Balancing nitrogen fertilization in apples: Boosting plant growth or European canker?

Abstract: European canker remains one of the most threatening diseases of apple in countries
with a temperate climate. The cause of the disease, the fungal pathogen Neonectria ditissima,
infects the host through several types of wounds, colonizes the bark tissues, eventually leading to tree death. Nitrogen(N)-containing products are commonly used in apple tree nurseries and orchards to boost plant growth. These products have also been related to an increased level of European canker (Campbell et al., 2018; Dryden et al., 2016; Vorster et al., 2021). This research aimed to determine if the increase of disease is caused by increased activity in N. ditissima growth and germination or by the boosted growth of the trees. Laboratory assays growing isolates on enriched PDA with 0 %, 0.5 %, 1 %, 3 % and 5 % of urea demonstrated no effect on the hyphal growth or spore germination percentage. Freshly grafted cv. Gala trees were potted in soil supplemented with 0, 50, 150 and 300 kg N/ha over the growing season. The trees were inoculated in the autumn by hanging sporulating cankers above trees or by artificial inoculation of 4 wounds per tree (N = 10). Results were obtained in spring. The percentage of diseased leaf buds when hanging cankers above trees were 2 ± 1 %, 39 ± 7 %, 37 ± 9 % and 61 ± 10 % for the N concentrations of 0, 50, 150 and 300 kg N/ha, respectively. Interestingly, the average canker lesion length on the plants grown with 150 and 300 kg N/ha was 20 ± 2mm, about double the size on the trees grown on soil with 0 or 50 kg N/ha. The outcome of this research may help refine the use of nitrogen fertilizer for improved disease management strategies.