The infection of pyrola (Pyrola asarifolia; Pyrolaceae) by spruce cone rust (Chrysomyxa pirolata; Uredinales): Morphological correlates in the host and consequences for spore dispersal

Abstract

Sexual spores (i.e., basidiospores) of the spruce cone rust (Chrysomyxa pirolata; Uredinales) produced on leaves of a small perennial herb, the common pink wintergreen (Pyrola asarifolia; Pyrolaceae), must travel from the forest floor to the forest canopy, where they infect the female cones of white spruce (Picea glauca; Pinaceae). Asexual spores (i.e., urediniospores) must move among pyrola plants on the forest floor. Spore transmission is complicated by relatively calm wind conditions within the forest, and the short critical period during which the cones of white spruce can be infected. Here, we document a correlation between the orientation of leaves in the pyrola and presence of rust pustules on the leaf’s ventral surface. We experimentally address the consequences to spore dispersal of this change in leaf position and presence of pustules. Transmission of rust spores can increase with higher leaf angles in two ways: (i) by raising the pustules further above the ground surface, thereby increasing the probability that wind will carry spores further (confirmed in experiment 1), or (ii) by increasing the probability that flying insects will alight on the pustules (confirmed in experiments 2 and 3), and either carry asexual spores (i.e., urediniospores) to other pyrola plants, or carry basidiospores to spruce cones. Vertical orientation increased insect attraction to both real and artificial leaves. Presence of rust increased insect attraction to natural leaves, an effect that could not be mimicked with coloured artificial leaves. Colour alone cannot explain attraction of insects (confirmed in experiments 2 and 3). We infer that both wind and flying insects, as potential agents of spore dispersal, are the likely targets of the rust’s alteration of the orientation and structure of the ventral surface of leaves of its pyrola host.