Nitrogen sources affect productivity, desiccation tolerance and storage stability ofBeauveria bassianablastospores

Innovative sustainable bioreactor-in-a-granule formulation of Trichoderma asperelloides

The advancement of fungal biocontrol agents depends on replacing cereal grains with low-cost agro-industrial byproducts for their economical mass production and development of stable formulations. We propose an innovative approach to develop a rice flour-based formulation of the beneficial biocontrol agent Trichoderma asperelloides CMAA1584 designed to simulate a micro-bioreactor within the concept of full biorefinery process, affording in situ conidiation, extended shelf-life, and effective control of Sclerotinia sclerotiorum, a devastating pathogen of several dicot agricultural crops worldwide. Rice flour is an inexpensive and underexplored byproduct derived from broken rice after milling, capable of sustaining high yields of conidial production through our optimized fermentation-formulation route. Conidial yield was mainly influenced by nitrogen content (0.1% w/w) added to the rice meal coupled with the fermentor type. Hydrolyzed yeast was the best nitrogen source yielding 2.6 × 109 colony-forming units (CFU)/g within 14 days. Subsequently, GControl, GLecithin, GBreak-Thru, GBentonite, and GOrganic compost+Break-Thru formulations were obtained by extrusion followed by air-drying and further assessed for their potential to induce secondary sporulation in situ, storage stability, and efficacy against Sclerotinia. GControl, GBreak-Thru, GBentonite, and GOrganic compost+Break-Thru stood out with the highest number of CFU after sporulation upon re-hydration on water-agar medium. Shelf-life of formulations GControl and GBentonite remained consistent for > 3 months at ambient temperature, while in GBentonite and GOrganic compost+Break-Thru formulations remained viable for 24 months during refrigerated storage. Formulations exhibited similar efficacy in suppressing the myceliogenic germination of Sclerotinia irrespective of their concentration tested (5 × 104 to 5 × 106 CFU/g of soil), resulting in 79.2 to 93.7% relative inhibition. Noteworthily, all 24-month-old formulations kept under cold storage successfully suppressed sclerotia. This work provides an environmentally friendly bioprocess method using rice flour as the main feedstock to develop waste-free granular formulations of Trichoderma conidia that are effective in suppressing Sclerotinia while also improving biopesticide shelf-life. KEY POINTS: • Innovative "bioreactor-in-a-granule" system for T. asperelloides is devised. • Dry granules of aerial conidia remain highly viable for 24 months at 4 °C. • Effective control of white-mold sclerotia via soil application of Trichoderma-based granules.

Advances in submerged liquid fermentation and formulation of entomopathogenic fungi

Entomopathogenic fungi (EPF) can be defined as beneficial multifunctional eukaryotic microorganisms that display pivotal ecological services in pest management, with some species possessing the special ability to establish mutualistic relationships with plants. Mass production of these fungi is critical to support affordable widespread commercialization and worldwide field application. Among the mass production methods explored mainly by industry, submerged liquid fermentation is a robust and versatile technology that allows the formation of different types of propagules designated for various applications in pest control. Many hypocrealean EPF are easily culturable on artificial substrates by producing single-celled structures (hyphal bodies, blastospores, and submerged conidia) or multicellular structures (mycelium and microsclerotia). Less frequently, some EPF may form environmentally resistant chlamydospores, but these structures have almost always been overlooked. A continued research pipeline encompassing screening fungal strains, media optimization, and proper formulation techniques aligned with the understanding of molecular cues involved in the formation and storage stability of these propagules is imperative to unlock the full potential and to fine-tune the development of robust and effective biocontrol agents against arthropod pests and vectors of diseases. Finally, we envision a bright future for the submerged liquid fermentation technology to supplement or replace the traditional solid substrate fermentation method for the mass production of many important EPF. KEY POINTS: • Submerged liquid fermentation (SLF) allows precise control of nutritional and environmental factors • SLF provides a scalable, robust, and cost-effective platform for mycopesticide production • Enhancing formulation, shelf life, and field efficacy of submerged propagules remain crucial • Understanding the molecular mechanisms behind submerged propagule formation is key to advancing SLF technology.

Complex nitrogen sources from agro-industrial byproducts: impact on production, multi-stress tolerance, virulence, and quality of Beauveria bassiana blastospores

We investigated the impact of various complex organic nitrogen sources on the submerged liquid fermentation of Beauveria bassiana, a versatile entomopathogenic fungus known for producing hydrophilic yeast-like single cells called blastospores. Specifically, we examined yeast extract, autolyzed yeast, inactive yeast, cottonseed flour, corn bran, and corn gluten meal as nitrogen compounds with different carbon-to-nitrogen (C:N) ratios. Our comprehensive analysis encompassed blastospore production, tolerance to abiotic stresses, shelf stability after drying, and virulence against mealworm larvae, crucial attributes for developing effective blastospore-based biopesticides. Notably, cottonseed flour emerged as the optimal nitrogen source, yielding up to 2.5 × 109 blastospores/mL within 3 days in a bioreactor. These blastospores exhibited the highest tolerance to heat stress and UV-B radiation exposure. The endogenous C:N ratio in blastospore composition was also impacted by nitrogen sources. Bioassays with mealworm larvae demonstrated that blastospores from cottonseed flour were the most virulent, achieving faster lethality (lower LT50) and requiring a lower inoculum (LC50). Importantly, blastospores produced with cottonseed flour displayed extended viability during storage, surpassing the retention of viability compared to those from autolyzed yeast over 180 days at 4°C. Despite differences in storage viability, both nitrogen sources conferred similar long-term blastospore bioactivity against mealworms. In summary, this research advances our understanding of the crucial impact of complex organic nitrogen selection on the phenotypic traits of blastospores in association with their intracellular C:N ratio, contributing to the production of ecologically fit, shelf-stable, and virulent propagules for effective pest biocontrol programs.

IMPORTANCE

Biological control through entomopathogenic fungi provides essential ecological services in the integrated management of agricultural pests. In the context of submerged liquid fermentation, the nutritional composition significantly influences the ecological fitness, virulence and quality of these fungi. This study specifically explores the impact of various complex organic nitrogen sources derived from agro-industrial byproducts on the submerged liquid fermentation of Beauveria bassiana, a versatile entomopathogenic fungus known for producing hydrophilic yeast-like blastospores. Notably, manipulating the nitrogen source during submerged cultivation can influence the quality, fitness, and performance of blastospores. This research identifies cottonseed flour as the optimal low-cost nitrogen source, contributing to increased production yields, enhanced multi-stress tolerance, heightened virulence with extended shelf life and long-term bioactivity. These findings deepen our understanding of the critical role of nitrogen compound selection in liquid media formulation, facilitating the production of ecologically fit and virulent blastospores for more effective pest biocontrol programs.

Beauveria bassiana submerged spores for control of two-spotted spider mite (Tetranychus urticae Koch (Acari: Tetranychidae)): production, stability, and virulence

In this study, IBCB 66, IBCB 868, and CBMAI 1306 isolates of Beauveria bassiana were grown in liquid culture for 4 days, leading to elevated submerged spores (SS) levels. The influence of the addition of different glycerol concentrations (0, 3, and 6%) (v/v) in the liquid culture was investigated regarding the stability (at 4 and 27 °C) of dried formulations. The virulence of SS was compared with aerial spores (AS) against Tetranychus urticae (Koch) (Acari: Tetranychidae). The results demonstrate the potential of using SS to control T. urticae. CBMAI 1306 and IBCB 868 isolates caused T. urticae mortality rates of 91.11% and 88.89% 5 days after treatment, respectively, when applied at concentrations of 1 × 108 SS mL-1. The median Lethal Time (LT50) values for these strains were 2.64 and 2.61 days, respectively. The dried formulations showed potential acaricidal activity. Higher glycerol concentrations in the liquid culture medium reduced formulation stability at 27 °C.

The Potential of Metarhizium anisopliae Blastospores to Control Aedes aegypti Larvae in the Field

Entomopathogenic fungi are promising as an environmentally benign alternative to chemical pesticides for mosquito control. The current study investigated the virulence of Metarhizium anisopliae blastospores against Aedes aegypti under both laboratory and field conditions. Virulence bioassays of conidia and blastospores were conducted in the laboratory, while field simulation bioassays were conducted under two conditions: totally shaded (TS) or partially shaded (PS). In the first bioassay (zero h), the larvae were added to the cups shortly after the preparation of the blastospores, and in the subsequent assays, larvae were added to the cups 3, 6, 9, and 12 days later. The survival of the larvae exposed to blastospores in the laboratory was zero on day two, as was the case for the larvae exposed to conidia on the sixth day. Under TS conditions, zero survival was seen on the third day of the bioassay. Under PS conditions, low survival rates were recorded on day 7. For the persistence bioassay under PS conditions, low survival rates were also observed. Metarhizium anisopliae blastospores were more virulent to Ae. aegypti larvae than conidia in the laboratory. Blastospores remained virulent under field simulation conditions. However, virulence rapidly declined from the third day of field bioassays. Formulating blastospores in vegetable oil could protect these propagules when applied under adverse conditions. This is the first time that blastospores have been tested against mosquito larvae under simulated field conditions, and the current study could be the basis for the development of a new biological control agent.

Improved Diaphorina citri (Hemiptera: Liviidae) Adults Biocontrol in Citrus by Hirsutella citriformis (Hypocreales: Ophiocordycipitaceae) Gum-Enhanced Conidia Formulation

Hirsutella citriformis Speare is the only entomopathogenic fungus involved in Diaphorina citri Kuwayama natural epizootics. The aim of the present study was to evaluate different protein sources as supplements to stimulate Hirsutella citriformis growth, improve conidiation on solid culture, and evaluate its produced gum for conidia formulation against D. citri adults. Hirsutella citriformis INIFAP-Hir-2 strain was grown on agar media enriched with wheat bran, wheat germ, soy, amaranth, quinoa, and pumpkin seed, in addition to oat with wheat bran and/or amaranth. The results demonstrated that 2% wheat bran significantly (p < 0.05) promoted mycelium growth. However, 4% and 5% wheat bran achieved the highest conidiation (3.65 × 10⁷ conidia/mL and 3.68 × 10⁷ conidia/mL, respectively). Higher conidiation (p < 0.05) was observed on oat grains supplemented with wheat bran, as compared with culturing on oat grains without supplements (7.25 × 10⁷ versus 5.22 × 10⁷ conidia/g), after a 14 d instead of 21 d incubation period. After supplementing synthetic medium or oat grains with wheat bran and/or amaranth, INIFAP-Hir-2 conidiation increased, whereas production time was reduced. After using Acacia and Hirsutella gums to formulate conidia produced on wheat bran and amaranth at 4%, field trial results showed that the highest (p < 0.05) D. citri mortality was achieved by Hirsutella gum-formulated conidia (80.0%), followed by the Hirsutella gum control (57.8%). Furthermore, Acacia gum-formulated conidia caused 37.8%, whereas Acacia gum and negative controls induced 9% mortality. In conclusion, Hirsutella citriformis gum used to formulate its conidia improved biological control against D. citri adults.

Optimization of Submerged Culture Parameters of the Aphid Pathogenic Fungus Fusarium equiseti Based on Sporulation and Mycelial Biomass

Fusarium equiseti (JMF-01), as an entomopathogenic fungus, can effectively control agricultural pests and has the potential to be a biocontrol agent. To promote mycelial growth and sporulation, we investigated the optimal submerged culture conditions for F. equiseti. In this study, we used the single-factor method and Box-Behnken design and determined the virulence of the submerged culture against Myzus persicae after optimization. As a result, the highly significant factors affecting the spore concentration of strain JMF-01 were the primary inoculum density and the initial pH, and the highly significant factor affecting the mycelial biomass was the medium-to-flask ratio. The highest mycelial biomass value was 0.35 g when the incubation time was 5.68 days, the initial pH was 5.11, the medium-to-flask ratio was 0.43, and 1 mL of the primary inoculum with spore density of 0.97 × 10⁷ conidia/mL was added. When the incubation time was 6.32 days, the initial pH was 4.46, the medium-to-flask ratio was 0.35, the primary inoculum density was 1.32 × 10⁷ conidia/mL of 1 mL, and the highest spore concentration of 6.49 × 10⁸ blastospores/mL was obtained. Compared with the unoptimized medium conditions, the optimized submerged culture had the highest mycelial biomass and spore concentration, which were 3.46 and 2.06 times higher, respectively. The optimized submerged culture was highly pathogenic toward M. persicae, reaching a 95% mortality rate. Our results provide optimal submerged culture conditions for F. equiseti and lay the basis for later research to expand production for pest control.

Development of novel spray-dried and air-dried formulations of Metarhizium robertsii blastospores and their virulence against Dalbulus maidis

The present research addressed spray-drying and air-drying techniques applied to Metarhizium robertsii blastospores to develop wettable powder (WP) formulations. We investigated the effect of co-formulants on blastospore viability during drying and assessed the wettability and stability of formulations in water. The effect of oxygen-moisture absorbers was studied on the shelf life of these formulations stored at 26 °C and 4 °C for up to 90 days. Additionally, we determined the virulence of the best spray-dried and air-dried formulations against the corn leafhopper Dalbulus maidis. While sucrose and skim milk played an essential role as osmoprotectants in preserving air-dried blastospores, maltodextrin, skim milk, and bentonite were crucial to attain high cell survival during spray drying. The lowest wettability time was achieved with spray-dried formulations containing less Ca-lignin, while charcoal powder amount was positively associated with formulation stability. The addition of oxygen-moisture absorbers inside sealed packages increased from threefold to fourfold the half-life times of air-dried and spray-dried formulations at both storage temperatures. However, the half-life times of all blastospore-based formulations were shorter than 3 months regardless of temperature and packaging system. Spray-dried and air-dried WP formulations were as virulent as fresh blastopores against D. maydis adults sprayed with 5 × 10⁷ blastospores mL^-1 that induced 87.8% and 70.6% mortality, respectively. These findings bring innovative advancement for M. robertsii blastospore formulation through spray-drying and underpin the importance of adding protective matrices coupled to oxygen-moisture absorbers to extend cell viability during either cold or non-refrigerated storage. KEY POINTS: • Cost-effective wettable powder formulations of M. robertsii blastospores were developed. • Bioefficacy of formulations against the corn leafhopper was comparable to fresh blastospores. • Cold storage and dual oxygen-moisture absorber are critical for extended shelf life.

Enhanced xylanase and endoglucanase production from Beauveria bassiana SAN01, an entomopathogenic fungal endophyte

This study was undertaken to explore alternative applications of the widely known entomopathogenic/endophytic fungus, Beauveria bassiana, besides its sole use as a biocontrol agent. B. bassiana SAN01, was investigated for the production of two glycoside hydrolases, xylanase and endoglucanase under submerged conditions. Among the different biomass tested, wheat bran provided the best results for both xylanase and endoglucanase, and their production levels were further enhanced using response surface methodology. Under optimised conditions, heightened yields of 1061 U/ml and 23.03 U/ml were observed for xylanase and endoglucanase, respectively, which were 3.44 and 1.35 folds higher than their initial yields. These are the highest ever production levels reported for xylanase and endoglucanase from any B. bassiana strain or any known entomopathogenic fungi. Furthermore, the efficacy of xylanase/endoglucanase cocktail in the saccharification of sugarcane bagasse was evaluated. The highest amount of reducing sugar released from the pretreated biomass by the action of the crude Beauveria enzyme cocktail was recorded at 30°C after 8 h incubation. The significant activities of the hydrolytic enzymes recorded with B. bassiana in this study thus present promising avenues for the use of the entomopathogen as a new source of industrial enzymes and by extension, other biotechnological applications.

Differential susceptibility of blastospores and aerial conidia of entomopathogenic fungi to heat and UV-B stresses

We investigated the comparative susceptibility to heat and UV-B radiation of blastospores and aerial conidia of Metarhizium spp. (Metarhizium robertsii IP 146, Metarhizium anisopliae s.l. IP 363 and Metarhizium acridum ARSEF 324) and Beauveria bassiana s.l. (IP 361 and CG 307). Conidia and blastospores were produced in solid or liquid Adámek-modified medium, respectively, and then exposed to heat (45 ± 0.2 °C) in a range of 0 (control) to 360 min; the susceptibility of fungal propagules to heat exposures was assessed to express relative viability. Similarly, both propagules of each isolate were also exposed to a range of 0 (control) to 8.1 kJ m^-2 under artificial UV-B radiation. Our results showed that fungal isolates, propagule types and exposure time or dose of the stressor source play critical roles in fungal survival challenged with UV-B and heat. Conidia of ARSEF 324, IP 363, IP 146 and IP 361 exposed to heat survived significantly longer than their blastospores, except for blastospores of CG 307. Conidia and blastospores of IP 146 and IP 363 were equally tolerant to UV-B radiation. We claim that blastospores of certain isolates may be promising candidates to control arthropod pests in regions where heat and UV-B are limiting environmental factors.

Growth kinetic and nitrogen source optimization for liquid culture fermentation of Metarhizium robertsii blastospores and bioefficacy against the corn leafhopper Dalbulus maidis

The cosmopolitan entomopathogenic and root endophytic fungus Metarhizium robertsii has a versatile lifestyle and during liquid fermentation undergoes a dimorphic transformation from hyphae to conidia or microsclerotia, or from hyphae to blastospores. In all cases, these processes are mediated by environmental and nutritional cues. Blastospores could be used in spray applications to control arthropod pests above ground and may serve as an attractive alternative to the traditional solid-grown aerial conidial spores of Metarhizium spp. found in commercial products. Nitrogen is a vital nutrient in cell metabolism and growth; however, it is the expensive component in liquid cultures of entomopathogenic fungi. Our goals in this study were to optimize nitrogen sources and titers for maximum production of M. robertsii blastospores cultured in shake flasks at highly aerated conditions and to further determine their virulence against the corn leafhopper Dalbulus maidis, an important vector of serious pathogens in maize crops worldwide. Our fermentation studies revealed that the low-cost corn steep liquor (CSL) was the most suitable nitrogen source to improve blastospore growth in M. robertsii. The growth kinetic assays determined the optimal titer of 80 g L^-1 and a yield up to 4.7 × 10⁸ cells mL^-1 within 5 days of cultivation (3 days preculture and 2 days culture), at a total cost of US$0.30 L^-1. Moreover, the blastospore growth kinetic was strongly dependent on glucose and nitrogen consumptions accompanied by a slight drop in the culture pH. Insect bioassays evidenced a high virulence of these blastospores, either as dried or fresh cells, to D. maidis adults fed on maize plants. Our findings provide insights into the nutritional requirements for optimal and cost-efficient production of M. robertsii blastospores and elucidate the potential of blastospores as an ecofriendly tool against the corn leafhopper.

Virulence of Some Entomopathogenic Fungi Isolates of Beauveria bassiana (Hypocreales: Cordycipitaceae) and Metarhizium anisopliae (Hypocreales: Clavicipitaceae) to Aulacaspis tubercularis (Hemiptera: Diaspididae)and Icerya seychellarum (Hemiptera: Monophlebidae) on Mango Crop

Six fungal isolates of Beauveria bassiana (Balsamo) Vuillemin and one isolate of Metarhizium anisopliae (Metschnikoff) Sorokin were isolated and evaluated for their pathogenicity to Icerya seychellarum (Westwood) and Aulacaspis tubercularis Newstead. There is a positive correlation between the concentration of the fungal blastospore concentrations and the percentage of mortality. Bio-efficacy increased significantly after inoculation with increasing concentration of blastospores and elapsed time up to 12 d after inoculation. The mortality of nymphs exposed to fungal isolates at various concentrations varied between 2.5 and 88.8%. Probit analysis of data at 95% confidence limits of LC50 and LT50s showed significant differences in the susceptibility of nymphs of I. seychellarum and A. tubercularis to the tested fungal isolates. The fungal isolates of Egy-6 and Egy-9 were the most effective against I. seychellarum and A. tubercularis, respectively. They had the lowest LC50 (4.20 × 105 and 5.71 × 103 blastospore ml-1) and LT50 (ranged from 4.61 to 9.79 and 4.84 to 8.71 d), respectively. The current study showed that all the fungal isolates yielded moderate mortality rates of nymphs and adult female populations of both the tested insect pests. To our knowledge, this is the first report of bio-efficacy of Beauveria and Metarhizium isolates against members of the Diaspidadae and Monophlebidae family insects. These results establish that the use of these native entomopathogenic fungi isolates of B. bassiana (Egy-3, Egy-4, Egy-6, Egy-7, Egy-9, and Egy-10) and M. anisopliae (Egy-5) could be considered for further development as microbial control agents of the mealybug and scale insects as a potential biological agent for use in an IPM program.

A nonconventional two-stage fermentation system for the production of aerial conidia of entomopathogenic fungi utilizing surface tension

AIM

To describe a new approach in which production of conidia of an entomopathogenic fungus takes place on the surface of an unstirred shallow liquid culture kept in nonabsorbent wells distributed in plastic sheets resembling a honeycomb.

METHODS AND RESULTS

First, liquid incubation time and medium composition for production of Beauveria bassiana aerial conidia were optimized. Wells inoculated with Sabouraud dextrose yeast extract produced 2·2 × 10⁸ conidia per cm² of liquid surface following 5 days of incubation. Finally, tests were carried out in a prototype comprised of stacked plastic sheets in a cylindrical container. Conidia production on liquid culture surface varied from 1·2 to 1·6 × 10⁹ conidia per ml of fermented broth. Germination rates and insect activity towards Tenebrio molitor larvae were not negatively affected when compared to conidia produced on solid medium.

CONCLUSIONS

The two-stage fermentation process here described, based on a simple nonabsorbent inert support, has potential for the application in the production of aerial conidia of B. bassiana and other fungi.

SIGNIFICANCE AND IMPACT OF THE STUDY

Aerial conidia are the most extensive propagule type used in commercial mycopesticides, traditionally produced by solid-state fermentation (SSF). The industrial applications and other important benefits of the two-stage fermentation process here described may overcome some hurdles inherent to SSF aiming for the production of aerial conidia. Additionally, production consistency is increased by the use of chemically defined medium, and the better control of the environmental conditions could allow for more reproducible industrial batches.