One hypothesis for plant invasion success is the “enemy release hypothesis”, where an invasive species leaves behind natural enemies adapted to attack it. Over time fresh enemies should emerge and hamper invasive species, as resident species adapt to utilize invaders. Pathogens in particular could have rapid evolutionary responses permitting use of novel hosts. What conditions favor the emergence and persistence of novel pathogens, and in particular those whose virulence is sufficient to check the spread of an invader?
We addressed this question by examining emergence and persistence of fungal pathogens on host plants, with a focus on a little-studied mode of disease transmission -- leaf litter. Our motivation comes from fungal pathogens recently emerged on Microstegium vimineum, an invasive annual grass of the eastern USA. Our model describes annual plant growth. Biomass at the end of the season is converted to litter, which interferes with seedling establishment. Pathogens have two modes of spread: parasitic (characterized by growth within a plant and plant-to-plant transmission), and saprophytic (characterized by fungal growth in litter). These two modes are linked through incidental infection of seedlings as they emerge through litter in early spring and later by the death of infected annuals, generating infected litter.
Results/Conclusions
We used R0, the number of secondary infections per primary infection, to assess emergence potential. Pathogen R0 cleaves into components reflecting saprophytic and parasitic modes of spread, which likely differ in their effectiveness across environmental conditions. The parasitic component increases, but the saprophytic component decreases, with increasing plant:litter ratio. This is because fungal parasites require susceptible plants to spread whereas fungal saprophytes require uncolonized litter to spread,. Environments with faster decomposition rates have higher plant:litter ratios and therefore are more likely to favor parasitic pathogens.
Transmission mode also influences likely virulence. Highly virulent pathogens -- which are of most interest in control of invasive species -- are more likely to emerge as saprophytes. Disease prevalence is higher for parasites than saprophytes. Virulent fungal parasites thus have much larger host population effects than saprophytes. Such strong population effects mean that parasites are more at risk of driving their host extinct than saprophytes. Hence, virulent parasites themselves are more prone to extinction. The most virulent saprophytes often cause cyclic plant population dynamics, inducing stochastic extinctions. Our results suggest that saprophytic fungal pathogen spillover via incidental infection could help keep in check plant invasions.