Abstract
Anthracnose, one of the major diseases in the onion, is caused by the fungus Colletotrichum gloeosporioides f. sp. cepae. The main strategy for controlling this fungus is by the use of fungicide, with this technique being used on the majority of crops. Currently, methods of disease control are being studied that are less aggressive to the environment, with the use of essential oils proving to be an alternative. As such, the aim of this study was to evaluate the direct effect of the essential oils of basil (Ocimum basilicum), lemongrass (Cymbopogon citratus (DC) Stapf.), sage (Salvia officinalis), thyme (Thymus vulgaris) and citronella (Cymbopogon winterianus) as an alternative in the control of the fungus C. gloeosporioides f. sp. cepae, in addition to determining its effect on the physiological quality of onion seeds. The essential oils of lemongrass and sage did not affect the physiological quality of the onion seeds, and all the essential oils used helped to inhibit spore germination in C. gloeosporioides f. sp. cepae. The essential oils of thyme, lemongrass and citronella at a concentration of 2000 ppm completely inhibited mycelial growth in the fungus. Treatment with the essential oil of lemongrass in seeds inoculated with the pathogen gave greater protection against fungal attack.
Highlights
Anthracnose is considered a disease of great importance in the onion (Allium cepa L.), the causative agent being the fungus Colletotrichum gloeosporioides (Penz.) Penz
The essential oil of citronella (Cymbopogon nardus (L.) Rendle.) contains the compounds citronellal, geraniol and limonene, while the major component of lemongrass essential oil (Cymbopogon citratus Staph.) is citral, which is considered responsible for the allopathic activity of the plant (GUIMARÃES et al, 2008)
(1000 ppm and 2000 ppm), and citronella and basil at 2000 ppm, showed 82%, 71%, 78% and 78% of germinated seeds respectively at the first reading, with no statistical difference, but did show a difference in relation to the control. These results differed from those found by Araújo Neto and Araújo (2012) in fennel (Foeniculum vulgare Mill), where treatment with a 2.5% concentration of citronella essential oil increased the percentage of germinated seeds at the first count
Summary
Anthracnose is considered a disease of great importance in the onion (Allium cepa L.), the causative agent being the fungus Colletotrichum gloeosporioides (Penz.) Penz. There has recently been a growing search for alternative methods of disease control, such as the use of essential oils, due to their low environmental impact (XAVIER et al, 2012). These have allopathic characteristics, i.e. they have the capacity to interfere directly or indirectly in plants or in microorganisms through the release of chemical compounds (SIMONETO; CRUZ-SILVA, 2010) that are produced via secondary metabolism and act as allelochemicals. These compounds may interfere with the physiological quality of the seeds and with normal plant development (SOYLU; KURT; SOYLU, 2010). The essential oil of basil (Ocimum basilicum L.) is composed of linalool, estragole, farnesene, eugenol and cineole, which have antifungal properties (MARTINEZ-VELAZQUEZ et al, 2011)
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