Field Crops Research 139 (2012) 9–19 Contents lists available at SciVerse ScienceDirect Field Crops Research jou rn al h om epage: www.elsevier.com/locate/fcr Yield response to fungicide of spring barley genotypes differing in disease susceptibility and canopy structure I.J. Bingham a,∗ , S.P. Hoad a , W.T.B. Thomas b , A.C. Newton b a Crop & Soil Systems Research Group, SRUC, Kings Buildings, West Mains Road, Edinburgh EH9 3JG, UK b The James Hutton Institute, Invergowrie, Dundee DD2 5DA, UK a r t i c l e i n f o Article history: Received 16 August 2011 Received in revised form 8 October 2012 Accepted 9 October 2012 Keywords: Barley Canopy structure Disease tolerance Powdery mildew Radiation interception Yield a b s t r a c t Field experiments were conducted over two years to quantify the yield response to fungicide of spring barley (Hordeum vulgare L.) genotypes differing in disease severity and canopy structure. Fourteen geno- types were selected from a doubled-haploid mapping population to represent three distinct canopy phenotypes; tall with prostrate leaves, medium height with prostrate leaves and medium height with erect leaves. The fungicide treatments were based on a triazole plus strobilurin and chlorothalonil pro- gramme applied at the start of stem extension and during booting. Untreated plots served as controls. The major foliar diseases and disorders were powdery mildew and brown spotting in 2006 and pow- dery mildew, rhynchosporium barley leaf blotch and brown spotting in 2007, and genotypes differed significantly in the severity of each. Fungicide reduced powdery mildew and rhynchosporium symp- toms in susceptible genotypes, but had little effect on spotting. The yield response to fungicide averaged across genotypes was approximately 1 t ha -1 and there was no significant difference in response between canopy types or genotype. The increase in yield was mostly via an increase in grain numbers m -2 with smaller or non-significant effects of fungicide on mean grain weight. When effects of fungicide on radia- tion interception were plotted against those on yield, a yield response of 0.8–1.4 t ha -1 was indicated in the absence of visible disease. The results show that the effects of fungicide treatment on grain numbers cannot be explained just in terms of protection of green area and increase in radiation interception. Nor were they the result of a delay in leaf senescence. Other possible explanations include control of symp- tomless infection at critical developmental stages, or direct effects of fungicides on plant metabolism. These findings have important implications for fungicide stewardship and disease management in bar- ley, because the requirement for fungicide, as defined in terms of the likely yield response of the crop, cannot be predicted just from an assessment of the amount visible disease present or the risk of a disease epidemic developing. © 2012 Elsevier B.V. All rights reserved. 1. Introduction In an effort to increase the sustainability of cropping practices and minimize their impact on the environment, there is a drive to reduce the unnecessary use of pesticides. Responsible stewardship of fungicides dictates that they only be applied to those crops where a significant increase in crop yield or quality is likely to result. Pre- dicting the requirement for fungicide requires an understanding of the effects of disease on the crop and of the response of both pathogen and crop to fungicide application. It has been reported that the yield response of cereals to application of fungicide can vary between cultivars and often does not relate well to the amount of visible disease present (Schafer, 1971; Kramer et al., 1980). ∗ Corresponding author. Tel.: +44 1315354000. E-mail address: ian.bingham@sruc.ac.uk (I.J. Bingham). Currently, the mechanisms underlying this variation have not been studied extensively, although a number of factors may contribute. Cultivars differ in a range of morphological and physiological traits that could influence the tolerance of disease (defined as yield loss per unit of visible disease) (Parker et al., 2004; Foulkes et al., 2006; Bingham et al., 2009). For example varieties with more prostrate leaves and a greater canopy light extinction coefficient may be more tolerant of foliar disease and suffer a smaller reduction in growth rate and yield under a given disease severity than those with erect leaves. This is because the lower leaves, where disease can be more severe, contribute less to canopy photosynthesis in varieties with prostrate leaves than those with a more erect habit (Bingham and Newton, 2009; Bingham and Topp, 2009). Such prostrate varieties might be expected to show a smaller yield response to application of fungicide and control of disease. Fungicides may also have effects on plant metabolism and growth in addition to those resulting from the control or prevention 0378-4290/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fcr.2012.10.004