Essential oils for managing anthracnose in mango (Mangifera indica): laboratory results do not translate into field efficacy

Essential oil–based products with broad plant disease control claims are commercially available and may be a practical alternative to copper fungicides for crop protection in organic mango orchards. We evaluated the disease control efficacy and crop safety of thyme oil, savory oil, and tree tea oil through replicated in vitro, in vivo (detached leaf and potted trees), and field assays. Three Colletotrichum species associated with mango anthracnose were tested in vitro, whereas only C. siamense was used for in vivo assays. Within the range of concentrations tested in vitro (62.5 to 2,000 μl active ingredient [a.i.]/liter), thyme and savory oil displayed fungicidal activity, whereas no fungistatic or fungicidal activity was observed with tea tree oil. In the in vivo assays, none of the treatments based on a preventive application rate of thyme (1,150 μl a.i./liter), savory (2,000 μl a.i./liter), or tea tree oil (342 μl a.i./liter) were effective in preventing the development of anthracnose on wounded and artificially inoculated leaves. Although field applications of thyme or tea tree oil did not result in phytotoxicity or negative impacts on fruit yield, they were ineffective in reducing the incidence and severity of naturally occurring anthracnose. Applications of copper hydroxide approved for organic agriculture were effective in controlling anthracnose in the field, and no added benefits were found by premixing this compound with thyme oil. Results indicate that essential oil products based on thyme or tea tree oil are inefficient at controlling anthracnose in mangoes.