Date of Award

Spring 1986

Project Type


Program or Major


Degree Name

Doctor of Philosophy


The airborne dispersal of Venturia inaequalis ascospores and Lycopodium sp spores from a point source was examined in an apple orchard. Twenty-five volumetric spore traps were constructed, and a system developed that governed intervals of trap operation by leaf wetness. The estimated inoculum density was equal to several unsprayed trees, much greater than inoculum densities in well-managed commercial orchards.

Dispersal gradients were analyzed using the equation Y = a + bX(,1) + cX(,1)('2) + dX(,1)('3) + eX(,2), where Y is the log of spores trapped, X(,1) is the log of distance from the source, and X(,2) is a constant to relate trap location to wind direction. Statistically significant (p = 0.05) regressions were calculated when data from all ascospore release periods were combined, but not for individual releases. Ascospores were randomly dispersed beyond 30 m from the source, and it was proposed that beyond 30 m an equivalent inoculum source within or outside of a commercial orchard would not contribute significantly to the inoculum level in that orchard. Lycopodium spore gradients were steeper than ascospore gradients, possibly reflecting differences in terminal velocity.

The ratio of airborne ascospores per developing lesion on unsprayed foliage was calculated by dividing area dose (ascospores cm('-2) air) by lesion density (lesions cm('-2) leaf tissue), where area dose = spores m('-3) x wind speed (m s('-1)) x sampling period (s)/10,000. The ratio was greatest for leaves infected at the beginning of the primary season. The percentage of scabbed fruit on trees unprotected with fungicide prior to pink bud phase was not statistically greater than on trees protected for the entire primary scab season. On potted McIntosh seedlings placed in the orchard, the relationship was described by the equation Y = 1.79 + 0.0073X, where Y is the lesion density and X is the airborne ascospore density when the airborne ascospore density was greater than 5000 ascospores m('-3) air. Below that figure, there was no relationship.

The deposition efficiency (percentage of area dose) of ascospores deposited on coverslips affixed to leaves in a glasshouse study was 3.8, 2.5, and 0.8% at wind speeds of 0.23, 0.45, and 0.89 m s('-1), respectively. The low number of lesions recorded in the orchard was thus partially explainable by the small proportion of ascospores deposited on foliage.