Abstract
Pseudocercospora griseola, the etiologic agent of angular leaf spot of common bean (Phaseolus vulgaris), is an important disease in all bean-producing regions worldwide and may cause extremely high yield losses. The control of this disease is made more difficult by the pathogen's genetic variability and the inefficiency of fungicides. In this study, of 26 essential oils tested at different concentrations, 25 demonstrated efficiency in affecting the germination of strains 63-31 and 63-63 of the pathogen, reaching inhibition levels of between 80% and 100%. Cymbopogon citratus and Cymbopogon martinii inhibited conidia germination at all concentrations; Eugenia caryophyllata, Cinnamomum sp., Thymus vulgaris, Matricaria recutita, Cordia verbenacea, Origanum vulgare, Cymbopogon nardus, at 0.1 and 0.5%; and Zingiber officinale, Mentha arvensis, Chamaecyparis pisifera, Lavandula officinalis, Ocimum basilicum, Pimpinella anisum, Ocimum selloi, Baccharis dracunculifolia, Laurus nobilis, Citrus sinensis, Melaleuca alternifolia and Eucalyptus globulus, at 0.5%. The main constituents identified were cinnamaldehyde in Cinnamomum sp.; eugenol in E. caryophyllata; trans-β-farnesene in M. recutita; pulegone in C. verbenacea; thymol in T. vulgaris; geranial and neral in C. citratus, and geraniol in C. martini. Through transmission electron microscopy (TEM), it was verified that C. citratus, C. martini and E. caryophyllata presented direct fungitoxic action on P. griseola, causing severe damage to the cellular ultrastructure of the conidia, invalidating germination. These results indicated that essential oils are a promising alternative strategy for the control of angular leaf spot in bean, representing less risk to human health and the environment.
Highlights
Angular leaf spot (ALS), caused by Pseudocercospora griseola (Sacc.) Crous & U
The antifungal activity of essential oils in the germination of P. griseola strains, expressed by the percentage of conidial germination inhibition, was calculated based on average number of conidia that germinated from each strain, 63-31 (46.67) and 63-63 (31.75) and the untreated control (Table 1)
Essential oils of C. citratus, C. martini and E. caryophyllata were more effective than the other treatments, showing at least 98% inhibition of conidial germination of strains 63-31 and 63-63 of P. griseola at all tested concentrations
Summary
Angular leaf spot (ALS), caused by Pseudocercospora griseola (Sacc.) Crous & U. Braun, is the most widespread disease in all bean-producing regions worldwide. Institute (CMI), this disease occurs in more than 60 countries (SARTORATO; RAVA, 1994) and, under favorable environmental conditions, yield losses may reach 80% (JESUS-JÚNIOR et al, 2001; STENGLEIN et al, 2003; PAULA-JÚNIOR; ZAMBOLIM, 2006). Control plant diseases, their continuous and indiscriminate use has caused contamination by waste, environmental pollution and the selection of resistant populations of pathogens. Chemical control carries considerable environmental and financial costs (MIKLAS et al, 2006). Due to the high genetic variability of P. griseola (BROCK, 1951; PYNDJI, 1993; PASTOR-CORRALES et al, 1998; OROZCO; ARAYA, 2005; STENGLEIN; BALATTI, 2006; SILVA, et al, 2008), genetic control through the use of resistant cultivars is very difficult
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