SIMULATION OF LEAF WETNESS DURATION FOR FIELD CORN

Computer models of the evaporation of dew and the drying of water drops on plant leaves were combined to simulate leaf wetness duration at a depth within a com canopy that is critical for the onset of disease. These models, for use in the field, were adapted to run on hourly climate observations over a 5 month period (May-September) at different locations in Canada. The models were used to compute the probability of particular episodes occurring in any month, the return period of extreme episodes and the average temperature associated with different lengths of leaf wetness duration. The formation of dew was inconsequential in all but a few of the episodes of leaf wetness. The simulation did not exhibit any observable pattern between average temperature and the length of surface wetness. This is not surprising since evaporation in these models is driven by radiation and the vapour pressure deficit. However, analyses of episodes of leaf wetness duration exceeding 1 day provided evidence for a geographical and seasonal variation in leaf wetness. This variation was explained with reference to monthly normals at each location and by an examination of the synoptic patterns associated with extreme episodes. The analysis of monthly normals revealed the occurrence of high wind speeds at Saint John that act to prolong leaf wetness through the advection of sea fog.