Multivariate analysis of apple cultivar susceptibility to Venturia inaequalis under greenhouse conditions

Apple scab, caused by Venturia inaequalis, is a disease of great importance in apple production regions in eastern North America. However, little is known regarding the relative susceptibility of North American cultivars to this disease since the majority of previous researches on apple scab were conducted on a highly susceptible cultivar, 'McIntosh'. We evaluated 21 cultivars commonly grown in eastern and central Canada for their responses to V. inaequalis by measuring the following components of partial resistance: the number of lesions per leaf, the number of lesions per square centimetre of leaf, the lesion surface area per leaf, the proportion of leaf area diseased, the number of conidia per lesion, the number of conidia per square centimetre of lesion, the incubation period, and the latent period. All of the components examined were effective in assessing relative cultivar susceptibility, but those related to disease severity, latent period, and conidia production were the most important and accounted for 86% of the variation among cultivars. Numbers of lesions per leaf and per square centimetre of leaf were highly positively correlated with principal component 1 (PC1). Incubation and latent periods were highly negatively correlated with PC1. 'McIntosh' and 'Vista Bella' were found to be highly susceptible whereas 'Golden Russet', 'Idared', 'Paulared', 'Red Delicious', and 'Sunrise' were the least susceptible.