Potato scab management with Brassica biofumigation and effect of volatiles on Streptomyces growth

Common scab of potato is an economically important soilborne disease caused by various Streptomyces species. The disease is very difficult to control, but biofumigation has recently shown some potential. Biofumigation consists of the incorporation of Brassica spp. crop residues containing glucosinolates that upon cell disruption are hydrolysed by the enzyme myrosinase to yield a diversity of biologically-active hydrolysis products, of which volatile isothiocyanates are the most toxic to soil microbes. In this study common scab was significantly reduced through soil incorporation of fresh and air-dried residues of Brassica oleracea var. capitata (cabbage) when applied prior to two consecutive potato plantings. The in vitro effect of volatile emissions from various Brassica species on pathogenic and non-pathogenic Streptomyces isolates was also evaluated using a bioassay method. In the chamber bioassay freshly macerated Brassica tissue, (B. oleracea var. capitata and B. juncea/S. alba mix) suppressed sporulation but not hyphal growth of the 79 evaluated Streptomyces isolates. The chamber bioassay also showed that the Brassica tissue volatiles were bacteriostatic, since isolates re-grew when removed from the chamber and transferred to fresh media. The results suggest that soil biofumigation could significantly influence composition of the Streptomyces community in agricultural soils and lead to significant suppression of common scab on potato.