Thinning reduces the effect of rust infection on jewelweed (Impatiens capensis)

We examined the relationship between plant density and the probability of infection by a rust (Puccinia recondita) in a natural population of jewelweed (Impatiens capensis). We also reduced plant density in field manipulations to determine whether there is any synergism between intraspecific competition and infection by the rust. The results showed no relationship between plant density and the probability of infection. However, we found that the effect of infection depends on plant density. Uninfected plants grew significantly faster than infected plants under natural conditions of high density, but infection had no significant effect on growth where density was experimentally reduced. Hence, the effect of disease on jewelweed depends on the plant's competitive environment. Such interactions between competition and disease are likely to be important to the population biology and breeding-system evolution of plants.