Effects of Clonostachys rosea f. catenula inoculum on the composting of cabbage wastes

Abstract

Globally, fungal inocula are being explored as agents for the optimization of composting process. This research primarily evaluates the efficacy of inoculating organic vegetable heaps with the entomopathogenic fungus, Clonostachys rosea f. catenula (Hypocreales), on the biophysicochemical properties of the end-product of composting. Six heaps of fresh cabbage wastes were inoculated with conidia of C. rosea f. catenula conidia and another six were not exposed to the fungus. The composted materials from fungus- and control-treated heaps were subsequently used as a medium to cultivate tomatoes (Solanum lycopersicum). The heaps were allowed to undergo composting. Heap temperature and moisture content were recorded weekly, and the chemical characteristics of the heaps were assessed after twelve weeks of composting. In addition, we evaluated the protective effect of the fungal inoculum against red spider mite (Tetranychus urticae) infestations of the tomatoes - conidial colonization of the plant tissue and the number of plants infested by the pathogen were recorded. Additionally, phytotoxicity tests, seed germination, and seedling toxicity on tomatoes were carried out. There were few significant variations (p < 0.05) in heap temperature or moisture level between treatments based on the weekly data; they ranged from 23.778 – 39.35 °C and 0.00 – 52.92% heap moisture readings from weeks 1-12, respectively. We found no significant differences in the levels of compost macronutrient N, P, K, Ca, Mg, C and Na) and micronutrient (Mn, Fe, Zn, Cu) constituents. Remarkably, the composted materials, when incorporated into a growth medium, from fungus-treated heaps induced a 100% endophytic tissue colonization in cultivated tomato plants. It was observed that the tomato plants grown in composted materials from heaps inoculated with C. rosea f. catenula had fewer red spider mite (Tetranychus urticae) infestations; however, the difference was not significant (χ2 = 0.96 and p = 0.32). Remarkably, the fungus was re-isolated from leaves of all the plants that were cultivated on the composted materials from the fungus-treated heaps. Moreover, the fungus treatment yielded composted materials that significantly (p ˂ 0.05) enhanced tomato seed germination. The phytotoxicity tests revealed that the composted samples from the heaps exposed to the C. rosea f. catenula inoculum was not toxic to tomato seeds and seedlings when the end-product of composting was used at a concentration of 25% v/v in a soil mix with 75% coarse sand. In conclusion, this study showed that the quality of composted materials containing C. rosea f. catenula was improved in terms of fungal endophytism, seed germination. and induced 100% endophytic tissue colonisation of tomatoes. However, C. rosea f. catenula did not confer protection against red spider mite infestations to tomatoes. C. rosea f. catenula inoculation did not significantly affect the composted materials' chemical constituents nor led to phytotoxicity of composted materials. Overall, the use of the entomopathogenic fungus, C. rosea f. catenula inoculum as a bio-enhancer in composting organic waste is beneficial to the organic composting process.