RELATIONSHIP BETWEEN AERIAL CONCENTRATION OF VENTURIA-INAEQUALIS ASCOSPORES AND DEVELOPMENT OF APPLE SCAB

The relationship between the aerial concentration of pathogenic spores and amount of disease produced on susceptible plants is fundamental to developing models for predicting the spread of airborne diseases. This relationship was examined for apple scab, a disease important in many parts of the world. Potted McIntosh apple trees were exposed in the field to Venturia inaequalis ascospores naturally discharged during rain. Ascospore concentration in the air, (C) (spores m-3), was monitored using spore samplers. Following exposure, trees were incubated under controlled conditions for infection and lesion development. In a separate experiment, individual leaves on potted McIntosh trees were quantitatively inoculated with V. inaequalis ascospore suspensions in the laboratory to determine the lesion-causing efficiency, (E), of ascospores. E varied with leaf age and was about 0.2 lesions per spore for the most susceptible leaves (approximately 3-5 days after unfurling). The number of lesions per shoot, N(S), was recorded for trees exposed in the field. A significant positive correlation (r2 = 0.54, P < 0.00001, d.f. = 293) was found between N(S) and exposure to ascospore concentration (C x exposure time). A physical model was used to estimate deposition, D (spores m-2), of ascospores onto leaves for a given exposure to C. These calculated values of D were combined with experimentally deter-mined values of E and the area of exposed leaves to estimate values of N(S). Values of N(S) estimated using the model were well correlated (r = 0.83, P < 0.01) with field observations.