The Effects of Habitat Fragmentation and Connectivity on Plant Disease

JOHNSON, BRENDA LYNN. The Effects of Habitat Fragmentation and Connectivity on Plant Disease. (Under the direction of Nick M. Haddad.) Within a large-scale habitat corridor experiment, I performed both experimental and observational studies to determine the effects of habitat fragmentation, habitat edge, and patch connectivity on the movement and incidence of fungal plant diseases. Increased spread of infectious disease is often cited as a potential negative effect of habitat corridors, and increases in the amount of habitat edge that are inevitable byproducts of corridor creation could also impact the incidence and development of plant disease. However, the impacts of corridors and habitat edges on plant disease dynamics remain empirically untested. Using sweet corn and southern corn leaf blight as a model plant-pathogen system, I experimentally tested the impacts of connectivity and habitat fragmentation on pathogen movement and disease development. I found that corridors do not facilitate the movement of wind dispersed plant pathogens, that connectivity of patches does not enhance levels of foliar fungal plant disease, and that edge effects are the key drivers of plant disease dynamics. Over time, less edgy patches had higher proportions of diseased plants, and distance of host plants to habitat edges was the greatest determinant of disease development. To test the effects of habitat connectivity and edge on the incidence of naturally occurring plant disease, I surveyed foliar lesions on three native Lespedeza species. I found that connectivity of habitat patches did not affect levels of disease and that incidence of wind dispersed foliar fungal diseases was significantly higher close to habitat edges, further demonstrating that edge effects play an important role in plant disease dynamics. I also found that density of host plants was significantly higher farther from habitat edges, contradicting previous studies that relate higher host densities to increased disease load. Environmental variables also showed strong edge effects, with significantly higher temperatures and light intensities at the interior of habitat patches, providing possible mechanisms for these disease patterns. Results from both studies show that worries over the potential harmful effects of connectivity on disease dynamics are misplaced, and that, in a conservation context, diseases can be better managed by mitigating edge effects. The Effects of Habitat Fragmentation and Connectivity on Plant Disease by Brenda Lynn Johnson A thesis submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the Degree of Master of Science Fisheries and Wildlife Sciences Raleigh, North Carolina 2010