The genus Beauveria is often used as a mycoinsecticides in many countries to control insect pests in agriculture. The conidia of entomopathogenic fungi (EPF) must retain high viability and virulence for effective biological control to ensue (Hong et al. 2002). Biocontrol strategies based on entomopathogenic fungi are not only dependent upon the interaction between host and pathogen but also on the environment to which they are exposed (Mishra et al. 2013). In this study, two B. bassiana strains differed somewhat in their virulence against H. cunea, which might be related to their biological properties. Multiple abiotic factors were shown capable of influencing conidiation in Bb10331 and Bb7725, including the medium type, carbon to nitrogen ratio, and the temperature. The two strains’ colony diameter, conidiation capacity, and their conidia germination rate were directly impacted by RH, illumination regime, and ambient pH.
Humidity is one of the most important environmental factors affecting the survival and activity of entomopathogenic fungi (Bugti et al. 2020). Not only concentration it affect fungal germination, it is also essential for EPF infection and subsequent sporulation on insect cadavers (Mishra et al. 2013). Early work by Ferron (1977) demonstrated that 92% RH is required for B. bassiana to get conidial germination in the absence of the host. However, in this study, when RH is 70%, the conidial germination rate of both strains could surpass 70%. That is to say, the relative humidity required for conidial germination of different fungal strains varies among them. In addition, the infection potential of EPF also depends upon the exposure time to favorable humidity conditions, given the positive correlation between them (Fargues and Luz 2000). Our results indicated that the optimum RH for the Bb10331 and Bb7725 strains was 95–100%. However, it is necessary to further compare the infection of these two strains against H. cunea at different RH levels in subsequent studies.
Illumination is one of the many signals that fungi use to perceive and to interact with their environment, providing them with critical information about their habitat (Corrochano 2007). Many studies have shown that illumination regimes have conspicuous effects on mycelial growth, conidial germination, and conidiation capacity (Fanelli et al. 2012; Dias et al. 2019). Fungal species can exhibit differential responses to light; for example, fungi sense light to protect themselves against DNA damage caused by certain color or solar ultraviolet wavelengths, by timing the release of spores or by inducing enhanced tolerance to stress for better physiological adaptation (Chelico and Khachatourians 2008; Braga et al. 2015). In the present study, the mycelium produced more conidia under light than dark conditions. This result is consistent with that of Dias et al. (2019). Although Bb10331’s colony diameter and conidiation capacity surpassed Bb7725’s, it had a lower conidial germination rate. This result suggests that Bb7725 has a higher effective conidia capacity. The phytochrome gene deletion in a strain of the insect pathogen B. bassiana generated higher osmosensitivity, an increased antioxidant capability, and weakened conidial thermotolerance (Qiu et al. 2014). Accordingly, the mycelial growth, conidiation capacity, and conidial germination rate of the two strains studied here differed under the same light treatment, likely due to their differential expression of phytochrome genes. Crucially, the lethal ability of either strain against H. cunea depends on their number of germinated conidia. Finally, the colors and types of light can also play an important role in the biological characteristics of B. bassiana strains (Yu et al. 2013; Qiu et al. 2014).
Ambient pH is an environmental stimulus capable of inducing a series of physiological and cellular events in microorganisms. Fungal adaptation to ambient pH relies on homeostasis of intracellular pH and the proper expression of permeases, secreted proteases, toxins, and antibiotics (Zhu et al. 2016). Work by Zhu et al. (2016) also confirmed that PacC and Pal partners regulate the growth and conidiation of B. bassiana in a pH-dependent manner and highlighted their importance for this fungal response to pH. Furthermore, pH variously affects different species of fungi; for example, the ambient pH has a major regulatory influence on conidiation in these species of Trichoderma atroviride and T. hamatum, whose conidiation and growth are favored by low pH (Steyaert et al. 2010). By contrast, in our study, the colony diameter and conidiation of both strains tended to increase initially but then diminished with a higher pH; this proves that their conidiation as well as growth are favored by a neutral pH. It has been speculated that the strong alkaline conditions may foster the production of fragmented vacuoles in hyphal cells, inducing high osmosensitivity in these cells (Antonio et al. 2010). Moreover, the responsive genes involved in various cellular events, such as growth, ion tolerance, cell differentiation, cell wall remodeling, secondary metabolism, and host infection are expressed under suitable pH conditions (Merhej et al. 2011; Cornet and Gaillardin 2014; O’Meara et al. 2014).
