Evaluation of fungicides for control of eyespot disease and yield loss nationships in winter wheat

Following the discovery of resistance to benzimidazole fungicides in the cereal eyespot pathogen Pseudocercosporella herpotrichoides in the UK in 1981, and an initial in vitro screen to select the fungicides with greatest activity against the pathogen, 40 field experiments were carried out between 1983 and 1986 to evaluate alternative fungicides for control of eyespot. At the majority of experimental sites, benomyl-resistant strains of the pathogen were present, and carbendazim did not control eyespot. Prochloraz was the most effective fungicide, reducing the eyespot index by 30–60%. There was no extra benefit from adding carbendazim to prochloraz. Flusilazole was almost as effective as prochloraz, but other fungicides had little or no effect. At sites with a high incidence of eyespot, prochloraz, with or without the addition of carbendazim, generally gave the largest yield increase. The mean yield increases each year were in the range 0·36–0·85 t/ha, and the greatest yield increase at any site was 2·27 t/ha. Most other fungicides increased yield, but carbendazim did not from 1984 to 1986. There were also yield increases at many sites with a low incidence of eyespot. Yield increases were associated with increases in thousand-grain weight at the majority of sites, but in only a few instances were there associated increases in specific weight. Prochloraz application at GS30-31 was cost effective at 71% of sites. At most sites, in regression of yield on eyespot, eyespot accounted for less than 25% of the variance in yield. The mean relationship between severe eyespot lesions and yield loss was such that each 1% increase in the percentage of tillers affected by severe eyespot was associated with a yield loss of 0·21 %. There was a significant positive correlation between eyespot at GS75 and GS30-31, and between yield increase from prochloraz treatment (at GS30-31) and eyespot at GS75, but not between yield increase and eyespot at GS30-31. The ADAS threshold for fungicide application of 20% tillers affected at GS30-31 was a reliable indicator of the cost-effectiveness of treatment at 60% of sites.