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Abstract

Conifer nurseries serve as reservoirs for a diverse suite of native and exotic pathogens, e.g., Fusarium and Phytophthora spp. These pathogens can move undetected through asymptomic, infected nursery stock, which can cause mortality in field-planted seedlings and risk pathogen introductions into novel landscapes (e.g., restoration areas). Once introduced, these pathogens can persist in landscapes and harm native plant communities due to costly and inefficient management practices. Broad-spectrum chemical treatments within nurseries and field sites are expensive and affect non-target, beneficial microorganisms in the soil. To mitigate pathogen introductions into field sites, targeted management strategies can be developed for nurseries by determining pathogen composition and pathogenicity/virulence. This project will address to: 1) evaluate the occurrence of Fusarium spp. in conifer tree nurseries across the U.S.A.; and 2) identify occurrence patterns among populations of pathogenic Fusarium, including host associations, geographic distribution, and potential movement. Because diverse Fusarium spp. can cause damping-off and/or root rot diseases of conifer seedlings in tree nurseries, Fusarium isolates were collected from western, midwestern, and southern U.S.A. to determine diversity of Fusarium spp. Based on DNA sequences of mitochondrial small subunit gene and elongation factor 1-alpha, F. oxysporum, F. commune, F. redolens, F. fujikuroi, F. proliferatum, F. solani, and F. equiseti were among the 10 species found from conifer tree nurseries in these regions. This project provides a framework for developing tools to quantify disease pressure by identifying pathogen populations and key mechanisms of disease development (i.e., roles of effector profiles) shared among different Fusarium spp.


Introduction

The genus Fusarium is ubiquitous in most container and bareroot nurseries on healthy and diseased conifer seedlings, in nursery soils, and on conifer seeds of several species, especially Douglas-fir (Pseudotsuga menziesii), western white pine (Pinus monticola), and ponderosa pine (Pinus ponderosa), and loblolly pine (James et al. 1990, Cram and Fraedrich 2009). Since the first report of this Fusarium root rot in forest nurseries, the major pathogen was previously identified as Fusarium oxysporum based on morphology (Bloomberg, 1981). However, selected Fusarium spp. isolates that had previously been characterized as pathogenic on Douglas-fir seedlings displayed a range of high, moderate, and low virulence (Stewart et al. 2006). Stewart et al. (2006) showed that all the highly virulent isolates were identified as F. commune, a recently named species (Skovgaard et al. 2003) based on DNA sequences. Further, F. proliferatum was recently identified as a pathogen of sugar pine (Pinus lambertiana) in California (Stewart et al. 2016). DNA sequences from the mtSSU and tef1 regions are useful for distinguishing Fusarium species.

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Colorado State University

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Fort Collins, CO 80523-1177

Email: John.Dobbs@colostate.edu

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