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Approaches for evaluation of resistance to European canker (Neonectria ditissima) in apple

Garkava-Gustavsson, L., Ghasemkhani, M., Zborowska, A., Englund, J. E., Lateur, M., Weg, E. van de
Acta horticulturae 2016 no.1127 pp. 75-82
Neonectria ditissima, apples, breeding, climate, conidia, crop production, cultivars, disease outbreaks, fungi, fungicides, greenhouses, horticulture, leaves, orchards, shoots, trees, Sweden
European canker is caused by the fungus Neonectria ditissima (Neonectria galligena, formerly Nectria galligena). The disease causes significant losses to apple production in Sweden and many other countries with a temperate wet climate. Application of fungicides and good horticultural practices do not prevent canker damage in nurseries and orchards. Disease outbreaks damage and even completely destroy trees. To date, complete resistance to N. ditissima is not known in apple, but cultivars differ considerably in their level of partial resistance. To be able to conduct breeding for resistance to European canker, reliable plant tests are urgently needed. Differences in resistance to N. ditissima (colonization rate; CR) in apple cultivars were evaluated by wound inoculations with a standardized number of conidia on cut shoots from mature trees placed in a climate chamber and on potted trees in an unheated greenhouse. Infection percentage (Inf%) was assessed by 'natural' inoculations in leaf scars under high infection pressure in the field. All the experiments were carried out over 2 years. Lengths of lesions were measured at regular time intervals on five occasions for shoots and seven occasions for trees. Considerable differences in CR and Inf% were found among the cultivars. The relative levels of resistance obtained are consistent with previous reports for most cultivars. Assessment of CR (area under curve) and Inf% thus proved to be useful tools for evaluation of resistance to European canker. Also, some potentially new sources of resistance were identified.