Effect of Soil Physical Conditions on Emission of Allyl Isothiocyanate and Subsequent Microbial Inhibition in Response to Brassicaceae Seed Meal Amendment

Generation of ally] isothiocyanate (AITC) in soil treated with residues of specific Brassicaceae species yields direct and indirect suppression of soilborne plant pathogens. Soil physical conditions demonstrably affected the quantity of AITC generated in response to soil incorporation of a Brassica juncea/Sinapis alba seed meal (SM) formulation. The concentration of AITC generated in SM-amended soil increased with an increase in temperature from 10 to 30 C. AITC emission was also elevated with an increase in soil water potential from-1,000 kPa through-40 kPa; however, a significant decrease in AITC emission was observed in a saturated soil environment (0 kPa). Peak AITC emission was obtained 2 to 3 h after SM amendment under optimal conditions but the peak was delayed in soils incubated at low temperature or in extreme moisture environments. Although AITC production varied significantly across different orchard soils, all three orchard soils yielded the same pattern of AITC release in response to SM amendment over the spectrum of soil water potentials examined in this study. Mycelia] growth inhibition in fungi and oomycetes isolated from apple roots was dependent on both AITC concentration and exposure time. Pythium ultimum exhibited sensitivity to AITC at concentrations ranging from 0.01 to 0.22 p.g g-1 of soil, whereas Hypocrea lixii was insensitive to AITC. Exposure to AITC at a concentration of 0.22 p.g CI of soil for a period of 2 h restricted hyphal growth of Rhizoctonia solani AG-5, Ilyonectria destructans, and Mortierella alpina. R. solani AG-5 exhibited significant growth inhibition when incubated at AITC concentrations of 0.008 to 0.011 p.g g-1 of soil for 10 h. These findings provide information that will be useful in the management of appropriate soil variables to obtain optimal yields of AITC in response to SM soil amendments and indicate that a standard soil moisture prescription may be suitable for use when applying this SM formulation for soilborne disease control.