Plant Pathogens, Host Specificity, and Cross-Kingdom Transmission Ecology

Foundational work

The foundational discoveries in this area came from work on Puccinia monoica in the meadows near RMBL. Roy (Roy, 1993) first described the striking case of floral mimicry by this rust fungus in Nature, showing that infected mustards produce flower-like structures that attract insects and aid fungal reproduction. A companion study that same year examined how rust infection was distributed across genetically diverse, mostly asexual populations of Arabis holboellii, and found — contrary to simple frequency-dependent expectations — that clonal diversity did not reliably predict reduced disease (Roy, 1993). Together these papers established the Gothic-area rust system as a model for studying pathogen ecology in natural plant populations.

Follow-up work in the mid-1990s fleshed out the mimicry mechanism and its community consequences. Roy (Roy, 1994) synthesized how fungi exploit pollinators and outlined the evolutionary stakes for floral biology. Experiments with artificial arrays showed that pseudoflowers and co-blooming wildflowers like buttercups and anemones can facilitate each other's insect visitation at some densities and compete at others (Roy, 1994) (Roy, 1996). Roy and Raguso (Roy & Raguso, 1997) then demonstrated that scent alone — not just visual resemblance — draws insects to pseudoflowers, sharpening understanding of how the mimicry actually works.

Key findings

A central thread across this body of work is that a single fungal pathogen can reshape the pollination ecology of an entire plant community. Pseudoflowers are not passive mimics of one model species but generalized flower-mimics that compete with and facilitate real flowers depending on density and the identity of the visiting insects (Roy, 1994). Insects can distinguish pseudoflowers from true flowers by fragrance, suggesting that some visitors actively choose them rather than being fooled (Roy & Raguso, 1997). Pseudoflowers can also influence the seed set of unrelated, non-host plants growing nearby (Roy, 1996) — a striking example of how a pathogen reaches beyond its direct host to affect community-wide reproduction.

Host biology turns out to matter as much as pathogen biology. Many of the Arabis hosts in the Gothic area reproduce asexually through pseudogamous apomixis, so their populations consist of mixtures of genetically identical clones with variable susceptibility to different rusts (Roy, 1995) (Roy, 1993). Reciprocal transplant experiments showed that local clones actually had less disease, less herbivory, and higher fitness than foreign clones — the opposite of what simple negative frequency-dependent selection would predict (Roy, 1998). Meanwhile, what looked like one rust species turned out to be several cryptic species in the Puccinia monoica complex (Roy et al., 1998), complicating any simple story about a single host-pathogen pair.

Pathogens in this system also ripple outward to herbivores and to other plant species. Fungal infection significantly reduces insect herbivory on infected hosts, with the strongest effect at the highest infection levels (Niedweicki, 2015). Work on Indian paintbrush (Castilleja) in the same meadows showed that pollinator behavior is context-dependent: each paintbrush species was preferred when it matched the surrounding plant community, and pollinator constancy broke down in mixed hybrid zones, likely promoting gene flow among species (Hersch & Roy, 2007). Subsequent genetic work confirmed that differences in chromosome number shape, but do not fully prevent, hybridization across paintbrush species (Hersch-Green & Cronn, 2009) (Hersch-Green, 2012).

Current frontier

Research in the 1990s established the Puccinia monoica system as a textbook example of pathogen-mediated floral mimicry. Recent studies since 2020 have shifted toward asking how environmental drivers — especially climate — modulate transmission in this and related systems. Ferrer (Ferrer, 2025) combined field surveys near Gothic with experimental drought treatments and found that rust infection prevalence rises with host plant density and with wet microclimates, that infected mustards tend to be physically closer to their alternate grass host than uninfected ones, and that experimentally imposed drought sharply reduces infection in grasses. These results connect long-standing questions about host density and life cycle to the very practical question of how warmer, drier summers will alter pathogen pressure in subalpine meadows.

Frontier work has also expanded beyond plant fungi to cross-kingdom transmission ecology. Pelzel-McCluskey et al. (Pelzel‐McCluskey et al., 2021) reviewed vesicular stomatitis virus outbreaks across the Rocky Mountain region, showing that the 2019 outbreak — the largest in 40 years, affecting more than 1,100 premises across eight states — was followed in 2020 by a wave that originated from overwintering of the same virus rather than a fresh incursion, with Rocky Mountain states curiously reporting zero confirmed cases that second year. New methods accompanying these studies include maximum-likelihood phylogenetic analysis of viral genomes, model selection for transmission pathways, and gravimetric soil moisture measurements paired with ordinal rust severity scoring in the field.

Open questions

Several important questions remain. How will shifting snowmelt and summer drought reshape the spatial overlap of multi-host pathogens like Puccinia monoica, given that infection depends on close proximity between mustards and grasses and on wet microclimates? Can the genetic architecture of mostly asexual host populations keep pace with evolving cryptic pathogen species, or will climate stress tilt the balance? How do livestock viruses overwinter in some Rocky Mountain locations but not others, and what role do insect vectors and weather patterns play in the gap years between outbreaks? Finally, integrating molecular tools — including surveys of microvirus diversity and other under-sampled microbes — with long-term field observations at RMBL offers a promising path for the next decade of work on how pathogens shape mountain ecosystems.

References

Ferrer (2025). Biotic and abiotic drivers of pathogen prevalence in a rust fungus with multiple plant hosts. →

Niedweicki (2015). Fungal Phytopathogens Decrease Plant-Insect Interactions. →

Hersch-Green (2012). Polyploidy in Indian paintbrush (Castilleja; Orobanchaceae) species shapes but does not prevent gene flow across species boundaries. American Journal of Botany. →

Hersch-Green, Cronn (2009). Tangled trios?: Characterizing a hybrid zone in Castilleja (Orobanchaceae). American Journal of Botany. →

Hersch, Roy (2007). Context-dependent pollinator behavior: An explanation for patterns of hybridization among three species of Indian paintbrush. Evolution. →

Pelzel-McCluskey et al. (2021). Review of Vesicular Stomatitis in the United States with Focus on 2019 and 2020 Outbreaks. Pathogens. →

Roy (1993). Floral mimicry by a plant pathogen. Nature. →

Roy (1993). Patterns of rust infection as a function of host genetic diversity and host density in natural populations of the apomictic crucifer, Arabis holboellii. Evolution. →

Roy (1994). The effects of pathogen-induced pseudoflowers and buttercups on each other's insect visitation. Ecology. →

Roy (1994). The use and abuse of pollinators by fungi. Trends in Ecology and Evolution. →

Roy (1995). The breeding systems of six species of Arabis (Brassicaceae). American Journal of Botany. →

Roy (1996). A plant pathogen influences pollinator behavior and may influence reproduction of nonhosts. Ecology. →

Roy (1998). Differentiating the effects of origin and frequency in reciprocal transplant experiments used to test negative frequency-dependent selection hypotheses. Oecologia. →

Roy et al. (1998). Cryptic species in the Puccinia monoica complex. Mycologia. →

Roy, Raguso (1997). Olfactory versus visual cues in a floral mimicry system. Oecologia. →