Pathogenicity Testing of Fungal Isolates Associated with Olive Trunk Diseases in South Africa

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; therefore, the aim of this study was to determine their pathogenicity toward 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 colonized 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 included Biscogniauxia rosacearum, Celerioriella umnquma, Coniochaeta velutina, Coniothyrium ferrarisianum, isolates of the Cytospora pruinosa complex, Didymocyrtis banksiae, Diaporthe foeniculina, Eutypa lata, Fomitiporella viticola, Neofusicoccum stellenboschiana, Neofusicoccum vitifusiforme, Neophaeomoniella niveniae, Phaeoacremonium africanum, Phaeoacremonium minimum, Phaeoacremonium oleae, Phaeoacremonium parasiticum, Phaeoacremonium prunicola, Phaeoacremonium scolyti, Phaeoacremonium 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 because of its high incidence from olive trunk disease symptoms in established orchards and its high virulence in pathogenicity trials.