ECOLOGICAL AND EVOLUTIONARY DYNAMICS OF PHLOX-ERYSIPHE CICHORACEARUM INTERACTIONS
Abstract
The research examined fundamental aspects of the evolutionary ecology of interactions between Erysiphe cichoracearum, an obligate fungal pathogen, and ten taxa of Phlox, its feral herbaceous host. Experiments focused on: (1) comparative disease development on resistant and susceptible plants, (2) ecogeographic and phylogenetic patterns of host resistance polymorphism, (3) effects of microclimate and host population composition on disease epidemiology, and (4) short-term effects of infections on host performance. The time course of pathogen development was highly variable among hosts. Complete resistance was expressed at four different stages in the pathogen life cycle. Some susceptible hosts exhibited forms of dilatory resistance. Hosts were polymorphic for response to each of two distinctive isolates of the pathogen. Despite differential responses by some plants, the pattern of resistance to each isolate was best correlated with the degree of habitat shading and other environmental features but not with host phylogeny or ploidy level. In experimental populations, disease was generally 5-20 times more severe in shaded versus exposed sites. However, the proportion of resistant plants did not significantly affect epidemiology on susceptible members of populations. In controlled environments, severe infections significantly reduced host survivorship and relative net productivity. Inhibition of flowering potential was also suggested. The research provides strong evidence that E. cichoracearum has been an important selective agent on Phlox and that the environment is the most important factor mediating the intensity of the antagonism. No other feral plant-pathogen system involving perennial herbs and obligate, biotrophic pathogens has been studied as comprehensively. The patterns of resistance polymorphism observed are consistent with hypothetical expectations for highly coevolved plant-pathogen systems, but the effects of host heterogeneity on disease epidemiology within populations and aspects of the degree of specialization of the pathogen are not. The significance of the findings for plant population biology and comparisons with cultivated plant-pathogen systems are discussed.
Degree
Ph.D.
Subject Area
Ecology
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