## 1 Introduction  
_Fusarium oxysporum_ Schltdl. emend. Snyder & Hansen is a dangerous forest plant pathogen and is listed among the ten most important fungal pathogens in plant pathology [1; 2; 3]. _Fusarium oxysporum_ colonise plants at different growth stages and cause various forms of disease in economically important plants around the world [1; 4; 5]. _Fusarium oxysporum_ can cause the damping-off, spotting, wilting and death of buds and whole plants with a fairly rapid progression, which can seriously reduce growth and yield. The species has been identified as the main cause of root rot in nurseries [4]. A mycelium can be observed on the surface of the infected plant, which can turn pink in an advanced stage due to the mass production of _Fusarium_ spores. _F. oxysporum_ can spread to neighbouring plants, so an infected plant can pose a significant threat to the entire crop [6; 7; 8]. _F. oxysporum_ can also colonise plants asymptmatically, making it difficult to identify the cause of the disease [9]. For this reason, it is particularly important to prevent the occurrence of pathogens. Although diseases caused by _Fusarium_ spread in the soil, they can also affect plants grown in a soilless system [10].  
The most commonly used agents to control _Fusarium_ are those containing chemicals from the group of triazoles [11], benzimidazoles [12], anilinopyrimidines [13] or quinazolines [14]. However, the chemical control of _Fusarium_ on infected plants is difficult [10; 15]. In addition, today's society pays particular attention to the excessive chemicalisation of the environment.  
The modern world faces the challenge of environmental pollution, including agriculture and forestry [16]. Legal regulations aim to limit excessive chemicalisation. Directive 2009/128/EC of the European Parliament and of the Council is based on the reduction in the use of chemical substances, the protection of biodiversity and the promotion of sustainable development. The most important measures include reducing the use of pesticides and restoring natural resources [17]. The reduction in chemicalisation can be achieved through the application of integrated pest management. This approach is based on minimising the use of pesticides and focuses instead on prevention. New, safer methods for the biological protection of plants against pathogens such as _F. oxysporum_ are therefore being sought [18]. The use of biopesticides, which are produced from metabolic products of microorganisms, plays an important role in integrated pest management [19].  
Some of the _Bacillus_ bacteria can be used as biological control agents (BCAs) against plant pathogens. They produce volatile organic compounds. Volatile organic compounds (VOCs) are low-molecular-weight organic compounds (<300 Da) that generally have lipophilic properties and a low boiling point [20]. VOCs can be produced by bacterial and fungal microorganisms [18; 20]. In this case, the term microbial VOCs (abbreviated mVOCs) is used to refer to this group of compounds. The importance of researching mVOCs is demonstrated by the fact that a database called mVOC Database 4.0 [21] has been created, which contains information on 3500 microbial VOCs produced by various species of bacteria and fungi. About 10% of all mVOCs can be assigned a biological function [22]; therefore, some mVOCs have the potential to be used in agriculture or forestry for the biological control of pathogens in the future. Microorganisms produce various inorganic and organic volatile compounds that can be widely utilised as infochemicals in interactions with plants, fungi and bacteria [16; 23; 24]The aim of the present study was to determine the effect of volatile organic compounds produced by three species of _Bacillus_ bacteria on the inhibition of _F. oxysporum_ growth.