2019 ASHS Annual Conference

Anaerobic soil disinfestation (ASD) is known to suppress multiple soilborne plant pathogens of horticultural crops. During ASD treatment, soil microbial activity is enhanced by labile carbon amendment to create anaerobic conditions resulting in production of volatile fatty acids (VFAs) and other antifungal volatiles. Our objectives were 1) to determine rates of VFA production during ASD with two soil amendment mixtures (wheat bran or dry molasses-based) at a range of C:N ratios (10:1, 20:1, 30:1 or 40:1), and 2) to evaluate survival of R. solani inoculum exposed to VFAs as a function of VFA, VFA concentration, and soil pH. For objective 1, a growth chamber pot study was conducted with sandy loam soil incubated with ASD amendments, soil samples collected during treatment and extracted with KCl, and VFA concentration quantified by HPLC. VFA production was highest at 20:1 and 30:1 C:N ratios for dry molasses amendment and 40:1 C:N ratio for wheat bran amendment. VFAs observed included acetic, propionic, n-butyric, isovaleric, valeric and isobutyric, and concentrations varied by amendment, soil type, amendment C:N ratio, and sampling time. For objective 2, we evaluated effects of VFA soil exposure on R. solani inoculum survival in an anaerobic chamber. VFAs of acetic, n-butyric, or valeric acids, a mixture of acetic and n-butyric acids at 4 or 8 mmol/kg soil were maintained at soil pH 4.5 or 5.5 in sandy or sandy loam soil for 5 days, and survival of R. solani enumerated by toothpick baiting technique. Controls (water or HCl) were included. Similarly, a pot study was conducted with 4 mmol/kg soil VFA concentration (acetic, propionic, n-butyric, isovaleric, valeric and isobutyric) at a soil pH of 5.0. ASD treatments (dry molasses or wheat bran-based amendments) and controls (HCl and unamended saturated/non-saturated) were also included, and suppression of R. solani and disease development on common bean evaluated. Suppression of R. solani inoculum was observed for all VFAs when compared to controls. There was a limited effect of VFA concentration and soil pH at the levels used in our study. In the pot study, lowest recovery of R. solani was observed from propionic, n-butyric, isovaleric and mixture of acetic and butyric acids (67-79% less than controls). ASD treatments reduced R. solani propagules by 31-53% compared to unamended controls. Bean emergence and root disease rating was lower in all treatments when compared to the non-saturated control, but was not different from the HCl or the saturated control.