New research by a recent PhD graduate of Stellenbosch University has shed light on what olive tree farmers can do to curb olive trunk diseases in their orchards. Among these tips are to prune trees when humidity levels are low, to apply wound protection within the first week after pruning, and to remove shredded branches from orchards.

The research was done by Dr. Meagan van Dyk of the Strand, as part of her PhD in Plant Pathology. Her research was funded by ARC Infruitec-Nietvoorbij, the National Research Foundation and the Department Trade and Industry’s THRIP programme in collaboration with SA Olive Industry Association. She was based at the Agricultural Research Council’s Infruitec-Nietvoorbij campus in Stellenbosch while completing her four-year research project under the supervision of Prof Francois Halleen (ARC Infruitec-Nietvoorbij), Prof Lizel Mostert (SU Department of Plant Pathology) and Dr. Chris Spies (ARC Plant Health and Protection).

“The olive industry in South Africa produces an excellent quality olive oil. However, the Olive Sector Development Plan of the Department of Trade and Industry identified low production and the lack of local research as weaknesses of the olive industry in South Africa. The effective management of pests and diseases, including olive trunk diseases, forms an integral part in improving the lifespan and yield of olive trees, and increasing olive production in especially the Western Cape,” says Prof Halleen of the ARC, who also holds the position as an extraordinary associate professor in the SU Department of Plant Pathology.

Prof Halleen says that trunk diseases typically take a long time to develop. It starts with the dieback of shoots and then spreads to the rest of the tree. He reckons that good harvests can still be taken off tries that farmers have managed well.

Dr. Van Dyk identified at least 33 fungal species as potential olive trunk pathogens. These were found in established orchards, and worryingly, some also in nursery stock. A new widespread Pseudophaeomoniella species was fingered as one of the most important olive trunk pathogens. It was found in orchards and in lower incidences in nurseries.

Pruning is an essential part of maintaining olive production, and it is also a way of controlling diseases. It helps to increase light distribution and aeration in an orchard, which in turn helps to ensure that fungicide spray is applied more uniformly all over trees.

It is, however, also a period on an olive farm during which trees are highly susceptible to infection. Pruning wounds were most susceptible within the first week after pruning. However, in some cases, Pseudophaeomoniella species infections occurred up to 42 days after pruning.

No clear-cut season (either winter or spring) stood out during which it is better to do pruning in an effort to prevent trunk disease. Higher humidity levels did, however, play a role.

In terms of pruning processes, Dr van Dyk, therefore, advises that:

  • Pruning is done while humidity levels are low, irrespective of the season.
  • Wound protection is applied to pruning wounds within the first week after pruning.
  • Farmers should not only remove and burn larger diseased branches from infected trees and orchards. Smaller branches, which are generally shredded, should also be removed because these can still sustain microbial communities and cause new infections.

In terms of the use of pruning wound protectants, Dr van Dyk found that:

  • Under high inoculum pressure, Tree Seal and Coprox Super/Bendazid consistently provided the best protection against Pseudophaeomoniella sp. infections.
  • Tree Seal provided protection of the pruning wounds during low inoculum pressure.
  • Under low inoculum pressure MT1, a Trichoderma-based product currently being developed by the SU Department of Plant Pathology was also able to reduce the incidence of Pseudophaeomoniella sp. within the first week after pruning.

Dr van Dyk’s studies with MT1 were done on “Frantoio” olive trees.

“The results suggest that MT1 can effectively be used to manage olive trunk diseases of the pruning wounds of ‘Frantoio’ olive trees in well-maintained orchards that are expected to have a low inoculum pressure,” she adds.

She’d like to see further evaluations of pruning wound protectants on other olive cultivars in different climatic and geographic areas too and against other potential olive trunk pathogens, such as Botryosphaeriaceae and Phaeoacremonium spp.

Two olive tree nurseries were sampled during the course of her research. Some young potted olive trees were found to be planted in soil infected with Phaeoacremonium parasiticum and Pleurostoma richardsiae.

“The fact that some nursery material was found to be infected with notable trunk pathogens highlights the need for more stringent disease management practices to ensure that pathogen-free material is delivered to producers,” says Dr van Dyk. “Further investigations of these pathogens occurring in the soil and 1–2-year old trees should be performed to establish whether this was an isolated event or if it is a chronic occurrence,” she notes.

Most of the olive trunk pathogens that Dr van Dyk found in nursery stock were detected from the parts of the plant that was buried in potting soil.

“This suggests that the majority of the infections developed from the basal end of the cuttings that were in direct contact with perlite, soil and water,” she adds. “I’d like to do further investigations to determine which of these components are contaminated, and how one would be able to prevent or decontaminate it.”

“This study led to new knowledge with regards to olive trunk diseases, their pathogenicity, detection, epidemiology and control which can be used for the development of improved management strategies of olive trunk diseases in South Africa,” says Prof Lizel Mostert of the SU Department of Plant Pathology.

More information about Dr. Van Dyk

Dr. Meagan van Dyk grew up in Tierfontein, a farming community near Malmesbury. She matriculated from La Rochelle Girls’ High School, Paarl. Dr van Dyk completed a BSc in Crop Protection and Breeding from Stellenbosch University, before starting her postgraduate studies in Plant Pathology.

Dr. Meagan van Dyk had the opportunity to present some of her findings at the 2018 International Congress of Plant Pathology in Boston in the US. Photo: Francois Halleen

A brown V-shaped canker was observed in an infected olive branch, typically caused by one of the trunk disease pathogens that Dr van Dyk investigated. Photo: Francois Halleen

Dr. Meagan van Dyk (middle) during one of the nursery surveys conducted, along with some of her study leaders. They were Dr Chris Spies of the Agricultural Research Council and Prof Lizel Mostert of the Department of Plant Pathology at Stellenbosch University. Photo Francois Halleen

For media enquiries only:
Prof Francois Halleen
ARC Infruitec-Nietvoorbij and extraordinary associate professor in the Department of Plant Pathology, Stellenbosch University
083 415 9152


April 2020
Article written by Engela Duvenage, for the Faculty of AgriSciences, Stellenbosch University