The Effects of Carvacrol and Thymol on Growth of Guignardia citricarpa, the Causal Agent of Citrus Black Spot on ‘Valencia' Orange Fruit

Citrus black spot (CBS), caused by Guignardia citricarpa, is a fungal disease in Florida that was first discovered in Southwest Florida in 2010 and is slowly spreading within the state. Lesions on the fruit can develop after harvest and become a grade defect at destination markets. Essential oils are aromatic oily liquids obtained from plant organs that have been used to control plant diseases. Essential oils may have direct antifungal activity and/or induce plant systemic acquired resistance. This study evaluated the activity of thymol and carvacrol against G. citricarpa in vitro and on naturally infected ‘Valencia’ orange fruit. In amended agar medium, the in vitro test demonstrated that both essential oils and a 1:1 mixture of the two were effective in inhibiting mycelial growth of G. citricarpa, with the highest efficacy displayed by thymol. Half-inhibition concentration of thymol, carvacrol, and the 1:1 mixture against mycelium growth was 0.017, 0.057, and 0.028 mg/mL, respectively. The same essential oil concentration were mixed in water or a commercial shellac wax to test in vivo efficiency against black spot development on fruit. After harvest, fruit were divided into asymptomatic or symptomatic groups, treated, and evaluated separately. After treatment, fruit were kept at 12 °C with 90% relative humidity and exposed to 5–10 ppm ethylene and continuous light to speed lesion appearance. On asymptomatic fruit, no treatment significantly affected disease incidence, but treatments did significantly affect the disease severity. Dipping fruit in water with 0.5 mg/mL of thymol, carvacrol, or the 1:1 mixture for one minute inhibited lesion appearance by 52%, 40%, and 48%, respectively, compared to control. Fruit coated with shellac containing 2 mg/mL of thymol, carvacrol, or the combination developed 59%, 49%, and 57%, respectively, fewer lesions than the control during storage. None of the treatments significantly inhibited lesion development on fruit that were symptomatic prior to treatment. Both essential oils and the 1:1 mixture at 2 mg/mL in water resulted in phytotoxic peel injury, with the most severe damage caused by thymol that also caused peel injury at 1 mg/mL . No injury was detected in treatments using the same concentrations in the shellac wax. Future tests will test higher essential oil concentration in the wax treatments.