Identification of Fungicides in Hevea brasiliensis Latex Serum Extracts and Inhibition Against Phytophthora palmivora, a Black Rot Disease Pathogen in Orchids

The para rubber tree (Hevea brasiliensis) is an economic crop cultivated worldwide, particularly in Southeast Asia. Their latex serum is commonly treated as waste in the rubber industry. We aimed to investigate the latex serum for its antifungal activity. Phytophthora species are major fungal pathogen causing substantial economic losses in crops worldwide, especially Phytophthora palmivora which causes black rot disease in orchid farms. Crude serum extracts from H. brasiliensis latex clone RRIT251 and RRIM600 were evaluated for inhibitory activity against the mycelial growth of P. palmivora. The serum extracts from both clones showed complete inhibition (100%) against the pathogen at a concentration of 20,000 mg/L. A dose–response experiment at a lower concentration of 10,000 mg/L demonstrated greater effectiveness of extracts from RRIT251. Two major volatile organic compounds, 1,2-Dithiane-4,5-diol (DTD) and 2,4-Di-tert-butylphenol (2,4-DTBP), were identified in these serum extracts by gas chromatography-mass spectrometry. Inhibition studies using synthetic DTD and 2,4-DTBP confirmed their fungicidal activities against P. palmivora. The combined treatment of DTD and 2,4-DTBP showed a trend towards synergistic inhibition. However, combined treatment using synthetic DTD and 2,4-DTBP at concentrations similar to those in crude serum exhibited lower efficacy than the crude extracts. The results suggest that the two compounds may work synergistically with other defensive proteins present in the serum extracts, contributing to antifungal activity. Thus, the latex serum waste of H. brasiliensis demonstrates potential use as a natural fungicide for the management and control of black rot disease in orchid farms.