LEAF BLOTCH SEVERITY ON SPRING BARLEY INFECTED BY ISOLATES OF RHYNCHOSPORIUM-SECALIS UNDER DIFFERENT TEMPERATURE AND HUMIDITY REGIMES

The infection efficiency and severity of leaf blotch on spring barley inoculated with three pathotypes of Rhynchosporium secalis from centra l Norway were studied under different temperature and humidity regimes . Seedlings of the cultivar Arve were subjected to two constant temper atures, 13 degrees or 18 degrees C. Dry periods of 8 h or longer befor e or after a wet period of 4 h, carried out in the first 48 h postinoc ulation, reduced disease severity assessed 16 days after inoculation. The effect of dry periods of up to 24 h was nullified when plants were subjected to high humidity for 48 h after the dry treatment. The dise ase developed most rapidly when the wet period was 48 h and the temper ature 18 degrees C. At or near the optimum temperature for R. secalis (18 degrees C), leaf wetness duration as short as 2 h resulted in cons iderable disease. Isolates reacted differently to temperature. The mos t aggressive isolate caused severe disease irrespective of temperature (56-70% of the leaf area infected); however, disease severity caused by the least aggressive isolate was significantly higher at the optimu m temperature compared with a lower temperature (13 degrees C). This i nformation can facilitate evaluation of weather data in relation to pr edicting leaf blotch for advisory purposes.