Abstract
The aim of this study was to evaluate the potential of essential oils derived from Hyptis marrubioides , Aloysia gratissima and Cordia verbenacea for controlling Asian soybean rust . The inhibitory activities of the essential oils (applied in a concentration range of 0.05 - 2%) on the germination of Phakopsora pachyrhizi urediniospores were investigated using in vitro assays. The curative and eradicating properties of the oils (applied in a concentration range of 0.05 - 0.5%) were studied under greenhouse conditions using the P. pachyrhizi -susceptible soybean cultivar MG/BR 46 (Conquista). Scanning electron microscopy was employed to investigate the effects of the essential oils on the morphology of the P. pachyrhizi urediniospores. The treatment with the essential oils at all concentrations tested led to the total inhibition of spore germination in vitro . The curative application of the essential oils reduced the disease severity, expressed as the area under the disease-progress curves, by 33 to 41%, whereas the commercial fungicide (pyraclostrobin + epoxyconazole) employed as a positive control reduced the severity by 61%. The treatment of infected plants with the essential oils resulted in morphological alterations in the fungal structures that were similar to those produced by the commercial fungicide, namely, a shrivelling of the urediniospores, appressoria, germ tubes and paraphyses.
Highlights
The yield and profitability of a soybean crop can be significantly affected by diseases caused by plant pathogens
The commercial fungicide inhibited germination by 100%. These results demonstrate that essential oils from the traditional Brazilian medicinal plants H. marrubioides, A. gratissima and C. verbenacea contain components that are toxic to the urediniospores of P. pachyrhizi
A chromatographic analysis revealed a considerable diversity in the composition and in the proportions of the various chemical classes present in essential oils derived from A. gratissima, C. verbenacea and H. marrubioides, such as alcohols, aldehydes, esters, ethers, hydrocarbons and ketones (SILVA et al, 2012a)
Summary
The yield and profitability of a soybean crop can be significantly affected by diseases caused by plant pathogens. The Asian soybean rust caused by the fungus Phakopsora pachyrhizi Syd. Syd (YANG et al, 1990, 1991) is one of the most destructive diseases in soybean. According to studies conducted in various soybean-growing regions, the reductions in yield caused by Asian rust can vary between 10 and 100% (HARTMAN et al, 1991; YORINORI et al, 2005; MCLAREN, 2008). 159-166, Apr.-June, 2014 it has been estimated that the cumulative total economic losses, including reduced yield and revenue plus the cost of disease control, due to soybean rust have reached US $ 10.1 billion since the first widespread epidemics were observed in the country (CONSÓRCIO ANTIFERRUGEM, 2008) In Brazil, Maringá, v. 36, n. 2, p. 159-166, Apr.-June, 2014 it has been estimated that the cumulative total economic losses, including reduced yield and revenue plus the cost of disease control, due to soybean rust have reached US $ 10.1 billion since the first widespread epidemics were observed in the country (CONSÓRCIO ANTIFERRUGEM, 2008)
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