Edge effects, not connectivity, determine the incidence and development of a foliar fungal plant disease

Abstract

Abstract. Using a model plant-pathogen system in a large-scale habitat corridor experiment, we 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. Increased spread of infectious disease is often cited as a potential negative effect of habitat corridors used in conservation, but the impacts of corridors on pathogen movement have never been tested empirically. Using sweet corn (Zea mays) and southern corn leaf blight (Cochliobolus heterostrophus) as a model plant-pathogen system, we tested the impacts of connectivity and habitat fragmentation on pathogen movement and disease development at the Savannah River Site, South Carolina, USA. 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. Variation in average daytime temperatures provided a possible mechanism for these disease patterns. Our results show that worries over the potentially harmful effects of conservation corridors on disease dynamics are misplaced, and that, in a conservation context, many diseases can be better managed by mitigating edge effects.

Can dispersal mode predict corridor effects on plant parasites?

“…Further experiments are needed to test the mechanisms supporting our observed relationship between dispersal mode and landscape connectivity (Johnson and Haddad 2011), and to test the generality of our results in other species and landscapes. For example, other parasite transmission systems such as pollinator-transmitted fungi (Thrall and Antonovics 1995) could be considered.…”

Southern leaf blight disease severity is correlated with decreased maize leaf epiphytic bacterial species richness and the phyllosphere bacterial diversity decline is enhanced by nitrogen fertilization

“…In a mapping study, for example, un-inoculated plant SLB disease scoring was only possible a week later than inoculated plant scoring, though the genetic architecture results were similar for each type of inoculation (Balint-Kurti et al, 2007) and the un-inoculated and inoculated plots were adjacent to each other. Transmission of SLB has also been shown to vary by plot configuration (Johnson and Haddad, 2011). Variation in space as well as in time increases the complexity of infection patterns and complicates experimental design.…”

Microevolutionary Effects of Habitat Fragmentation on Plant-Animal Interactions

“…Besides the consequences of the disruption of plant-animal mutualisms, a less obvious scenario arises from the modification of plant-animal antagonisms in fragmented habitats. Small fragments are more exposed to herbivores and plant pathogens due to microclimate conditions favorable for their proliferation [9,49]. Despite the fact that herbivory rates often decrease with fragment size [50], small fragments are more exposed to novel antagonists (e.g., fungal diseases, parasitoids, galling, and mining insects) that arrive from the matrix and are strengthened by edge effects [49,51,52].…”

“…Small fragments are more exposed to herbivores and plant pathogens due to microclimate conditions favorable for their proliferation [9,49]. Despite the fact that herbivory rates often decrease with fragment size [50], small fragments are more exposed to novel antagonists (e.g., fungal diseases, parasitoids, galling, and mining insects) that arrive from the matrix and are strengthened by edge effects [49,51,52]. The spread of antagonists from the matrix could be strengthened by the use of biological corridors as a conservation strategy, since they may act as dispersion routes for diseases [49,52].…”

“…Despite the fact that herbivory rates often decrease with fragment size [50], small fragments are more exposed to novel antagonists (e.g., fungal diseases, parasitoids, galling, and mining insects) that arrive from the matrix and are strengthened by edge effects [49,51,52]. The spread of antagonists from the matrix could be strengthened by the use of biological corridors as a conservation strategy, since they may act as dispersion routes for diseases [49,52]. This situation is of critical importance when remnants of native habitat are surrounded by agricultural matrices, from which large generalist herbivores may act as pathogen vectors [53,54].…”