Fungal pseudoflowers can influence the fecundity of insect-pollinated flowers on Euphorbia cyparissias

Euphorbia cyparissias is often infected by a rust fungus from the species complex Uromyces pisi. Infected plants do not form flowers but pseudoflowers, rosettes of yellow leaves upon which the fungus presents gametes in a sweet-smelling sugary nectar. Insects feed on the nectar and transfer fungal gametes between mating types. Here we show that pseudoflowers and the flowers of non-infected hosts overlap in "flowering" for more than one month, even though pseudoflowers start "flowering" one month earlier than true flowers. As the fungus and its host also share insect visitors, we hypothesized that they might interact either by facilitating each others' insect visits or by competing for "pollinators". We addressed this question by weekly grid-mapping an Euphorbia population near Zermatt in the Swiss Alps and relating the average density and frequency around hosts and pseudoflowers during their "flowering" period to their fitness (success in seed set and spore production). The seed set of uninfected Euphorbia plants was significantly higher when they were surrounded by fewer pseudoflowers. The fungus, on the other hand, was not obviously influenced by the presence of host flowers. Instead, the reproductive success of single pseudoflowers decreased with a higher density of pseudoflower-neighbors. Our results suggest that the fungus might be a pollinator-competitor for Euphorbia flowers.