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To avoid the risk of harmful effects from synthetic fungicides on human and natural environments, our study aimed to evaluate the potential use of the indigenous Thai Myrtaceae plants as a harmless means for controlling the plant diseases. Four Myrtaceae extracts showed no phytotoxicity towards lettuce cultivated in hydroponic conditions, with notable plant-growth-promoting potential observed in lettuce treated with Callistemon viminalis (EECV). With regard to a direct effect, Melaleuca cajuputi (95% EEMC) showed the strongest inhibition of mycelial growth of A. brassicicola (85–100%), followed by 95% EECV (40–63%), while both extracts significantly inhibited spore germination (80–100%). A GC/MS and LC/MS analysis revealed terpenoids as the predominant compounds at approximately 55% and 37% in EEMC and EECV, respectively, with high concentrations of phytol (14%) and 1,8-cineole (14%) being particularly notable. As an inducer of an indirect effect of the plant extracts, the results from the experiment with 95% EEMC (15,000 and 50,000 ppm) as well as 50 and 95% EECV (15,000 and 50,000 ppm) applications was in good agreement, demonstrating significant reductions in disease severity in hydroponically grown lettuce ranging from 50 to 70% together with the induction of defense enzymes (specifically β-1,3-glucanase, chitinase, and peroxidase) compared to those in the inoculated control and fungicide treatment groups. Furthermore, a correlation analysis revealed a negative correlation between disease severity and the three defense enzymes. Our findings underscore the considerable efficacy of the three botanical fungicides tested here (i.e., 95% EEMC as well as 95% and 50% EECV) for the management of Alternaria leaf spot in lettuce.
Somnuek et al. (Tue,) studied this question.