The presence of sporulating lesions on seedlings inoculated with the apple scab fungus in the greenhouse was the best criterion for discarding susceptible individuals, and provided a means of selecting other factors for resistance in addition to the V f gene. Short photoperiods (8 hr) and low light intensities (1000 ft-c) significantly reduced the percentage of resistant seedlings. This emphasized the importance of growing the seedlings under uniform conditions. Progenies from crosses of resistant by susceptible parents (V f v f × v f v f ) were grown under various controlled regimes of light, temperature, and humidity. Inoculation with the scab fungus resulted in significantly different percentages of resistant seedlings, indicating genetic differences in resistance between these progenies. When inoculation results of progenies of susceptible varieties were summarized, the percentages of resistant seedlings varied with the observed susceptibilities of the parents. These genetic differences should be taken into consideration in a breeding program.
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Lamb et al. (1969) studied this question.