Management of Naturally Occurring Diseases by Supernatant from <i>Chlorella</i> Cultures in Pepper

The large-scale culture of <i>Chlorella</i>, a genus of microalgae, generates valuable products used to improve human health and produce biofuel. Such commercial applications utilized only the microalgal cells. However, the process produces tons of supernatant waste that require detoxification and disposal. A previous study demonstrated that cell-free supernatants from <i>Chlorella fusca</i> culture primes plant immunity in the model plant <i>Arabidopsis thaliana</i>, suggesting its potential for use as a bioprotectant. The current study investigated the effects of treatment with <i>Chlorella</i> supernatant on crop plants in an agricultural setting. Supernatants from <i>Chlorella</i> sp. strains HS2 and ABC001 were drench-applied to pepper (<i>Capsicum annum</i> L.) seedlings under greenhouse and field conditions. The disease control capacity and growth of plants were evaluated, as well as the diversity of the rhizosphere microbiota. Application of either supernatant reduced the severity of bacterial leaf spot disease caused by <i>Xanthomonas axonopodis</i> pv. <i>vesicatoria</i> and enhanced pepper seedling growth in the greenhouse. Under field conditions, cell-free supernatants of strains HS2 and ABC001 not only reduced the severity of natural-occurring viral and bacterial diseases and insect infestation but also increased fruit yield. Additionally, drenching with <i>Chlorella</i> supernatants improved species diversity in the rhizosphere microbiota. The application of <i>Chlorella</i> supernatant to pepper therefore offered protection against diverse field pathogens and promoted seedling growth and productivity. Our finding provides insight into novel methods of sustainable agriculture utilizing recycled cell-free waste supernatants from the industrial culture of <i>Chlorella</i>.