A temporal limit to the association between flag leaf life extension by fungicides and wheat yields

Three field experiments, each repeated over two or three seasons, on winter wheat investigated a possible limit to the association between grain yield and flag leaf life, as extended by fungicide application. The experiments involved up to six cultivars and different application rates, timings and frequencies of the strobilurin azoxystrobin and the triazole epoxiconazole. In the 2000/01 and 2001/02 seasons, the relationships between the thermal time to 37% green flag leaf area (m) and yield deviated from linearity. 'Broken stick' models were fitted to cultivar × experiment combinations within each season and the limit to the benefit to yield associated with extending flag leaf life was 700 °C days (S.E. = 20.7) and 725 °C days (S.E. = 9.33) after anthesis in 2000/01 and 2001/02, respectively. In 2002/03, the relationship between yield and m did not deviate significantly (P > 0.05) from linearity, but in this latter year the fungicide application failed to increase m past 700 °C days.