Crop and Environment Research Centre, Harper Adams College, Newport, Shropshire TF10 8NB, UK.

Background and objectives
Fusarium ear blight (FEB) is recognized to be a significant disease of wheat throughout the world [1]. Internationally, the disease has increased in recent years mainly as a result of an increase in the incidence of maize/wheat rotations . In the upper Midwest of the United States, FEB has been widespread between 1993 and 1996 and was responsible for an estimated loss of $1 billion in the state of Minnesota during 1993 [2]. The objective of this study was to determine a mathematical relationship between the severity of FEB symptoms caused by Fusarium culmorum and yield components of the winter wheat cultivar Avalon.

Materials and methods
During three successive growing seasons (1994-96), natural epidemics of FEB caused by F. ;culmorum were allowed to develop from discrete point sources of inoculum throughout six field plots (32X2 m) of winter wheat (cv. Avalon). All plots were mist irrigated from GS 65-80 in order to provide conditions conducive to disease development. At GS 80, each plot was divided into 16 subplots (2X2m) and disease severity scored for 25 ears selected at random for each subplot. Throughout each trial, 1200 single ears showing a range of disease severities were tagged. Disease severity was assessed for each tagged ear at GS 80. All tagged single ears, and all ears found within a 1m2 quadrat were harvested from each subplot. All ears harvested were threshed and the number of grains per ear and individual grain weight determined for each single ear and each subplot at 15% moisture content. Simple linear regression analysis was undertaken on both single ear and subplot data to determine if significant relationships existed between the severity of FEB and number of grains per ear and individual grain weight. In order to compare both subplot and single ear approaches for determining disease-yield loss relationships, analysis of parallelism was undertaken on all regression relationships.

Results and conclusions
The restricted development of secondary foci within wheat plots suggested that FEB caused by F. ;culmorum may be monocyclic in nature. The fact that infection of wheat ears by Fusarium spp. is optimum during the period of anthesis is well documented. Given that the latent period of F. ;culmorum is 8-19 ;days, the resistance of ears to infection will have increased significantly by the time secondary inoculum is produced and as such would explain the monocyclic nature of the disease. For subplot data, a significant and strong relationship between the incidence and severity of FEB was determined following linear regression analysis in all three years of the study with up to 94% of variance accounted for. This would suggest that incidence of FEB, assessed as the percentage of ears infected, can give a strong indication of disease severity, assessed as percentage of spikelets infected, and, as such, could be used as a quick and easy approach to disease assessment. Using either subplot data and single ear data, regression analysis failed to identify any significant relationship between the severity of FEB and the number of grains per ear. However, significant relationships were determined between the severity of FEB and individual grain weight using both subplot and single ear approaches in each of the three years. Comparing relationships revealed that although linear relationships varied from year to year, during each year, relationships obtained from subplot and single ear approaches were not significantly different. This would suggest that either approach can be used to determine a disease-yield relationship in the case of FEB.

1. Parry DW, Jenkinson P, McLeod L, 1995. Plant Pathology 44, 207-38.
2. Dublin HJ, Gilchrist L, Reeves J, McNab A, 1997. Fusarium Head Scab: Global Status and Future Prospects. Mexico, D.F.:CIMMYT.