Mechanisms by which fungicides increase grain sink capacity and yield of spring barley when visible disease severity is low or absent

Fungicides are a central component of disease management programmes in many cereal production systems worldwide, but intensive use has led to problems with the evolution of resistance in pathogen populations, harmful effects on non-target organisms and residues in the food chain and environment. Better targeting of fungicide applications, according to the likely response of the crop, would help improve the sustainability of disease management, but decision making is hindered by our poor understanding of the mechanisms by which fungicides influence crop yield. This study investigated possible mechanisms responsible for yield increases following fungicide treatment of spring barley with little or no visible disease. Field experiments conducted over six site-years in the UK quantified the effects of the protectant fungicide chlorothalonil (Chl), and systemic fungicides prothioconazole (Pro) and pyraclostrobin (Pyr), applied on their own or in combination. The severity of visible disease in the absence of fungicide treatments was minimised by growing varieties with good disease resistance. Contrasting fungicide chemistries led to different effects on yield. There were two components to the yield increases. The first involved an increase (4.3 to 7.5 %) in grain number m-2 and was elicited by Pro and Pyr, but not Chl. The second was a smaller increase (3.7 to 4.6%) in mean grain weight (MGW) and was elicited by each of the fungicides tested. The effect of the triazole (Pro) and strobilurin (Pyr) on grain number was not the result of the control of visible disease pre-anthesis or an increase in photosynthetically active radiation (PAR) interception. Nor was it the result of the control of asymptomatic pathogen infection or possibly leaf surface saprophytes, because Chl gave equivalent control without increasing grain number. Our results are consistent with the hypothesis that triazoles and strobilurins can increase grain sink capacity through direct physiological effects in the field. These apparent physiological effects differ from those reported for wheat as they occur pre-anthesis and are not associated with delayed leaf senescence and resulting increased PAR interception. Application of Pro + Pyr increased radiation use efficiency (RUE), estimated over the whole season, by around 10% compared to untreated controls and 6% compared to Chl treated crops. Evidence is presented to show that whilst the average yield response to Pro+Pyr in the absence of disease was around 3%, the response varied widely between sites and years. Where effects on grain number do occur, they can be elicited in a range of varieties by a single application made at booting. We argue that benefiting from physiological effects of triazole and strobilurin fungicides on spring barley yield, through appropriate timing of application, is compatible with strategies to target and reduce fungicide use in this crop