A greenhouse study was set up in January 2011 to investigate the effect of soil myrosinase concentration and Bacillus spp. on the release efficiency of isothiocyanates (ITCs) from freeze-dried ‘Pacific Gold’ mustard leaf tissue after incorporation into soil. Seventeen diverse soil samples collected from various regions of South Carolina were used in the study. A correlation analysis was conducted to determine if soil myrosinase was positively correlated with ITC concentrations after mustard incorporation. ITC concentrations were then analyzed with soilborne Bacillus spp. populations, bean emergence, disease incidence, and shoot weight to identify significant correlations. Finally, the effect of freeze-dried ‘Pacific Gold’ mustard leaf tissue on populations of soilborne Rhizoctonia solani, Sclerotium rolfsii, and Pythium spp., as well as the effect on bean emergence, disease incidence, and shoot weight was investigated using two different soil samples. Soil myrosinase activity ranged from -0.71 μg glucose g-1 soil 4 h -1 to 5.87 μg glucose g-1 soil 4 h-1 and was not significantly correlated with ITC concentrations following mustard incorporation. ITC concentrations ranged from 1.8 μg g-1 soil to 13.4 μg g-1 soil and were not significantly correlated with soilborne Bacillus spp. populations, bean emergence, disease incidence, or shoot weight. Incorporation of freeze-dried ‘Pacific Gold’ mustard leaf tissue into soil did not significantly affect populations of soilborne R. solani, S. rolfsii, or Pythium spp., nor did it significantly affect bean emergence, disease incidence, or shoot weight.
A three-season field experiment was set up to determine if biofumigation with oilseed radish, ‘Pacific Gold’ mustard, and winter rapeseed would reduce populations of the soilborne plant pathogens R. solani, S. rolfsii, and Pythium spp. Soil ITC concentrations were quantified following incorporation of each brassica treatment at 4 hours and two days during the summer 2011 pepper experiment, and 4 hours, 2 days, and 4 days during the fall 2011 pepper experiment. The effect of brassica cover crops on pepper vigor and yield, and incidences of pepper mortality caused by root rot and Southern blight were also investigated. Additionally, the relative pathogenicity of five Pythium spp. isolates on pepper was determined in a greenhouse experiment. After incorporation of the three brassica cover crops, soil was covered with virtually impenetrable film (VIF). The two control treatments included weedy fallow (+) VIF and weedy fallow (-) VIF. Brassica cover crops were planted in Spring 2010, Fall 2010, and Spring 2011 with pepper crops planted Summer 2010, Spring 2011, and Summer 2011, respectively. ITCs concentrations were highest following incorporation of ‘Pacific Gold’ mustard. Winter rapeseed yielded the second highest ITC concentrations. Oilseed radish yielded very low ITC concentrations in spring 2011, and none during summer 2011. ITCs were detected in low concentrations in fallow (+) VIF treatments. All (+) VIF treatments reduced populations of R. solani compared to (-) VIF, with no differences between brassica treatments and fallow (+) VIF. Brassica treatments generally did not reduce populations of Pythium spp. compared to fallow (+) VIF, although results were inconsistent across repetitions. Population estimates of S. rolfsii were generally low with few significant differences among treatments. Pepper stunting was generally significantly lower in all (+) VIF treatments compared to fallow (-) VIF. In general, there were no significant differences in pepper stunting between brassica treatments and fallow (+) VIF. Brassica treatments did not consistently affect pepper death due to root rot or Southern blight compared to either control. Pepper yields were consistently highest in brassica treatments compared to fallow (+) VIF, and (+) VIF yields were higher than (-) VIF yields. Only one of five Pythium spp. isolates recovered from soil in the pepper field used in the third repetition was pathogenic on pepper in the greenhouse, and it was identified as P. aphanidermatum.