Date of Award

Spring 1984

Project Type


Program or Major


Degree Name

Doctor of Philosophy


A theoretical model (Prot. Ecol. 5:103-125) was the framework of a study of factors affecting the number of Venturia inaequalis ascospores per m('2) orchard floor per season (PAD) in New Hampshire apple orchards. PAD was calculated as the product of scab lesions per m('2) leaf tissue at leaf fall, pseudothecia per lesion, asci per pseudothecium, the proportion of the orchard floor covered by leaf litter at bud break, and ascospores per ascus. Disease incidence and the proportion of lesions that formed mature ascocarps were strongly correlated; lesion fertility was directly proportional to disease incidence at leaf fall. Most lesions from commercial orchards did not produce mature ascocarps. However, even when disease incidence was near zero, approximately 3% of the lesions formed mature ascocarps. Mechanisms were discussed by which sexual reproduction could occur when the probability of pairing of compatible mating types was remote. Infertile lesions were associated with aborted ascocarps as observed by Keitt and Palmiter (Amer. J. Bot. 25:338-345). The number of asci per ascocarp and the number of ascocarps per lesion did not differ on McIntosh, Cortland, or Delicious leaves. Leaves decayed primarily during winter and only slowly from bud break to petal fall. PAD varied from 13 to 44,544 ascospores/m('2)/season in orchards where 0.04% and 9.32% of the leaves were infected the previous autumn, respectively. A model proposed by Van der Plank was used to compute time differences ((DELTA)t) in the onset of scab epidemics based on differences in PAD. Spray programs for apply scab could be delayed by 2-22 days in commercial orchards depending on PAD and the value of Van der Plank's infection rate (r).