The Norwegian Crop Research Institute, Kvithamar Research Centre, N-7500 Stjordal, Norway

Background and objectives
Leaf blotch caused by the imperfect fungus Rhynchosporium secalis is one of the major diseases of barley in Norway. This region has low summer temperatures and high precipitation levels. Yield losses up to 40% have been registered in central Norway. Control of leaf blotch in Norway is mainly based on sanitation and crop management practices, including use of fungicides. NORPRE is a decision support system (DSS) for crop protection which was established in 1982 at the Norwegian Plant Protection Institute[1]. Leaf blotch is one of the diseases that this system deals with. One objective of this study was to evaluate advice concerning leaf blotch control suggested by NORPRE. At the same time we were able to study the effect of fungicide treatment at different growth stages and disease levels. The influence of leaf blotch on yield of two common Norwegian cultivars was also investigated.

Materials and methods
A single fungicide treatment (propiconazole) was carried out on spring barley at different growth stages (g.s.), and the effect on leaf blotch severity and grain yield was studied in 16 field experiments in central Norway in 1993-1995. All fields were assessed for leaf blotch severity at least five times during the growing season. Assessments started at tillering (g.s. 22-25) and were terminated at milky ripe stage (g.s. 75). Area under the disease progress curve (AUDPC) is known to be closely related to yield loss for barley crop. This area was calculated by the formula defined by Shaner and Finney [2].

Results and conclusions
Leaf blotch occurred in all fields and the range of percentage leaf area infected was from 7.3% to 90.2% in untreated control plots. According to disease progress curves, disease severity increased rapidly at flowering. Early treatments (g.s. 26-39) reduced diseased leaf area effectively, but the yield increase following these treatments was less than the yield increase achieved by late treatments (g.s. 40-60). A fungicide treatment at about ear emergence (g.s. 45) can be evaluated in central Norway since leaf blotch severity increased at this time in all experimental fields. Yield losses were from 7% to 28%. Yield was negatively correlated with leaf blotch severity integrated over the registration period (AUDPC) in seven out of 16 fields and coefficients varied from -0.46 to -0.80. The time of treatments suggested by NORPRE was, for 25% of the fields, the proper time for spraying. In the rest of the fields the effect of the NORPRE-suggested treatment was intermediate between the early and late treatments concerning both disease control and yield. The NORPRE-system has great potential by monitoring the disease level at different growth stages and can offer an appropriate warning for each farm connected to this system. The threshold values for leaf blotch control, however, should be adjusted on the basis of forecasted weather data.

1. Magnus HA, Munthe K, Sundheim E, Ligaarden J, 1991. Danish Journal of Plant and Soil Science 85, 1-6.
2. Shaner E, Finney RE, 1977. Phytopathology 67, 1051-1056.