Pear scab: Components of potential ascospore dose and validation of an ascospore maturity model

Variables necessary to calculate potential ascospore dose (PAD) for Venturi a pirina, the causal agent of pear scab, were measured: area of individual leaves averaged 20.0 cm(2), 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 n umber 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 mowe d 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 p er m(2) in a commercial and an unsprayed orchard, respectively. For validat ion of a previously published model relating ascospore maturity to degree-d ays with a 0 degrees C base, a linear regression equation was developed usi ng 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.