Control of pitch canker using essential oils from medicinal and aromatic plants

Pitch canker is a tree disease caused by the fungus Fusarium circinatum (teleomorph = Gibberella circinata), known to affect mainly conifers, namely susceptible Pinus species and Douglas fir (Pseudotsuga menziesii). This fungal phytopathogen is usually transported from infected trees to susceptible healthy pines by water (e.g., rain), wind or vectored by bark beetles, and induces damping-off, resin-streaming cankers on main stems and lateral branches, shoot dieback, needle chlorosis or discoloration, cone death, and increased tree mortality. Severity of the disease is heavily dependent on the pine host and biotic and abiotic conditions, such as temperature, humidity, soil properties, insect pathogens, etc. A much higher incidence is detected at pine nurseries and plantations than on wild pine stands. In nurseries, pest management focuses on proper hygiene and phytosanitary measures that can include the use of insecticides and fungicides for direct control. Nevertheless, many active biocides have been recently withdrawn in Europe, which prompted research on natural compounds, such as essential oils (EOs). These complex mixtures of secondary metabolites are mainly comprised of terpenes and phenolic compounds, and can show very high biologic activities. The present work reviewed the available literature on EOs analyzed against Fusarium circinatum. The use of EOs in fungicidal formulations against pitch canker is still preliminary. The EOs from 62 plant species were reported against this pathogen, with over 50% of plants belonging to the Myrtaceae, Compositae and Apiaceae families. The highest activities (lowest minimum inhibitory concentrations) were obtained for the EOs of Cinnamomum verum, Cymbopogon citratus, Foeniculum vulgare, Syzygium aromaticum and Thymus vulgaris. Nevertheless, few reports detail EO chemical composition, which hinders the assessment of structure-activity correlations. A higher investment on the screening of natural compounds as eco-friendly fungicides against pitch canker is necessary to promote more sustainable disease control measures.