The biology and ecology of cryptic life history strategies of the pitch canker pathogen Fusarium circinatum in Pinus radiata seedlings and alternate grass hosts

Fungal pathogens of plants can be cryptic associates both in hosts that may eventually become diseased, and in plants that will never develop symptoms. Such cryptic associations have important implications for unintended movement of plant pathogens and predictions of disease outcomes. Fusarium circinatum is a pathogen of many pine species worldwide, causing a disease known as pitch canker. Pinus radiata, one of the most susceptible and economically important hosts, is damaged by pitch canker in native forests, and in production systems in South Africa, Chile and Spain. The first objective of this project was to determine if grasses can serve as alternate hosts of F. circinatum, by characterizing endophytic life history strategies in corn (Zea mays) (Chapter 1) and conducing surveys that identified cryptic infection of grasses in native pine forests in California (Appendix 1) and plantations in South Africa (Appendix 2). These studies document the first non-coniferous hosts for F. circinatum. The second objective was to characterize the manner in which F. circinatum colonizes P. radiata seedlings (Chapter 2). The results revealed a commensal to potentially beneficial association within roots, which leads to disease only when colonization extends into shoot tissue. These studies offer a reinterpretation of this fungus as a hemibiotrophic, rather than a necrotrophic, pathogen. Moreover, cryptic root infections in pine seedlings were found to induce a systemic response that rendered plants more resistant to subsequent shoot infections by the same fungus in a dose dependent fashion that did not reduce seedling fitness (Chapter 3). This indicates that regenerating populations may be able to adaptively respond to pathogen infection and thereby persist, contrary to the current concept that seedling survival is contingent upon escape. Cumulatively, these studies point to the possibility that pathogenicity to pines may not be central to the life history of F. circinatum, but may instead be an artifact of introduction into new host environments.