## 1 Introduction  
Entomopathogenic fungi in the form of microscopic spores adhere to the outer surfaces of insect bodies. When the spores germinate, they penetrate the host body and proliferate in the body cavity, ultimately killing the host [1]. The fungi are recognized as environmentally safe natural-mortality agents for the biological control of insects and other arthropod pests [2]. They are also considered rich sources of structurally novel biologically active substances [3]. The common and important entomopathogenic fungus is the teleomorphic phase of the genus _Cordycepus_, which includes anamorphic phase names of _Beauveria_, _Isaria_, _Hirstella_, _Metarhizium_, and _Nomuraea_. To apply these insect-killing fungi to biological control and other biotechnology areas, an understanding of their ability to tolerate diverse environmental stresses is required. The effects of ionizing radiation, plasma, ultraviolet (UV) light, and heat-treatment stresses on fungal cell inactivation have been previously studied. Nonthermal-atmospheric-pressure plasma jets (APPJs) and dielectric-barrier-discharge plasma have been used for various plasma treatments, such as cancer treatment, wound healing, sterilization and inactivation of biological materials [4; 5; 6; 7; 8; 9; 10; 11]. However, the effects of plasma treatment on eukaryotic microbiological materials, such as entomopathogenic fungal cells, have not been studied thoroughly. There have been only two reports in which oxidative stresses generated by an APPJ device were applied to _Cordyceps bassiana_ and _C. pruinosa_ spores [12; 13]. In these previous studies, both plasma radicals and an electric field were applied to the fungal sample by positioning it between a needle-shaped powered electrode and a surface electrode attached to the bottom of a microwell plate. If the plasma radicals are generated through discharge between the coaxial electrodes, the radicals can be applied to the fungal samples without an electric shock. But, it has not been thoroughly investigated how the fungal cells can be stressed by plasma radicals in the absence of electric shocks. Reactive oxygen and nitrogen species (RONS) from the plasma itself and plasma-activated water depend on parameters such as the plasma devices, plasma working gases, and background medium. Therefore, the types and amounts of plasma-generated RONS can be determined by selecting an appropriate plasma working gas. The atmospheric-pressure air plasma jet generates unique RONS different from those of the atmospheric-pressure _Ar_ plasma jet. If an electric shock-free, atmospheric-pressure air plasma jet is used to treat _C. pruinosa_ spores in an aqueous solution, the cellular response of the air plasma-treated fungal spores will be different to that of _Ar_ plasma-treated fungal spores due to the difference in plasma-induced RONS. This study was undertaken to elucidate the death mechanism of fungal spores using an electric shock-free, atmospheric-pressure air plasma jet, simply called soft plasma jet. For this aim, we measured the viability of the plasma-treated spores and investigated how the degree of DNA damage and cell wall rigidity affect the viability of plasma-treated _C. pruinosa_ spores. Viability of the plasma-treated spores was investigated by counting the number of living and dead spores. Plasma-generated reactive species were examined by optical emission spectra, and intracellular ROS was confirmed by measuring the fluorescence of \(H_{2}\)_DCF-DA_-stained _C. pruinosa_ spores. Cell shapes and wall surfaces of plasma-treated spores were imaged using a scanning electron microscope (SEM), and the loss of cell wall integrity was measured using the fluorescence intensity of propidium iodide (PI)-stained spores. The effect of plasma treatment on _C. pruinosa_ DNA was determined by measuring phosphorylation of the histone protein from double-strand breaks.