The ability to determine the concentration of pathogenic spores in the air surrounding host plants is fundamental to the development of models for predicting airborne spread of disease. The aerial concentration of Venturia inaequalis ascospores, C (ascospores m(-3)), was monitored during two seasons using Rotorod samplers deployed at several heights above the ground in a young orchard of dwarf apple trees. C decreased rapidly with height above the ground, and, on average, values of C at 3.0 m height were only about 6% of C measured at 0.15 m. A spore dispersal model is presented that suggests that this rapid decrease of C with height was due mainly to a rapid increase of wind speed and turbulent eddy diffusivity with height above the ground. The model described adequately the general shape of the vertical variation of C. During conditions of light wind, however, the model tended to underpredict relative values of C both near the ground and near the top of the trees. Improved model performance during light wind and rain will require better methods for measuring and modeling wind speed and turbulent eddy diffusivity under these conditions.
