Chalcones to control Alternaria alternata in murcott tangor fruits

Resumo

Alternaria brown spot (ABS), caused by the fungus Alternaria alternata pathotype tangerine, is one of the main phytosanitary problems for mandarin growers. About 15 applications per year of harmful fungicides are required for controlling ABS disease in citrus orchards. As chalcones seem to be less toxic to humans and environment than the commercial fungicides in use, this study initially aimed at synthesizing 137 chalcones through aldolic condensations between benzaldehydes and acetophenones. The resulting chalcones were screened for activity against A. alternata through a fungal growth assay that was carried out in 96-cell polypropylene plates, using the same concentration to all studied substances. The four active chalcones underwent conidia germination and mycelial growth, which confirmed the antifungal activity of the compounds. These chalcones were then poured onto Murcott tangor fruit that had been inoculated with conidia of the fungus. All four chalcones reduced the ABS progress to values significantly smaller (P≤0.05) than that observed for the control. Statistical calculations showed that the best results were afforded by two compounds, bearing a 2,4,5-trimethoxyphenyl group at position 3 of prop-2-enal and a 3-nitro- or 3-hydroxyphenyl group at position 1 of the aldehyde. Such compounds reduced the incidence of the disease in Murcott tangor fruit to values that did not differ statistically from those obtained with a commercial fungicide.