Plant population heterogeneity and pathogen and herbivore levels: a field experiment.

The identity of neighbouring plants in a population is thought to affect the disease levels of an individual plant: disease levels are expected to be higher for susceptible plants surrounded by other susceptible plants as compared to susceptible plants that have resistant plants as neighbours. To explore this idea, plants in a natural population of the annual sunflower, Helianthus annuus, were scored in 1988 for levels of the rust pathogen Puccinia helianthi. Progeny of plants with high rust levels (families predicted to be susceptible to the disease) were grown in 1989 in a series of field plots at either high frequency (all plants in a plot of one family) or in low frequency (a plot with plants of many families). There was a positive correlation between rust levels of the parent plants in 1988 and their progeny in 1989, but the frequency of a family in the experimental plots did not affect rust levels. Generally low rust levels at the site, as well as other factors, probably contributed to the absence of a frequency effect. Herbivore levels and plant survival and reproduction varied among plots, but were not affected by the plot composition.