Production of defensive metabolites by Pinus patula X Pinus tecunumanii  hybrids in response to Fusarium circinatum infection

The pathogen Fusarium circinatum, causal agent of pitch canker disease, is currently one of the biggest fungal threats to pine health worldwide. Symptomatic infections are associated with a high mortality rate, particularly for nursery and established plants. Pitch canker disease results in reduced growth and significant annual losses in the South African forestry industry. Pines respond to insect damage and fungal infection by forming traumatic resin ducts, as well as upregulating the production of defence compounds. These phytochemicals include terpenes, the main chemical constituents of pine resin, and phenolics, produced in specialized cells of the secondary phloem. Many of these compounds belonging to these two phytochemical groups are known to have inhibitory or lethal effects on pine pests and pathogens. Although most Pinus species are susceptible to F. circinatum, there is variation in susceptibility to this pathogen among the different species and their hybrids. Resistance of a pine species to the pitch canker fungus is a major determining factor in the species' value to the pine industry; however, the underlying mechanisms of resistance are poorly understood. To explore defense responses between resistant and susceptible pines, we used GCMS and LCMS to characterize the phytochemical changes in young P. patula X P. tecunumanii hybrid clones in response to F. circinatum infection. A significant increase was observed in the concentration of terpenes and phenolics between five- and 14-days post-inoculation. More resistant hybrids plants showed moderate disease symptoms as well as lower concentrations of defensive phytochemicals, both constitutively and in response to F. circinatum infection. These findings suggest that increased concentrations of terpenoid oleoresin and phenolics are not part of the defence strategy of pine against infection by F. circinatum. Understanding F. circinatum’s ability not only to overcome, but seemingly benefit from a more severe phytochemical defense response, will help explain the enduring pervasiveness of this global pine pathogen.