A Novel Fusarium Species Causes a Canker Disease of the Critically Endangered Conifer, Torreya taxifolia

Smith, J. A., O’Donnell, K., Mount, L. L., Shin, K., Peacock, K., Trulock, A., Spector, T., Cruse-Sanders, J., and Determann, R. 2011. A novel Fusarium species causes a canker disease of the critically endangered conifer, Torreya taxifolia. Plant Dis. 95:633-639. A canker disease of Florida torreya (Torreya taxifolia) has been implicated in the decline of this critically endangered species in its native range of northern Florida and southeastern Georgia. In surveys of eight Florida torreya sites, cankers were present on all dead trees and 71 to 100% of living trees, suggesting that a fungal pathogen might be the causal agent. To identify the causal agent, nuclear ribosomal internal transcribed spacer region (ITS rDNA) sequences were determined for 115 fungi isolated from cankers on 46 symptomatic trees sampled at three sites in northern Florida. BLASTn searches of the GenBank nucleotide database, using the ITS rDNA sequences as the query, indicated that a novel Fusarium species designated Fsp-1 might be the etiological agent. Molecular phylogenetic analyses of partial translation elongation factor 1-alpha (EF-1) and RNA polymerase second largest subunit (RPB2) gene sequences indicate that Fsp-1 represents a novel species representing one of the earliest divergences within the Gibberella clade of Fusarium. Results of pathogenicity experiments established that the four isolates of Fsp-1 tested could induce canker symptoms on cultivated Florida torreya in a growth chamber. Koch’s postulates were completed by the recovery and identification of Fsp-1 from cankers of the inoculated plants. Canker disease of Florida torreya, known as gopherwood or stinking cedar (Torreya taxifolia Arn.), hereafter referred to as CDFT, appears to have contributed to this plant being listed as critically endangered by the U.S. Fish and Wildlife Service. Currently, this endemic taxaceous conifer is restricted to bluffs and ravines along the Apalachicola River in Gadsden and Liberty counties in Florida and Decatur County in Georgia (16). T. taxifolia is considered the rarest conifer in North America and one of the most endangered species in the world (4). Although the decline of Florida torreya was first observed in the late 1930s (1), the tree was still common in its habitat in northern Florida and southeastern Georgia through the 1950s. The rapid decline of the species in the early 1900s was initially attributed to an unknown fungal disease based on the abundance of leaf spots and stem cankers (7). Due to decline, and to the lack of seed-bearing trees (1), Florida torreya was considered to be destined for extinction (7). In addition to the decline, this species has been negatively impacted by changes in hydrology, forest structure, heavy deer browse, and a loss of reproductive capability (17). Florida torreya stems killed by disease often re-sprout in a manner reminiscent of American chestnut following Chestnut Blight. Florida torreya has declined by more than 99% over the past century from an estimated population of 357,500 individuals in 1914 to approximately 1,350 in the 1990s (19,20), to current estimates of 400 to 600 individuals (T. Spector, personal communication). Trees in their native range have not reproduced from seed for several decades (18). Despite several attempts to conclusively determine the causal agent of Florida torreya decline, disease etiology has not been elucidated (1–3,8,22). In the first pathology studies conducted on T. taxifolia (1), it was noted that leaf spots, needle necrosis, defoliation, and stem lesions were common on native and cultivated T. taxifolia. Several pathogens were commonly isolated from symptomatic needles (Macrophoma sp., Rhizoctonia solani, Sphaeropsis sp., and Sclerotium rolfsii); however, no pathogens were isolated from cankered stems and Koch’s postulates were not completed. Subsequently, El-Gholl (3) reported Fusarium lateritium as a causal agent by demonstrating this species’ capacity to cause leaf spots. Alfieri et al. (2) completed Koch’s postulates with F. lateritium as a leaf spot pathogen, but did not address whether this species could induce the canker disease. While Schwartz et al. (22) implicated Pestalotiopsis microspora as the causal agent of the canker disease, no information was given on canker development, morphology, or ability to cause mortality. Artificial inoculations using P. microspora resulted in stem canker development (8), but stem mortality was not observed. These reports are considered to be inconclusive given that Pestalotiopsis spp. are considered to be weak opportunistic pathogens (23). Subsequent studies implicated a Scytalidium sp. due to frequent isolation from cultivated and naturally occurring Florida torreya. Artificial inoculations resulted in small lesions on needles, but cankers were not observed. In addition to biotic causes of decline, several studies have reported on changes in soils, drought, global warming, sunlight exposure, and fire regime as possible causes of decline (21). Some of these environmental changes are thought to have occurred because of the building of the Woodruff Dam along the Apalachicola River in 1957, and changing land uses in the surrounding areas. However, none of these environmental hypotheses has been demonstrated as a cause of the decline. Current efforts to manage this endangered species have been hindered by a lack of understanding of the current and historic causes of disease of Florida torreya. As a result, various agencies have taken different approaches to manage Florida torreya depending on which cause the decline is attributed to. For these reasons, more information is needed about the etiology of CDFT in order to develop sound management practices. Accordingly, the present study was conducted to: (i) assess canker incidence among natural populations of T. taxifolia, and (ii) identify the causal agent of CDFT. Corresponding author: Jason A. Smith, E-mail: [email protected] Accepted for publication 26 January 2011. doi:10.1094 / PDIS-10-10-0703 © 2011 The American Phytopathological Society