Survey of stem cankers and dieback symptoms of young apple trees and possible inoculum sources

Recently, a higher occurrence of canker development on 1-year-old apple trees shortly after establishment was observed in apple orchards in the Western Cape Province of South Africa. Knowledge regarding latent infections of canker or wood rot pathogens in certified nursery trees in South Africa is lacking. This study aimed to assess the phytosanitary status of nursery trees in relation to canker and wood rot pathogens and to investigate propagation material as possible inoculum sources

Thirteen 1-year-old apple orchards showing canker or dieback symptoms were sampled from three apple producing areas in the Western Cape. Certified nursery apple trees were collected from four nurseries as well as scion and rootstock mother plant material, which are used during propagation of apple trees. The propagation material sampled include cankers and pruning wounds from apple scion mother blocks, 1-year-old scion shoots, rootstock layer blocks, and diseased 1-year-old rootstock shoots. Isolations were made from the discolouration observed in the vascular tissue of the plant parts and from asymptomatic material. Causal fungal species involved in the infection of the apple plant material were identified. Pathogenicity trials were done with 72 fungal isolates representing 39 species inoculated onto two-year-old branches of 14-year-old Golden Delicious trees. Lesions lengths were measured and fungi re-isolated after 5 months.

Similar canker and wood rot fungi were isolated from 1-year-old diseased apple trees, nursery apple trees and from the propagation material. A total of 44 fungal species associated with canker or wood rot symptoms reported on fruit trees or other woody hosts were identified in this study. Didymosphaeria rubi-ulmifolii s.l. was the dominant fungal specie causing latent infection in the certified nursery apple trees and was also found causing dieback in 1-year-old apple trees. Other canker and wood rot pathogens isolated belonged to the Basidiomycetes, Botryosphaeriaceae and Diatrypaceae. It also included species in the genera Didymella, Didymosphaeria, Phaeoacremonium, Diaporthe, Cadophora, Coniochaeta, Cytospora and Truncatella angustata. Sixty-five percent of certified nursery apple trees were infected with canker and wood rot pathogens. The bud unions and pruning wounds on the rootstock were most infected in the nursery trees, suggesting that aerial spores are also sources of inoculum. Basidiomycete and Ascomycete fruiting structures were found on the scion mother block trees, 1-year-old apple trees, and the 1-year-old nursery rootstock shoots which can contribute to aerial inoculum. Propagation material is also a source of inoculum with scion shoots used for budding and rooted rootstock cuttings from layer blocks having respective 5% and 21% latent infections of canker and wood rot pathogens. Neofusicoccum viticlavatum, D. foeniculina and D. perniciosa caused the longest lesions, while all the other species caused lesions that were significantly longer than the control. Twenty-five new records for apples world-wide and 25 new records for apples in South Africa were found.

This study confirmed the presence of canker and wood rot pathogens in apple propagation material as well as certified nursery apple trees. Management should focus on i) the sanitation of orchards and nursery fields of dead shoots and branches; ii) the production of cleaner propagation material and iii) improved wound protection of nursery trees.