In vitro phytopathogenic fungal inhibition assays and composition of the essential oils obtained from Conyza species belonging to the Argentine flora

STEGMAYER, MI; ÁLVAREZ, NH; DELLAFERRERA, IM; REUTEMANN, AG; DERITA, MG

Conferencia; Virtual International Conference Plant productivity and food safety: Soil science, Microbiology, Agricultural Genetics and Food quality; 2021

The genus Conyza belonging to the Asteraceae family is native to South America and it is widely distributed around the world. In Argentina, species of this genus are known to be important weeds but also they are used in traditional medicine by infusions as febrifuge, anthelmintic and insecticide, among others. Despite the extensive number of medicinal properties attributed, its use has not been proven against phytopathogens that affect fruits of worldwide importance. In this work, the essential oils (EO) obtained from Conyza bonariensis var. angustifolia, Conyza bonariensis var. bonariensis and Conyza sumatrensis were evaluated against five destructive phytopathogenic fungi: Botrytis cinerea, Colletotrichum acutatum, Fusarium semitectum, Monilinia fructicola and Rhizopus stolonifer.The essential oil was extracted from leaves (100 g) of each species and variety, using a Clevenger-type distiller. The obtained products were evaluated using the agar diffusion method adapted for volatile samples: Petri dishes of 6 cm diameter were covered with 10 mL of PDA media; once solidified, a conidia suspension of 104 Colony Forming Units/mL was inoculated inside a well located in the center. Once the water from the inoculated conidia solution evaporated, 15 µL of each EO (treatment) or sterile water (control) was deposited on the center of the Petri dishes top. This quantity of EO applied represented a concentration of 1000 ppm. The Petri dishes prepared in this way were incubated upside down so that the EO, once evaporated, came in contact with the culture medium where the fungal strains were growing. Once the mycelium of the control plates completely covered the surface of the media (approximately 7 days), the measurements of the mycelium diameter developed in each plate previously treated with EO were carried out by scanning the plates for later analysis with ImageJ® software. Assays were performed by triplicate, and the percentage of fungal growth inhibition was calculated according to the following equation: I % =100(C-M)/C, where I% represents the inhibition percentage, C the average of the three control plates mycelia area and M the average of the three treated plates mycelia area. ANOVA was performed and statistical differences were analyzed by Tukey test. Results showed that the highest content of EO (0.12%) was obtained for C. sumatrensis followed by C. bonariensis var. bonariensis (0.09%) and C. bonariensis var. angustifolia (0.07%). Regarding fungicidal activities, C. bonariensis var. bonariensis EO showed the highest fungal inhibition against M. fructicola (93%) and R. stolonifer (89%). Furthermore, it was moderately active against B. cinerea and F. semitectum displaying 55% and 57% of growth inhibition, but resulted almost inactive for C. acutatum (21% of inhibition). C. sumatrensis EO showed high antifungal specificity against M. fructicola (90% of inhibition), being inactive against the rest of the fungi panel; while C. bonariensis var. angustifolia EO did not inhibit any evaluated phytopathogen by statistical comparison with the control growth. In addition, the chemical profile of each EO was analyzed by GC-MS and the major compounds were determined; in all the samples, (E)-ß-Farnesene, Caryophyllene, (-) Germacrene-D, D-Limonene, Lachnophyllum ester, a-Cadinol and ß-Ocimene were found.