Use of the systemic fungicide propiconazole for oak wilt management: an assessment of uncharacterized host - pathogen - fungicide interactions.

Abstract

Propiconazole is a systemic fungicide widely used for the control of oak wilt, however, the long-term efficacy of this fungicide has not been well established and treatment effectiveness may be below levels that justify its use in many situations. To date, it is not known if propiconazole applications prevent root graft transmission of Ceratocystis fagacearum nor if they can completely eradicate this pathogen from the root system of an infected tree. Significant translocation of propiconazole into the roots from the point of injection has not been demonstrated and fungitoxic concentrations of the fungicide have not been determined in vivo. Furthermore, symptom development is not induced by pathogen colonization of the root system; oaks respond to infection only when C. fagacearum spreads above ground in the vascular system. Therefore, it is impossible to determine the extent of pathogen distribution in the root systems of trees near disease centers. Treatment failure may result from poorly informed management decisions that do not take into account pathogen distribution or the capabilities of the systemic fungicide being utilized. This dissertation presents the results of three investigations conducted to examine use of propiconazole to control root graft transmission of C. fagacearum in red oaks (Section Lobatae). The distribution of propiconazole in the root system of treated trees was examined using gas chromatography-mass spectrometry. While substantial movement of the fungicide into the root system did occur following injection, the inability of propiconazole to prevent infection or eradicate the pathogen from root-inoculated oaks was demonstrated. The spatial and temporal spread of C. fagacearum in the root system of oaks in or near disease centers, and the role of self- and inter-tree root grafts in pathogen spread was examined. Distribution of C. fagacearum in the root systems of wilted and wilting trees was sporadic, and the prevalence and importance of self-grafts was noted. Finally, the effects of propiconazole on the growth and anatomy of treated oaks was investigated. Plant growth regulating properties of the fungicide suggest that fungitoxicity alone may not be responsible for propiconazoleinduced disease protection. The implications of these previously unknown aspects of the host-pathogen-fungicide interactions in propiconazole treated trees, and suggestions for improved treatment efficacy and oak wilt management are discussed.