We develop a model to examine the interaction between resource competition and infection by parasites and mutualists that affect resource uptake. The model is a biomass Susceptible-Infected model. The model demonstrates that unless the R* of the infected class is higher than the R* of the susceptible class, the susceptible class will quickly be excluded from the system. We postulate that the difference between these R*s will predict the range of transmissibility (β) that will allow both infected and susceptible plants to co-exist. We then demonstrate that reproduction may be critical to allowing susceptible plants to continue to exist in the system. In contrast, the continued vegetative growth of infected plants allows infected plants to exist and dominate the system at relatively low transmission rates. This may have important consequences for re-establishing mutualistic interactions in systems that have been damaged, as evolution may not have favored high transmission rates for plant infections, particularly mutualistic ones.