Duration of control of two soilborne pathogens following incorporation of above- and below-ground residues of Brassica juncea into soil

The persistence of control of primary infections caused by two soilborne fungal plant pathogens, Rhizoctonia solani and Gaeumannomyces graminis var. tritici , following the incorporation of above-ground parts (AP), below-ground parts (BP) or both (AP+BP) of Brassica juncea into soil was examined through an experiment in controlled conditions. Control was quantified by measuring disease incidence in bioassays where inoculum was introduced at different dates after the incorporation of plant residues. All types of residue showed an unexpected long-term persistence that lasted at least 13 days, while the predominant glucosinolates contained in AP (20·9  µ mol sinigrin g⁻¹ dry matter) and BP (2·3  µ mol gluconasturtiin g⁻¹ dry matter) were hydrolysed in less than 3 days. Temporal trends in the efficacy of the residues behaved mostly in a quadratic manner, suggesting that the noxious effect of residues may be attributable to the release of isothiocyanates during the first days following incorporation, but that other mechanisms are most likely to contribute to lasting persistence. Across all treatments, AP and AP+BP suppressed R. solani by 54 and 63%, respectively, and G. graminis var. tritici by 40 and 40%, respectively, compared with controls. While BP did not cause any additional detectable effect when combined with AP, they had a significant effect when incorporated alone (approximately 20% suppression of both species), suggesting the existence of a complex interaction between these two types of residue.