Natural Flora Is Indiscriminately Hosting High Loads of Generalist Fungal Pathogen Colletotrichum gloeosporioides Complex over Forest Niches, Vegetation Strata and Elevation Gradient

Abstract

Crop pathogenic fungi may originate from reservoir pools including wild vegetation surrounding fields, and it is thus important to characterize any potential source of pathogens. We therefore investigated natural vegetation's potential for hosting a widespread pathogenic group, Colletotrichum gloeosporioides species complex. We stratified sampling in different forest environments and natural vegetation strata to determine whether the fungi were found preferentially in specific niches and areas. We found that the fungi complex was fairly broadly distributed in the wild flora, with high prevalence in every study environment and stratum. Some significant variation in prevalence nevertheless occurred and was possibly associated with fungal growth conditions (more humid areas had greater prevalence levels while drier places had slightly lower presence). Results also highlighted potential differences in disease effects of strains between strata components of study flora, suggesting that while natural vegetation is a highly probable source of inoculums for local crops nearby, differences in aggressiveness between vegetation strata might also lead to differential impact on cultivated crops.

Keywords: Colletotrichum gloeosporioides species complex; anthracnose risk; fungal niche; natural vegetation; vegetation strata.

Figure 1

Study area. Study plots were sampled in the island of Guadeloupe.

Figure 2

Some isolated strains. Scale bars are 20 µm. Strain from (1) a leaf with typical fungal necrosis from Ipomea setifera (floor strata in edge environment, ‘diseased’ leaf), (2) a diseased leaf from Phyllanthus mimosoïdes tree (canopy strata deep forest environment), (3) a healthy leaf from Philodendron lingulatum (understory strata from riparian environment), and (4) a healthy leaf from Psychotria urbaniana (understory strata from hill environment).

Figure 3

Global prevalence of species from C. gloeosporioides complex in the different forest environments. Hill forest plots had marginally lower prevalence (p < 0.1), mostly due to fairly lower variation in prevalence compared to other niches, and riparian forest plots had significantly greater prevalence (p < 0.05). Each environment is estimated via three plots.

Figure 4

Global prevalence of species from C. gloeosporioides complex in the different vegetation strata. ‘Floor’ and ‘understory’ have slightly greater and marginally significant prevalence (p < 0.1) compared to ‘canopy.’ Each stratum is estimated via 12 plots.

Figure 5

Effect of altitude on global Colletotrichum prevalence (p < 0.0345, Table 1). For the sake of clarity, the average for each spot is reported, though data are binomial at plant level.

Figure 6

Scheme of significant correlations for all Colletotrichum prevalence, at global and strata levels. Only significant correlations are illustrated. Dash lines associate plot prevalence; grey lines associate strata prevalence from either diseased or healthy leaves estimates to their global equivalent; and black lines associate diseased or healthy estimates to their counterpart correlate. All correlations were positive. Coefficient values are reported in Table 2.