## 1 Introduction  
Olive is an extremely important crop for Mediterranean Basin countries, including Portugal [1]. Olive anthracose, mainly caused by diverse fungi clustering in the _Colletotrichum acutatum_ species complex, is a key constraint to olive production through its effects on olives. The yield loss caused by anthracose can reach 80-100% in some diverging regions of the world [2]. The control of this disease is challenging, being mostly based on the use of copper-based products [2]. Besides its limited efficacy, the use of these products has also led to several environmental problems [3]. Therefore, there is an urgent need to develop more effective and environmentally friendly tools to control anthracose. This strategy has long been promoted by the EU (Directive 2009/128/EC), and more recently, it has formed part of the European Green Deal, particularly the Farm to Fork Strategy, which aims to reduce pesticide use in the EU by 50% by 2030. One possible approach for meeting this demand is the development of biological control products based on the use of endophytes. There is increasing evidence that this microbial community, living in internal plant tissues, can help host plants deal with pests and diseases usingvarious mechanisms [4]. This beneficial effect is mainly ascribed to the production of secondary metabolites by the endophyte, including volatile organic compounds (VOCs) [4]. Endophyte-derived VOCs are active in disease control as direct antimicrobial agents and as resistance inducers, preventing plant colonization by pathogens [5]. They can also protect plants from pests and diseases by mediating intra- and interspecific interactions in a broad array of organisms [6]. For instance, endophytes can alter the composition of plant volatile organic compounds, making it more attractive to natural enemies of insect pests, and thus minimizing pest infestations [7]. Hence, this newly emerging but not yet fully understood role of endophytes provides opportunities for anthracose management. Compared to non-volatile metabolites, endophytic VOCs have significant advantages in biological control. Due to their chemical proprieties, VOCs can spread over large distances, thus having a long-distance range of action, and are easily degraded [5]. Several studies have investigated the fungal endophytic communities of olive tree tissues using cultivation-dependent [8; 9; 10; 11] and independent methods [12; 13]. Curiously, some of these studies have shown that fungal endophytes colonizing an olive tree may play an important role in protecting the host plant against anthracose [10; 11]. Indeed, olive tree cultivars with different susceptibilities to anthracose were shown to harbor distinct endophytic fungal communities under the same pathogen pressure [11]. Each olive cultivar apparently was selective towards specific endophytic taxa, which may confer these differences in host susceptibilities towards anthracose. The differences found in fungal ecological function profiles among olive tree cultivars, with some cultivars showing a high rate of beneficial endophytes than others, reinforce our hypothesis [12]. Some of the beneficial endophytes associated with olive trees belong to fungal genera with a recognized role in controlling plant pathogens. In this regard, both the _Aureobasidium_ and _Surocladium_ genera hold great promise in the protection of olive trees against diseases, due to their recognized ability to act as biocontrol agents against an array of plant pathogens [14; 15; 16; 17; 18; 19; 20]. Thus, the main aim of this study was to evaluate the ability of the endophytes _A. pullulans_ and _S. summerbellii_, previously isolated from an olive tree cultivar moderately resistant to anthracose, to inhibit _C. acutatum_ growth and anthracose symptoms. An important prerequisite for developing and applying effective microbial control agents is a solid knowledge of their mode of action.