Variables necessary to calculate potential ascospore dose (PAD) for Venturia pirina, the causal agent of pear scab, were measured: area of individual leaves averaged 20.0 cm2, number of lesions per infected leaf ranged from 1.8 to 2.2, 13.5 to 41.7% of lesions were fertile (contained pseudothecia), number of pseudothecia per fertile lesion ranged from 15.2 to 24.4, and number of asci per pseudothecium averaged 131.5. The proportions of orchard floor covered with leaves (leaf litter density) were 0.34 and 0.67 for mowed and nonmowed orchards, respectively. Incidence of infected leaves ranged from 0.1 to 32.3%. Calculated PAD ranged from 1,114 to 708,320 ascospores per m2 in a commercial and an unsprayed orchard, respectively. For validation of a previously published model relating ascospore maturity to degree-days with a 0°C base, a linear regression equation was developed using 11 data sets collected between 1981 and 1998. The slope of the line was not different (P = 0.951) from that of the original model.
Keywords: epidemiology.