529 Screening of Apple Cultivars for Resistance to European Canker, Neonectria ditissima L. Garkava-Gustavsson 1,2 , A. Zborowska 2 , J. Sehic 1 , M. Rur 3 , H. Nybom 1 , J.-E. Englund 4 , M. Lateur 5 , E. Van de Weg 6 and A. Holefors 2 1 Swedish University of Agricultural Sciences, Department of Plant Breeding and Biotechnology, Balsgård, Fjälkestadsvägen 459, 291 94 Kristianstad, Sweden 2 Swedish University of Agricultural Sciences, Department of Plant Breeding and Biotechnology, Box 101, 230 53 Alnarp, Sweden 3 Swedish University of Agricultural Sciences, Department of Plant Protection Biology, Box 102, 230 53 Alnarp, Sweden 4 Swedish University of Agricultural Sciences, Department of Agrosystems, Box 104, 230 53 Alnarp, Sweden 5 Walloon Agricultural Research Centre, Breeding & Biodiversity Unit, Rue de Liroux 4, 5030 Gembloux, Belgium 6 Plant Breeding-Wageningen University and Research Centre, PO Box 16, 6700AA, Wageningen, The Netherlands Keywords: Nectria canker, susceptibility, phenotyping Abstract European canker, caused by the fungus Neonectria ditissima, is a severe problem in apple production both in Sweden and in many other northern European countries. Even when applying fungicides and good horticultural practices, canker damage occurs almost yearly in nurseries and orchards. Some years, devastating outbreaks destroy numerous trees. To date, complete resistance to N. ditissima is not known in apple. For further research and plant breeding, heritable variation in quantitative resistance should be investigated by phenotyping large sets of cultivars. In the present project, 55 apple cultivars were screened for resistance to N. ditissima. One-year-old shoots from mature trees were inoculated in the greenhouse with a standardized volume and concentration of conidia suspension using different inoculation methods. Two-year-old trees of five cultivars were inoculated in the field. Length of the occurring cankers was measured at regular intervals throughout a period of up to three months. The investigated cultivars showed considerable differences in colonization rate. In cultivars known to be highly resistant, i.e., ‘Santana’, lesions progressed much slower compared to susceptible cultivars like ‘Cox’s Orange Pippin’ and ‘James Grieve’. Since the inoculation-based phenotyping is demanding in labour and time (duration), especially when the test is performed on grafted trees, qPCR-based assessment of fungal biomass at early stages of infection was explored as an alternative or complementary approach for phenotyping. INTRODUCTION European canker, a serious disease caused by the fungus Neonectria ditissima (Neonectria galligena, formerly Nectria galligena), creates large problems in apple production in Sweden as well as many other northern European countries. Canker damage occurs almost yearly in nurseries and orchards. Furthermore, the fungus causes rotting of fruits in storage (Brown et al., 1994). Damage on young trees, infected during propagation, is especially serious (McCracken et al., 2003). After a latent period of 3-5 years, systemic infections suddenly break out on these trees and damage the entire main stem. In some cases, epidemics have necessitated the removal of entire orchards (Jones and Aldwinckle, 1990). Despite fungicide application and good horticultural practices, trees regularly become infected in nurseries and orchards and, once infected by N. ditissima, the fungus stays in the vascular system and parenchyma tissue and continues to cause damage. Visible cankers on minor branches and side shoots can be removed by pruning. However, Proc. of the 13 th Eucarpia Symposium on Fruit Breeding and Genetics Eds.: K.M. Evans et al. Acta Hort. 976, ISHS 2013