Resistance-genes affect how pathogens maintain plant abundance and diversity
Stump et al., 2020

Specialized pathogens are thought to maintain plant community diversity; however, most ecological studies treat pathogens as a black-box. Here we develop a theoretical model to test how the impact of specialized pathogens changes when plant resistance (R) genes mediate susceptibility. This work synthesizes two major hypotheses: the gene-for-gene model of pathogen resistance and the Janzen-Connell hypothesis of pathogen-mediated coexistence. We examine three scenarios. First, R-genes do not affect seedling survival; in this case, pathogens promote diversity. Second, seedlings are protected from pathogens when their R-gene alleles and susceptibility differ from those of nearby conspecific adults, thereby reducing transmission. If resistance is not costly, pathogens are less able to promote diversity because populations with low R-gene diversity suffer higher mortality, putting those populations at a disadvantage, and potentially causing their exclusion. R-gene diversity may also be reduced during population bottlenecks, creating a priority effect. Third, when R-genes affect survival, but resistance is costly, populations can avoid extinction by losing resistance alleles, as they cease paying a cost that is unneeded. Thus, the impact pathogens can have on tree diversity depends on the mechanism of plant-pathogen interactions. Future empirical studies should examine which of these scenarios most closely reflects the real world. 

https://www.amnat.org/an/newpapers/Oct-Stump-A.html

.