The effects of grazing by large herbivores on biodiversity and ecosystem functioning have been extensively studied, whereas how grazing influence plant diseases, especially in natural grasslands, remains poorly understood. Therefore, we undertook a field study regarding a grazing trial in a temperate meadow steppe grassland to investigate mechanisms underlying grazing-host-pathogen interactions. The effects of cattle grazing at different grazing intensities of 0, 0.23, 0.34, 0.46, 0.69, and 0.92 AU/ha (where 1 AU= 500 kg of adult cattle) on the microenvironment, vegetation characteristics, and occurrence of diseases were evaluated. At the population level, the effects of grazing on grassland vegetation characteristics and disease varied with grassland plant species. Compared with nongrazing, grazing directly decreased the average density, coverage, and disease incidence of palatable and edible forages by 51.4, 62.4, and 82.4% in the 0.92 AU/ha treatment but increased the occurrence and prevalence of disease in remaining small herbs by 752.1%. At the community level, with the increase of grazing intensity, the pathogen load of the whole community in grassland was positively related to host coverage. In addition, there was a trend toward increased microtemperature and decreased microhumidity with increased grazing. Although occurrence of plant diseases in natural grasslands is influenced by a range of factors, comprehensive analysis highlighted the major role that cattle grazing intensity plays in the occurrence of plant diseases in natural grasslands. In addition to its direct effect, grazing also indirectly affects disease occurrence by shifting plant community structure and the microenvironment. However, direct effects of grazing intensity affected disease occurrence more than indirect effects.
Keywords: biodiversity; cattle grazing intensity; community pathogen load; grazing-host-pathogen interactions; microenvironment; seasonal changes; vegetation characteristic.