Neonectria ditissima cankers initiated in the second year of the nursery phase and at time of planting of apple trees

Abstract European canker developing in young apple orchards may have been initiated in the nursery. Artificial inoculation of Neonectria ditissima was carried out at time of heading back and manipulating of branch angles in the second year of nursery phase sized trees of 10 apple cultivars, either T-budded or grafted (with or without interstem). The trees were followed for two growing seasons and canker was found on 10% of the trees inoculated in wounds from heading back of the top shoots and on 24% of the trees inoculated in cracks from bending side branches. About 50% of those trees had visible cankers after the first growing season, clearly showing the risk of delivering non-symptomatic trees to customers when infected in year two of the nursery phase. Commercial trees of six cultivars were inoculated either in wounds (cut surfaces or cracks from bending), or on non-wounded trees at the time of planting and followed for two growing seasons. Inoculation in crack wounds from bending side branches resulted in a higher number of infected trees than cut surfaces, and both had a significantly higher incidence than non-wounded inoculated trees and their non-inoculated controls (97%, 67%, 14%, and 3 to 5%, respectively). All trees with canker after two growing seasons had symptoms after the first growing season. After two growing seasons, 50% of the trees with an infected scion had canker lesions on the rootstock. These results show that cankers discovered during the year of planting could have been newly initiated infections if the trees had been wounded and exposed to inoculum at planting.