Effect of N management on the combined damage of chocolate spot and rust diseases and yield loss of faba bean in wheat–faba bean intercropping system

Objectives The objectives were to explore differences of the degree of chocolate spot and rust combined disease and yield loss between faba bean monoculture and wheat with faba bean intercropping under different nitrogen levels, quantify yield loss caused by the combined damage of chocolate spot and rust, and evaluate contribution of intercropping control disease to yield advantage.

Methods In 2016, a field experiment was designed by using a randomized block design. There were four nitrogen levels, N0 (0 kg/hm²), N1 (45 kg/hm²), N2 (90 kg/hm²) and N3 (135 kg/hm²), two planting modes, faba bean monoculture and wheat with faba bean intercropping, and disease control and no control. The damage degree of chocolate spot and rust on faba bean and the yield loss were investigated.

Results The nitrogen application (N₁, N₂ and N₃) increased the chocolate spot and rust area under disease progression curve (AUDPC) 33.9% and 39.6% on average in the monoculture, increased the chocolate spot and rust AUDPC 27.1% and 69.3% on average in the intercropping, and the AUDPC was the highest under the N₃ level. Compared with the monocropping, the intercropping significantly reduced the chocolatespot and rust AUDPC 49.1%–53.6% and 39.6%–56.8% under N₀–N₃ levers, respectively. The nitrogen application (N₁, N₂ and N₃) aggravated the harm of the chocolate spot and rust and caused faba bean 100-seeed weight loss 28.1–32.4 g (monocropping) and 16.3–16.8 g(intercropping), grain yield loss of 1441–1770 kg/hm² (monocropping) and 815–1263 kg/hm²(intercropping), ranked as follows: N₃ > N₂ > N₁ > N₀. Compared with the monoculture, the intercropping could recover 46.8% 100-seeed weight loss and 36.9% grain yield loss of faba bean. Regression analysis of the combined damage of faba bean diseases and grain yield showed that for each unit increase in percent of the chocolate spot and rust AUDPC, there was a grain yield loss of 1.7 kg/hm². The chocolate spot was the main disease causing yield loss of faba bean, followed by rust. Appropriate nitrogen application could exert the advantage of intercropping better.

Conclusions Nitrogen application aggravates the combined damage of the chocolate spot and rust, and increases the yield loss of faba bean. Wheat with faba bean intercropping could significantly reduce the combined damage of the chocolate spot and rust and recover the yield loss. The effect of the intercropping could control disease and increase yield under full realization at appropriate rate of nitrogen fertilizer, and other effects of yield advantage of the intercropping could be maximized. In this experiment, considering the effects of disease control and yield increase and other effects of intercropping, the recommended nitrogen application rate of faba bean was 45 kg/hm².