Insect Dispersal of Fungal Disease: Effects of Ustilago Infection on Pollinator Attraction in Viscaria vulgaris

Transmission efficiency is an important part of plant-disease interactions. Here I report on differences in insect-attracting traits of healthy and fungus-infected plants, in a system whose pollinating insects disperse fungal spores. Viscaria vulgaris (Caryophyllaeae) infected by the anther smut, Ustilago violacea, bloomed earlier and remained open longer than healthy plants. Infected flowers produced significantly less nectar than did healthy ones, but total nectar production per inflorescence was similar in the two types, because infected plants produced more flowers per inflorescence. Sugar content of nectar was similar in infected and healthy flowers. I trapped few airborne anther smut spores, suggesting that pollinating insects are the important disease vector. Bumblebees, the major dispersers of pollen and spores, visited fewer infected flowers than expected by chance. The early flowering of infected flowers forces naive pollinators to visit infected flowers, thus they become disease vectors even though they prefer healthy plants later in the flowering season. Spore dispersal reached its peak before that of Viscaria pollen, suggesting a very efficient dispersal system.