Field studies on the biofumigation of take-all by Brassica break crops

Biofumigation refers to the suppression of soil-borne pathogens and pests b y biocidal compounds released by Brassica crops when glucosinolates (GSL) i n their residues decay in soil. We conducted field studies at 2 sites to in vestigate the hypothesis that biofumigation by Brassica break crops would r educe inoculum of the take-all fungus Gaeumannomyces graminis var. tritici (Ggt) to lower levels than non-Brassica break crops, and thereby reduce Ggt infection and associated yield loss in subsequent wheat crops. High and un iform levels of Ggt were established at the sites in the first year of the experiments by sowing wheat with sterilised ryegrass seed infested with Ggt . Ggt inoculum declined more rapidly under Brassica crops than under linola and this reduction coincided with the period of root decay and reduced roo t glucosinolate concentrations around crop maturity. There was no consisten t difference in inoculum reduction between canola (Brassica napus) and Indi an mustard (Brassica juncea), nor between cultivars with high and low root GSL within each species. Despite significant inoculum reduction attributabl e to biofumigation, there were no differences in the expression of disease and associated impacts on the yield of subsequent wheat crops across the si tes. Seasonal conditions, in particular the distribution of rainfall in bot h the summer-autumn fallow following the break crops and during the subsequ ent wheat crop, influenced inoculum survival and subsequent disease develop ment. In wet summers, inoculum declined to low levels following all break c rops and no extra benefit from biofumigation was evident. In dry summers th e lower inoculum levels following brassicas persisted until the following w heat crops were sown but subsequent development of the disease was influenc ed more by seasonal conditions than by initial inoculum levels. Significant extra benefits of biofumigation were observed in one experiment where whea t was sown within the break crops to simulate grass weed hosts of Ggt. Unde r these circumstances there was greater reduction in Ggt inoculum under can ola than linseed and an associated decrease in disease development. For hos t-dependent pathogens such as Ggt, we hypothesise that the benefits of biof umigation to subsequent wheat crops will therefore be restricted to specifi c circumstances in which inoculum is preserved during and after the break c rops (i.e. dry conditions, grass hosts present) and where conditions in the following wheat crop lead to significant disease development (early sowing , wet autumn and spring, dry periods during grain filling).