Antifungal activity against Alternaria solani and control of early blight in tomato by essential oil of bergamot orange

Abstract

The early blight (caused by the fungus Alternaria solani) results in significant damage to the tomato crop, directly affecting productivity. An alternative to the frequent use of pesticides is the use of essential oils, which can act in defense against phytopathogens. The objective of this work was to evaluate the toxic activity in vitro of the bergamot orange (Citrus aurantium ssp. bergamia) essential oil against A. solani, the control of the early blight, and the activity of defense enzymes in tomatoes treated with this oil and inoculated with A. solani. Mycelial discs of A. solani were added to dishes with V8 culture media to which essential oil at concentrations of 0, 500, 1000, 1500, 2000, and 2500 µL L-1, in addition to a standard fungicide treatment (azoxystrobin + difenoconazole, 200 + 125 g L-1, respectively) was added. The Petri dishes were incubated at 25 °C in the dark. Mycelial growth was evaluated daily for 19 days, when all treatments reached maximum growth. Sporulation analysis was performed thereafter. Tomato plants were treated with bergamot essential oil, 30 days after transplanting, in the concentrations and fungicide mentioned, in the second pair of leaves. After 72 hours, the pathogen was inoculated using a spray bottle, on treated leaves (second pair of leaves) and untreated leaves (third pair of leaves). The area under the disease progress curve (AUDPC) was calculated based on five severity assessments. The activity of peroxidase (POX), polyphenol oxidase (PPO), and phenylalanine ammonia-lyase (PAL) was evaluated in leaves treated with the concentration of 2500 µL L-1 of essential oil. The concentration of 2500 ?L L-1 reduced mycelial growth and sporulation of the pathogen by 68.15% and 29.48%, respectively. In treatments with application of essential oil, lower AUDPC was observed for a concentration of 2500 ?L L-1, which was statistically similar to that observed for fungicide application, both in treated and untreated leaves. A greater activity of PPO, POX, and PAL was found locally and systemically, both in the second and third leaves, at concentration of 2500 ?L L-1. The essential oil of bergamot can be an alternative for the control of early blight in tomato.