EFFICIENCY OF ASCOSPORES OF VENTURIA-INAEQUALIS IN PRODUCING SCAB LESIONS ON APPLE LEAVES

McIntosh apple trees at three early stages of leaf expansion were exposed to freshly discharged ascospores of Venturia inaequalis in a dew chamber at 18-19 C. Spores deposited on glass coverslips positioned next to the leaves at each node were counted. Spores on some coverslips were also tested for viability. Lesions were noted after the trees had been kept in a growth chamber (12 hr of light at 25 C followed by 12 hr of darkness at 20 C) for 11-27 days. The efficiency of lesion production was calculated as the ratio of the number of lesions per unit of leaf surface area at the time of inoculation to the number of spores per unit of coverslip area. Lesion-forming efficiency averaged 5, 6, and 14% for the three groups of trees.