Occurence of canker and woodrot pathogens in young apple trees and possible sources of inoculum

[摘要] ENGLISH ABSTRACT: Apples are one of the most important deciduous fruit crops cultivated in South Africa. The deciduous fruit industry identified a higher occurrence of young apple trees that died due to canker or wood rot diseases. The infected plant part starts to die back, rapidly killing the young tree shortly after establishment. Knowledge regarding the occurrence of canker or wood rot pathogens in healthy nursery trees in South African is lacking. Therefore, the aim of this study was to investigate the occurrence of stem canker and wood rot pathogens in young apple trees, as well as to identify possible inoculum sources which include mother material used for propagation.The sampling strategy was divided up into three phases namely diseased trees collected from 1-year-old commercial orchards, certified nursery apple trees and propagation material including scion mother block trees and rootstock layer blocks. Thirteen 1-year-old orchards, which exhibited dieback symptoms shortly after establishment were sampled (ten trees per orchard). One soil sample per orchard to a depth of 30 cm was also collected. The soil sample was collected near the roots of one of the diseased trees. A total of 480 certified nursery apple trees were collected from four nurseries to determine if seemingly healthy trees are infected with dieback pathogens. These certified trees adhere to the standards set out by the Scheme.Cankers, pruning wounds and green shoots from 310 trees in scion mother blocks were collected as well as asymptomatic 405 green shoots from rootstock layer blocks. Plant material was surface sterilised, cut open, and isolations were made from the internal vascular discolouration. Fungal cultures obtained during the study were identified to species level by DNA sequencing and phylogenetic analyses of either the ITS, β-tubulin or EF1α gene regions, within each specific taxonomic group.A total of 45 fungal species belonging to the taxonomic groups: Basidiomycetes, Botryosphaeriaceae, Diaporthales, Diatrypaceae, Dothideomycetes, as well as, Phaeoacremonium species and Truncatella angustata, were identified in this study. The species identified in the current study have all been associated with canker or wood rot symptoms found on fruit trees in other fruit growing regions of the world. Thirty-one canker and/or wood rot causing fungi found in this study are firstly reported in apples in South Africa, of which 27 species are also reported for the first time in apples worldwide. A latent infection level of 65% was found in certified nursery apple trees with the four most predominantly isolated pathogens, including Didymosphaeria rubi-ulmifolii s.l., Diplodia seriata, Diaporthe eres and Didymella pomorum. The majority of the pathogens were found from brown wood discoloration and white rot symptoms observed in the bud union and pruning wounds on the rootstock in nursery trees. The high infection rate in the bud union and pruning wound made on apple trees indicated that nursery trees got infected during the propagation process via aerial inoculum, which was present at the time of budding and pruning back.The same fungal species were found causing latent infections in nursery trees, which were later found to be diseased in newly established orchards. The soil analyses indicated that dieback of 1-year-old commercial trees was related to the stress conditions created when apple trees were established on sub-optimal soil. More soil samples should be collected in the future to confirm these findings. Green shoots of scion trees from mother blocks, from which buds are exiced for budding, presented 5% infection and the shoots from rootstock layer blocks had an infection of 21%. Thus, buds and rooted rootstock cuttings used during propagation can also contribute to infected nursery apple trees.Basidiomycete and Ascomycete fruiting structures were found on dead trees and cankers in the scion mother blocks, 1-year-old commercial orchards as well as in 1-year-old nursery blocks. These fruiting structures can contribute to the aerial inoculum present during propagation and establishment. However, spore trapping studies should be done in the nurseries, to determine the incidence and extend of the inoculum present during the propagation process in the nursery. This study has found that mother plant material can be the source of infected nursery trees and that a high percentage of certified nursery apple trees were infected with possible canker and wood rot pathogens resulting in the distribution of seemingly healthy apple trees to farmers.

[发布机构] Stellenbosch University