Assessing the threat of the polyphagous shot hole borer ambrosia beetle and its fungal symbiont – Fusarium euwallaceae, to Persea americana, in South Africa

Abstract

The polyphagous shot hole borer (PSHB) or Euwallacea fornicatus is an ambrosia beetle, originally from South-East Asia, that can cause extensive damage, to a large variety of trees, by acting as a vector to inoculate its pathogenic fungal partner, Fusarium euwallaceae, directly into host plants, causing Fusarium Dieback (FD) (Eskalen et al., 2012, Freeman et al., 2013). In a recent study in South Africa, this ambrosia beetle-fungal complex was detected, damaging Platanus x acerifolia (London Plane) trees (Paap et al., 2018). This observation caused concern to local agriculture, particularly to avocado growing communities, as this invasive beetle is internationally notorious for the damage it has caused specifically to avocado, in Israel (Mendel et al., 2012) and California, in the United States of America (Rugman-Jones & Stouthamer, 2016). This study was therefore commenced to survey, monitor and identify the beetles associated with disease on avocado and to identify fungal pathogens/symbionts connected to beetle-associated damage on avocado plants, in South Africa. During this investigation, the presence of several wood-boring beetles and fungi associated with damage to avocado were detected. Alongside several ambrosia beetles and ambrosial fungi that were detected, the causal agent for FD (F. euwallaceae) was detected causing damage to a residential avocado tree, in Johannesburg, in Gauteng. Molecular and phylogenetic identification confirmed the isolate’s identity as being genetically identical to the isolates from avocado, in Israel and California (Mendel et al., 2012, Freeman et al., 2013, O’Donnell et al., 2016, Na et al., 2018). A subsequent pathogenicity trial fulfilled Koch’s Postulates and confirmed the isolate’s pathogenicity to avocado. This finding was submitted for publication in The American Phytopathological Society Journal: Plant Disease - 10-18-1818-PDN. The impact of this discovery emphasises the introduction of this pest complex and indicates significant geographic spread from where it was first detected in South Africa, in Pietermaritzburg, in KwaZulu-Natal. This intention of this research was to highlight the threat posed by ambrosia beetles, particularly the PSHB, to local agriculture, specifically to that of the avocado industry as well as the biodiversity of native and indigenous plants. In South Africa, a lack of previous attention to beetle damage means that currentlyapplied agricultural practices are unequipped and uninformed to detect, diagnose and treat the disease and to prevent its spread. Therefore, this study also performed investigations towards trying to combat FD. This included screening the susceptibility of two avocado fruit cultivars, to two fungal symbionts associated with the PSHB. This study found evidence that suggests standalone pathogenicity for the two symbionts (F. euwallaceae and Graphium euwallaceae), that were assessed, to both cultivars and also found that a co-inoculation of the two symbionts resulted in substantially more severe disease symptoms than when inoculated on their own. These findings indicate the complexity of this disease and the importance of studying the overall microbial diversity associated with vector organisms, such as ambrosia beetles, towards better understanding the interactions between the organisms involved in the causation of disease. Additionally, the efficacy of different chemical and biological control agents, against F. euwallaceae, were assessed. In vitro assays proved that all of the chemical and biological agents that were tested in this study, indicated promising prospects, as their action led to a significant decrease in the growth of F. euwallaceae mycelia. For two of the chemical agents, complete mycelial inhibition was observed. However, when comparing the sensitivity of these agents to what is observed in planta (Mayorquin et al., 2018), to the results from this study, underestimations and limitations of effective in vitro concentrations were emphasised. These findings contribute towards the management of FD, by indicating increased pathogenicity of the combined effects of the fungal associates of the PSHB and also addresses the high demand for the consideration and application of potential chemical and biological agents for the treatment of FD, in South Africa. The findings and conclusions from this thesis provide a basis for the further study of the PSHB and other ambrosia beetles, in South Africa, particularly on avocado. Research towards ambrosia beetles should be aimed at developing existing knowledge and contributing towards sustainable, effective and holistic management systems.