Incidence of leaf pathogens in the canopy of a Mexican tropical wet forest
The patterns of leaf pathogen attack and mechanisms of infection in the canopy of a Mexican tropical rain forest are described based on a survey of 30 tree and liana species. The study included the identification of the causal agents of pathogenic damage, as well as a description of the magnitude, interspecific and seasonal variation of leaf damage. Foliage of all tree species was attacked by fungal pathogens that caused leaf spots. Sixty two percent of the leaves surveyed in the dry season (N = 4348), and 59% of the leaves surveyed in the wet season (N = 3077) showed evidence of damage by fungal pathogens. Overall, leaf area damaged per plant was very low and similar in both seasons (= 5% of leaf area damaged). Within the low levels of damage, there was considerable variation in the degree of infection among species: the percentage of diseased leaves ranged from 12 to 97.19, while leaf area damaged per plant ranged from 0.90 to 12.54%. A salient pattern of the survey is that 59% of the leaves in the dry season and 54% of the leaves surveyed in the wet season were concurrently attacked by herbivores and fungal pathogens, and only between 2 and 4% of the leaves bore damage by pathogens alone (dry and wet seasons respectively). Pathogenicity assays experimentally confirmed that the predominant mechanism of fungal establishment was wounding, such as that caused by herbivory and only rarely did infection occur through direct contact (without wounds). This study reveals that leaf fungal diseases are a prevalent component of the foliage in the tropical rain forest canopy, although the magnitude of affected leaf area is considerably low.