Date of Award

Fall 1992

Project Type


Program or Major


Degree Name

Doctor of Philosophy

First Advisor

William E MacHardy


The components of partial resistance to Venturia inaequalis and the expression of resistance were quantified in nine apple cultivars ('Delicious,' 'McIntosh,' 'Stayman,' 'Mutsu,' 'Paula Red,' 'Ida Red,' 'Golden Delicious,' 'Spartan,' and 'Rome') under field conditions. Ascospore productivity was influenced by cultivar, and was greatest on 'Rome' (476 spores/cm$\sp2$ leaf), but cultivar did not affect the rate of ascospore maturation significantly.

Relative conidium productivity/lesion was influenced by cultivar, and ranged from 30,126 conidia/lesion on 'Golden Delicious' leaves to 4,443 conidia/lesion on 'Delicious' leaves. The infectious period (length of time conidia are produced) was longest on 'Golden Delicious' (94 and 68 days) and shortest on 'McIntosh' (51 and 52 days). An early buildup of scab occurred on cultivars with lesions that had high conidium productivity during the first 10 days after lesion appearance. A non-destructive technique that allowed repeated removal of conidia from scab lesions is described.

Incubation period was not influenced by cultivar, but it was correlated with leaf age: the greater the age of a leaf when infected, the longer the incubation period. An ontogenic resistance mechanism was apparently operational in all cultivars by the 12th day after a leaf unfurled.

The area of exposed tissue at each fruit bud growth stage measured and the area of the youngest five leaves on a shoot differed among the cultivars. The rate of extension shoot growth (leaves emerged/week) did not differ among the cultivars. Each year, phenological development of the fruit buds differed among the cultivars on each observation date except the final observation at fruit-set.

The cultivars are ranked for resistance, based on incidence and severity of foliar scab as follows: least resistant are 'McIntosh' and 'Rome,' moderately resistant (disease reduced by 41-76 %, relative to 'McIntosh') are 'Stayman,' 'Mutsu,' 'Spartan,' 'Golden Delicious,' and 'Delicious,' and highly resistant (disease reduced 92 %, relative to 'McIntosh') are 'Ida Red' and 'Paula Red.'.