Effect of previous crops on crown rot and yield of durum and bread wheat in northern NSW

The effect of previous crops (oilseed, legume, and cereal) on the incidence and severity of crown rot (Fusarium pseudograminearum, Fp) and yield of wheat was investigated in 3 field studies in northern New South Wales. The experiments were designed to compare the effectiveness of the Brassica break crops canola (Brassica napus L.) and mustard (B. juncea L.) with chickpea (Cicer arietinum L.) on reduction of Fp in subsequent wheat crops. Responses to previous broadleaf and cereal crops were investigated in Fp-tolerant bread wheat (Triticum aestivum L.) and Fp-susceptible durum wheat [Triticum turgidum L. ssp. durum (Dest.)]. In all experiments, broadleaf break crops increased the yield of Fp-susceptible durum wheat compared with durum after cereals (by 0.24–0.89 t/ha). The same response was observed for the Fp-tolerant wheat at 2 of the 3 sites (0.71 and 0.78 t/ha), with a lower yield (0.13 t/ha) after break crops than after cereals at one site during a drought. The yield of the Fp-susceptible durum wheat was generally higher after brassicas than after chickpea (yield advantage 0.27–0.58�t/ha), whereas there was no such difference in the tolerant wheat variety. In most cases, these yield responses to the previous crops were closely related to the severity of Fp infection. Overall yield of susceptible durum wheat was reduced by 1% for each 1% increase in Fp severity at harvest. Residual water and nitrogen (N) did not explain responses to previous crops, although common root rot (Bipolaris sorokiniana) may have contributed to some of the responses at the sites. There was little evidence that the lower disease and higher yield following brassicas compared with chickpea was related to suppression of Fp by biofumigation. More plausible explanations are that residual cereal residues decomposed more rapidly under dense Brassica canopies thus reducing Fp inoculum, that Fp severity was increased following chickpea due to higher soil N status, or that brassicas resulted in soil/residue biology that was less conducive to Fp inoculum survival. Evidence for the latter was provided by consistently higher levels of Trichoderma spp. isolated from wheat following brassicas compared with chickpea or cereals. Irrespective of the mechanisms involved, the results demonstrate that Brassica oilseeds provide an effective break crop for crown rot in northern NSW. Furthermore, brassicas may provide an excellent alternative rotation crop to chickpea for high value durum wheat due to an apparent capacity to more effectively reduce the severity of crown rot infection in subsequent crops.