Evaluating the contribution of take-all control to the break-crop effect in wheat

Abstract

Break-crops such as legumes and oilseeds increase the yield of subsequent cereal crops by reducing the level of diseases and weeds that build in continuous cereal crops, and can also improve water and nitrogen supply. Although the term ‘break-crop’ originates from their role in breaking disease cycles of soil-borne diseases such as take-all (caused by Gaeumannomyces graminis var. tritici), the contribution of take-all control to the overall break-crop effect has not been separated in most studies. We re-analysed a range of medium- and short-term crop-sequencing experiments comprising 18 year × site combinations in diverse environments in southern Australia. An analysis using linear mixed effects models was conducted to: (i) define the agro-environments that lead to increases in take-all incidence in continuous wheat crop sequences, (ii) quantify the effect of take-all on wheat yield, and (iii) ascertain the contribution of the reduction in take-all following break-crops to the size of the total break-crop effect on wheat crop yield. Break-crop effects on wheat yield averaged 0.7 t/ha and ranged from 0 to 2.1 t/ha. On 14 of 18 occasions, take-all contributed to reduced wheat yield in continuous wheat rotations, although the estimated effect exceeded 0.1 t/ha on just six of those occasions. As a result, reduced take-all by break-crops contributed to <20% of the total break-crop effect in all but one instance, where the suppression accounted for 80% of the break-crop effect. In summary, although the break-crops improved wheat yield by 0.7 t/ha, the contribution from take-all control in the 14 locations where it could be quantified was just 0.1 t/ha. Correlation analysis revealed that take-all incidence in wheat was most likely to proliferate in colder, wetter environments. Take-all can severely damage crop yield, and the reduction contributes to the break-crop effect, but the average impact on wheat yield is small and poorly correlated with the potential yield of the wheat crop. The analytical approach helped to quantify the effect of take-all damage on crop yield, to provide further insight into the agro-environment that contributes to high levels of take-all incidence, and to demonstrate that take-all, like many other processes, operates in an episodic manner that is rare but, on occasions, severe.