Grazer facilitation of fungal infection and the control of plant growth in south-western Atlantic salt marshes

1. While great effort has been made in documenting the processes that drive plant-induced susceptibility after herbivore attack and it is widely accepted that herbivores can facilitate plant diseases, the relative importance of this interaction in controlling plant growth in natural systems remains largely unexplored. 2. In south-western Atlantic salt marshes, we investigated the importance of disease after herbivory by examining: (i) whether or not a herbivorous crab facilitates disease (i.e. fungus infection) in marsh plants (Spartina alterniflora and S. densiflora) when clipping off small portions of leaves and (ii) the separate and interactive effects of crab grazing but fungal infection in controlling marsh plant growth. 3. Our results show that crab grazing facilitates fungal infection in Spartina leaves. A factorial field experiment shows that both direct crab herbivory and fungal infection strongly suppress plant production (by more than 50%). 4. Synthesis. These experimental results demonstrate that fungal infection following herbivory attack can decrease salt marsh plant production and that increased disease susceptibility can be a fundamental factor in controlling plant production in natural ecosystems, even in cases where herbivores do not directly inoculate the pathogen but only damage plant tissue.