Fungi associated with dieback of apple trees to be excavated and the assessment of Diplodia seriata fruiting structures on chipped apple wood used for mulch

Excavated apple trees as part of orchard renewal are often chipped and used as mulch in younger orchards. Therefore, the extent of fungal pathogens associated with dieback in old apple orchards to be excavated was investigated, with an emphasis on Diplodia seriata, an important canker pathogen of Malus domestica in South Africa. Trees showing symptoms of dieback were sampled from 14 orchards in the Grabouw, Vyeboom and Kouebokkeveld apple-producing areas in South Africa. Isolations were made from surface sterilised internal tissue of dieback samples. For the detection of D. seriata inoculum from chipped apple wood pieces, samples were collected in October 2020 and from February to April 2021 of orchards in the Grabouw (2 orchards) and Vyeboom (1 orchard) production regions. Fungi associated with canker and wood rot symptoms were isolated from 118 of the 144 trees showing symptoms of dieback. Eutypa lata was most often isolated (from 18.6 % of trees) followed by D. seriata (9.3 %). The predominant wood rot pathogen associated with the dieback of apple trees was Trametes versicolor (from 33.1 % of trees). Visual inspections of mulch wood pieces indicated the presence of D. seriata pycnidia and or conidia from material assessed between October 2020 and April 2021. Viable D. seriata cultures were obtained from 87 % of the mulch wood pieces investigated. Diplodia seriata remained present and viable for a period of 20 months on the wood chips. This study showed that apple trees to be excavated and used for mulch, harbor important canker and wood rot pathogens. The presence of viable D. seriata inoculum on apple tree wood chips used for mulch in younger orchards illustrates that there is a risk involved in using wood chips made from old fruit trees. Infected tree parts and entire dead trees should thus be removed before the chipping of older apple trees to be used for mulch.