A recent olive trunk disease survey performed in the Western Cape Province, South Africa, identified several fungi associated with olive trunk disease symptoms, including species of Basidiomycota, Botryosphaeriaceae, Coniochaetaceae, Calosphaeriaceae, Diaporthaceae, Diatrypaceae, Phaeomoniellaceae, Phaeosphaeriaceae, Symbiotaphrinaceae, Togniniaceae and Valsaceae. Many of the species recovered had not yet been reported from olive trees and therefore the aim of this study was to determine their pathogenicity towards this host. Pathogenicity tests were first conducted on detached shoots to select virulent isolates which were then used in field trials. During field trials, 2-year-old olive branches of 15-year-old trees were inoculated by inserting colonised agar plugs into artificially wounded tissue. Measurements were made of the internal lesions after 8 months. In total, 58 isolates were selected for the field trials. Species that formed lesions significantly larger than the control could be considered as olive trunk pathogens. These include Biscogniauxia rosacearum, Celerioriella umnquma, Coniochaeta velutina, Coniothyrium ferrarisianum, isolates of the Cytospora pruinosa complex, Didymocyrtis banksiae, Diaporthe foeniculina, Eutypa lata, Fomitiporella viticola, Neofusicoccum stellenboschiana, Nm. vitifusiforme, Neophaeomoniella niveniae, Phaeoacremonium africanum, Pm. minimum, Pm. oleae, Pm. parasiticum, Pm. prunicola, Pm. scolyti, Pm. spadicum, Pleurostoma richardsiae, Pseudophaeomoniella globosa, Punctularia atropurpurascens, Vredendaliella oleae, an undescribed Cytospora sp., Geosmithia sp., two undescribed Neofusicoccum spp. and four Xenocylindrosporium spp. Pseudophaeomoniella globosa can be regarded as one of the main olive trunk pathogens in South Africa, due to its high incidence from olive trunk disease symptoms in established orchards and due to its high virulence in pathogenicity trials.
Trunk disease fungal pathogens reduce olive production globally by causing cankers, dieback, and other decline-related symptoms on olive trees. Very few fungi have been reported in association with olive dieback and decline in South Africa. Many of the fungal species reported from symptomatic olive trees in other countries have broad host ranges and are known to occur on other woody host plants in the Western Cape province, the main olive production region of South Africa. This survey investigated the diversity of fungi and symptoms associated with olive dieback and decline in South Africa. Isolations were made from internal wood symptoms of 145 European and 42 wild olive trees sampled in 10 and 9 districts, respectively. A total of 99 taxa were identified among 440 fungal isolates using combinations of morphological and molecular techniques. A new species of Pseudophaeomoniella, P. globosa, had the highest incidence, being recovered from 42.8 % of European and 54.8 % of wild olive samples. This species was recovered from 9 of the 10 districts where European olive trees were sampled and from all districts where wild olive trees were sampled. Members of the Phaeomoniellales (mainly P. globosa) were the most prevalent fungi in five of the seven symptom types considered, the only exceptions being twig dieback, where members of the Botryosphaeriaceae were more common, and soft/white rot where only Basidiomycota were recovered. Several of the species identified are known as pathogens of olives or other woody crops either in South Africa or elsewhere in the world, including species of Neofusicoccum, Phaeoacremonium, and Pleurostoma richardsiae. However, 81 of the 99 taxa identified have not previously been recorded on olive trees and have unknown interactions with this host. These taxa include one new genus and several putative new species, of which four are formally described as Celerioriella umnquma sp. nov., Pseudophaeomoniella globosa sp. nov., Vredendaliella oleae gen. & sp. nov., and Xenocylindrosporium margaritarum sp. nov.
Several fungal trunk pathogens are associated with olive trunk diseases in South Africa. Little is known regarding the inoculum sources of these pathogens in the olive industry and no specific management strategies are in place. The aim of this study was to investigate the status of olive nurseries in South Africa, with regard to the presence of trunk pathogens in olive plant material, to determine whether nursery material can be considered inoculum sources contributing to long distance dispersal of these pathogens. Isolations were made from asymptomatic cuttings from mother blocks (Stage 1), asymptomatic and symptomatic rooted cuttings (Stage 2) and 1- to 2-year-old trees (Stage 3) of eight cultivars in two nurseries. Known olive trunk pathogens of Nectriaceae, Diaporthaceae, Botrysphaeriaceae, Togniniaceae, Phaeomoniellaceae and Pleurostomataceae were recovered. Neofusicoccum australe was detected in a single Stage 1 cutting. Stage 3 material showed the highest incidence of fungi from these families, with Pleurostoma richardsiae having the highest incidence in both nurseries (82.2% and 36.7% of the 1- to 2-year-old trees). Phaeoacremonium parasiticum was present in 28.9% of the trees from one nursery (Stage 3). The remaining pathogens occurred in 13.3% or less of the material. These results indicate that nursery propagation material from mother blocks harbour low levels of trunk pathogens, and that additional infections occur during the nursery process. Management strategies should focus on the prevention and elimination of infections in mother blocks, as well as during the propagation process to ensure pathogen-free material is delivered to producers.
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