Screening of organic substrates for the development of effective biofungicides to manage cashew fusarium wilt disease.

Background: The biotechnology industry commonly utilizes synthetic media to grow biological control agents (BCAs); however, these media are often considered impractical, particularly in developing countries.

Objective: This study aimed to identify the most suitable locally available organic substrates for the cultivation of BCAs used against cashew fusarium wilt disease.

Materials and methods: Experiments were conducted in 2022 and 2023 on five substrates, namely rice husk (RH), rice bran (RB), hulled millet, parboiled rice, and their combinations, as growth media for combined Bacillus strains and Trichoderma asperellum. The carbohydrate and protein content of the substrates were quantified colorimetrically.

Results: Protein and carbohydrate contents ranged from 13.4 - 19.8% to 76.3 - 82.7%, respectively. The BCAs exhibited high colonization rates on all tested substrates, with combined substrates supporting the highest colonization, followed by RH and RB. Endospore formation and conidiation levels varied significantly over time across substrates and incubation temperatures (p ≤ 0.05). Population densities decreased over time under refrigerated, room temperature, and incubator conditions for most substrates in both seasons, except for combined substrates, RH, and RB. Final population counts were as follows: 2.1 × 10⁷ ± 4.9 × 10⁵ and 1.0 × 10⁷ ± 0.3 × 10⁵ colony-forming units (CFUs)/mL (combined substrates), 2.3 × 10⁷ ± 5.4 × 10⁵ and 5.7 × 10⁷ ± 1.1 × 10⁶ CFUs/mL (RH), 3.6 × 10⁵ ± 2.1 × 10⁴ and 3.3 × 10⁵ ± 1.6 × 10⁴ CFUs/mL (RB), while under refrigerated conditions, population densities remained relatively unchanged throughout the study period.

Conclusion: Based on the findings, it is recommended to use a mixture of substrates, RH alone, or RB alone as appropriate media for the large-scale production of the studied biofungicides.