Host Specificity of Pathogenic Pythium Species: Implications for Tree Species Diversity

Abstract

In the Janzen-Connell hypothesis, host-specific natural enemies enhance species diversity and influence the structure of plant communities. This study tests the explicit assumption of host specificity for soil pathogens of the genus Pythium that cause damping-off disease of germinating seeds and seedlings. We isolated Pythium spp. from soil of a tropical forest in Panama. Then, in an inoculation experiment, we determined the pathogenic!ty of 75 tropical isolates of unknown pathogenicity and seven pathogenic temperate isolates of Pythium on seeds and/or seedlings of eight tropical tree species. Only three tropical isolates, one identified as P. ultimum and two as P. dphanidermdtum, were pathogenic. Tropical pathogenic isolates were pathogenic on 4-6 of eight tree species. Temperate isolates were pathogenic on 0-4 of eight species, indicating that some tropical tree species are susceptible to novel isolates of Pythium. No tree species was susceptible to all isolates and two species were not susceptible to any isolate. Collectively, these results indicate that these Pythium isolates vary widely in their pathogenicity, causing differential mortality of potential host species; likewise, the tree species vary in their susceptibility to a given Pythium isolate. These differences in pathogenicity and susceptibility indicate some support for the Janzen-Connell assumption of host specificity. While they are not restricted to a single species, their intermediate level of specificity suggests that Pythium spp. have the potential to have some effect on forest community structure and diversity.Abstract in Spanish is available at http://www.blackwell-synergy.com/loi/btp.

Distance- and density-dependent seedling mortality caused by several diseases in eight tree species co-occurring in a temperate forest

“…These fungal species are considered facultative pathogens, which originally exhibited wide host ranges (Agrios 1997;Horst 2001). Although little is known of the causal relationships between infectivity and host mortality in most of the individual fungal species, particularly for host tree species (but see Sahashi et al 1995;Packer and Clay 2000;Augspurger and Wilkinson 2007;Seiwa et al 2008), the observed distant-dependent attacks by the predominant diseases (including several fungi) may suggest that generalist pathogens with broad host ranges may cause seedling mortality in a host-specific manner in this forest community. Recently, Sicard et al (2007) found that both infectivity and the degree of leaf damage by the pathogen Colletotrichum lindemuthianum differed among individual host populations and among host plants, because the pathogens were adapted to the local genotypes of the host plant.…”

Virulence of oomycete pathogens from Phragmites australis‐invaded and noninvaded soils to seedlings of wetland plant species

“…2010)), we have little understanding of the species composition, dynamics, and impacts of these pathogen communities on plant performance in their native and introduced ranges. From the few studies that have focused on pathogen populations, it is becoming clear that species within the oomycete genus Pythium can be significant regulators of native plant communities (Mills and Bever 1998; Packer and Clay 2000, 2003, 2004; Augspurger and Wilkinson 2007; Gómez-Aparicio et al. 2012) and, at the same time, facilitate invasiveness of a number of plant species (Reinhart et al.…”

Soil-borne pathogens restrict the recruitment of a subtropical tree: a distance-dependent effect

“…Interpreting host specificity An explicit assumption of the J-C hypothesis, the host specificity of natural enemies, has been examined in some studies (Augspurger and Wilkinson 2007;Gilbert and Webb 2007;Liu et al 2012a;Novotny et al 2002). Ideally, a robust test of host specificity should test the effects of pathogen isolates on multiple potential host species.…”