C6 - Diversity of endophytic fungi on cacao and impact on plant pathogenherbivorous insect interactions in relation to agroforestry diversity and management

Abstract
This subproject aims at assessing the diversity of endophytic fungi in cacao trees in relation to agronomic practices. Our central hypotheses are that (i) the diversity of endophytic fungi is declining from agroforestry to monoculture plantations, that (ii) the incidence of herbivorous pests, ants, and plant pathogens is related to the diversity of fungal endophytes, that (iii) trees harbouring a high diversity of endophytes are better adapted to ENSO droughts and that (iv) management intensification in cacao monocultures will reduce endophytic diversity. We will monitor endophyte diversity at regular intervals in cacao field plots and will establish field and laboratory experiments with nursery grown cacao plants to test these hypotheses.

Summary
Endophytic fungi form a substantial part of diversity in temperate and tropical plants. Studies from Central America, using cacao trees, indicate that endophytic fungi could be regarded hyperdiverse. Surprisingly, only very few studies to date have investigated the impact of endophytic fungi diversity in relation to the colonisation pattern of plant pathogens and herbivorous insects. Moreover, almost nothing is known as how agronomic measures will interact with the endophyte diversity resulting in cascading effects on pollinator, herbivore, and plant pathogen diversity, respectively. We will evaluate the ecosystem service mediated by endophytic fungi to plant health by analysing the diversity of endophytes colonising cacao trees using agroforestry plots already established during previous STORMA phases, differing in intensity of land use and agronomic impact. Diversity of endophytes will be analysed by collecting different parts of the cacao trees (roots, branches and leaves, respectively) and will be characterised either by morphotaxonomical criteria or by rDNA sequencing.

Growth conditions of cacao trees may influence the colonisation pattern by endophytic fungi. Intensification in cacao plantations through weeding, fertiliser application and removal of shading trees might negatively affect both the diversity and incidence of fungal endophytes colonising these trees. We will therefore establish fertilizer treatments (Urea nitrogen fertilizer at 0, 50 and 100 kg per ha and year, respectively) or artificial drought conditions (to mimic ENSO events) to test the hypothesis that environmental conditions impacts the diversity of endophytes and, via cascading effects, also the diversity and abundance of herbivorous insects and plant pathogens. We hypothesize that a loss in endophytic diversity will increase pathogen and herbivore infestations, resulting in reduced productivity. Colonisation of cacao trees by endophytic fungi might be related to the distance to source populations, thus we will manipulate trees to become devoid of endophytes. We will test, whether trees growing nearby pristine forest will be re-colonised more rapidly and will harbour a more diverse endophytic community as compared to trees grown in plantations located in the open landscape. Ants may be important vectors of endophytes. We will therefore test the effects of ant diversity and ant removal on endophyte-pathogen-herbivore interactions. Above and below ground diversity of endophytic fungi might be related to the diversity of mycorrhizas; thus assessment of mycorrhizal diversity by C5 will help in understanding the diversity pattern within endophytic fungi communities on cacao. Finally we argue that the diversity of endophytic fungi will influence ecosystem functions; thus assessment of insect herbivores, pollinators and belowground community pattern (C3) will be linked to diversity of endophytic fungi on cacao trees.