PARENT SESSION
Organized Oral Session 35: Mutualism, competition, and invasion: Applying ecological theory to agriculture
Organizer(s): LE Drinkwater, J Maul, DK Letourneau, and KL Monsen
Wednesday, August 10, 1:30 PM - 5:00 PM, Meeting Room 511a, Level 5, Palais des congrès de Montréal

Invasion by plant pathogens in relation to nutrient availability and biodiversity.

van Bruggen, Ariena^*,1, Termorshuizen, Aad¹, ¹ Wageningen University, Wageningen, the Netherlands

ABSTRACT- From invasion theory it is known that in simplified systems, like agroecosystems, invasibility decreases with increasing biodiversity, while in natural ecosystems there is an optimum biodiversity, above which invasibility may increase again. Invasion theory has been built primarily on invasions by exotic plants and animals. Invasions by microorganisms have been explored less. Here, we do not consider only exotic species as invaders but any pathogen species not yet present in a crop in a particular growing season. Organically managed agroecosystems are generally more diverse than their conventional counterparts due to the absence of pesticides and easily available plant nutrients, the enhancement of the terrestrial food web by addition of various organic materials, and frequently, greater plant diversity. From the theory of invasion biology we expect a reduced spread of diseases in organic compared to conventional farms. In several field experiments and bioassays with introduced pathogens we showed that soil-borne diseases are generally less severe in organic than conventional agroecosystems. In some cases we found a negative correlation between pathogen invasion and microbial diversity in soil, and a positive correlation between nitrogen availability and pathogen spread. Invasion by foliar plant pathogens is frequently slowed down by lower substrate quality and genetic complexity, but perthotrophic r-strategists are little affected by microbial or plant diversity and can be highly destructive in both organic and conventional agroecosystems. Among these pathogens are many Phytophthora species that have short generation times, high reproductive capacities, and are genetically highly variable and adaptable. Examples of endemic and exotic invasive plant pathogens will be presented.

Key words: biodiversity, invasibility, organic agriculture, plant pathogens