crop rotation use varied widely between fields. For example, soybean was grown eight of eight years in a single field while five fields had three years of continuous soybean. Planting two consecutive years of soybean was common. The cool, wet growing season during 2004 promoted diseases caused by pathogens with lower temperature requirements for infection and disease development.

Root diseases were widespread and often severe. Fusarium root rot (caused by Fusaria spp.) was most prevalent. Pythium root rot (caused by Pythium spp.), Phytophthora root rot (caused by Phytophthora sojae), and Rhizoctonia root rot (caused by Rhizoctonia solani) were also present, but to a lesser degree. Root rot was identified even if there had not been a soybean crop in the previous seven years (since 1997). The root rot ratings were higher in the second sampling (Figure 1 and 2). Root rot ratings increased with more years of soybean in the rotation.

Correlations indicate a significant relationship between root rot ratings from plants collected during July and cropping system (Figure 1), and significance between plants collected during August and cropping system (Figure 2). Fusaria spp. populations (pathogenic as well as saprophytic) were correlated with herbicide use, and Rhizoctonia spp. populations were correlated with cropping system. Over all the data collected there is not a clear trend in root rot ratings between the different weed management systems.