Browsing by Author "Jacobs, Vernon Guy"

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    Assessment of dieback pathogens of apple trees to be removed and the detection of diplodia seriata on chipped apple wood
    (Stellenbosch : Stellenbosch University, 2022-03) Jacobs, Vernon Guy; Mostert, Lizel; Halleen, Francois; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: The use of chipped fruit trees as mulch in apple orchards is a common practice in the deciduous fruit industry of South Africa. Apple trees destined to be removed are usually old, do not produce optimally or need to make way for new varieties. These apple trees often have visible dieback symptoms, caused by canker and wood rot pathogens. The use of mulch produced from infected trees holds a risk to young or newly established apple orchards that could get infected with the spread of diseased wood chips as mulch. However, the presence and ability of canker and wood rot pathogens to survive on mulch made from apple trees need to be determined. Therefore, the aim of this study was to assess the diversity of canker and wood rot pathogens present on apple trees destined to be removed and chipped, and to determine the presence and viability of the canker pathogen, Diplodia seriata, on chipped apple tree wood used for mulches. The diversity of canker and wood rot pathogens present in apple trees to be removed was determined. Branches or trunks showing dieback symptoms, cankers or wood rot were sampled from 14 orchards in the Grabouw, Vyeboom and Kouebokkeveld apple producing areas in the Western Cape of South Africa. Isolations were made from a diseased branch or trunk sample taken from ten trees of every orchard. Canker and wood rot pathogens were isolated from 118 of the 144 trees sampled. Known canker pathogens identified included Cytospora parasitica, Diaporthe eres, Diplodia seriata, Eutypa lata, Phaeoacremonium fraxinopennsylvanicum, Pm. minimum and Pm. viticola. Of the canker pathogens, E. lata was predominantly isolated, followed by D. seriata and species of Phaeoacremonium. Species not frequently reported were Cy. parasitica and Dia. eres. Lesser-known fungi reported included Coniochaeta ligniaria, C. velutina and Pleurostoma richardsiae. The most diverse taxonomic group identified was the wood rotting fungus Agaricomycetes. Trametes versicolor was predominantly isolated. Chondrostereum purpureum and Schizophyllum commune were reported less. Lesser-known wood rot species identified included Bjerkandera adusta, Coprinellus micaceus, Fomitiporia capensis, Fomitiporella americana, Fo. viticola, Inocutis spp. (Taxon 3), Oxyporus latemarginatus, Phanerochaeta chrysosporium, and Phlebia sp. Several fungal species were reported for the first time from apple trees: C. ligniaria, Pl. richardsiae, F. capensis and two species of Fomitiporella, Fo. americana and Fo. viticola. Diplodia seriata is an important canker pathogen of apple trees often associated with dieback symptoms of young and mature trees. The presence of D. seriata on chipped wood pieces was investigated. Chipped apple wood pieces were sampled from heaps and when spread onto the orchard floor of three orchards in the Grabouw and Vyeboom production regions. Wood chips were sampled four times from October 2020 (heaps) to April 2021 (orchard floor). Visual inspections were done on samples collected, whereas quantitative PCR (qPCR) analyses of DNA isolated from water washes of wood pieces were done on samples from the first and last sampling. Two apple wood chip heaps were generated in the winter of 2020 (F1-2020, F2-2020), and one heap from 2019 (F1-2019). The presence of D. seriata pycnidia and viable conidia was investigated on a selection of samples. Pycnidia and/ or conidia of D. seriata were present on all wood chips assessed for F1-2019, F1-2020 and F2- 2020 sampled in October 2020 and April 2021. Diplodia seriata cultures obtained from 52 of 60 wood chips were able to survive for 20-months (F1-2019). A qPCR assay was developed to detect D. seriata from mulch wood pieces. The species-specific primers developed was specific for D. seriata and the limit of detection and limit of quantification were 571 fg and 2859 fg, respectively. Diplodia seriata was detected from DNA extracted from water washes of wood chips sampled in October 2020 and April 2021 from all three orchards. From 120 samples, 84% tested positive for D. seriata. This study showed that apple trees, chipped and used for mulch, harbour important canker and wood rot pathogens. Many of these pathogens can form fruiting structures on the wood. Visual field observations during this study confirmed the presence of basidiocarps for wood rot pathogens such T. versicolor and S. commune and pycnidia of canker pathogen, D. seriata, from cankered areas on trees in orchards sampled from. Spores could easily be distributed from wood chips made from infected trees as inoculum could be present. The presence of viable D. seriata inoculum on apple tree wood chips used for mulch in younger orchards illustrates a risk of using wood chips made from old orchards. A similar risk could be expected for other canker and wood rot pathogens associated with dieback of apple trees. However, further investigation should be done to assess the risk of transmission of other known disease-causing pathogens. The level of dieback of older orchards further contributes to the risk of the mulch. Decision-making should, therefore, include assessing the health status of orchards to be removed and if it should be used as mulch. Alternatively cankered wood or dead wood can be removed before the removal of orchards and chipping. If apple tree wood chips are used as mulch other options can be explored such as composting and the application of heat treatments to ensure that mulch material does not contribute to inoculum of canker and wood rot pathogens.