The Dynamics of the Plant Lactuca sibirica and the Frequency of Its Rust Puccinia minussensis in River Valleys in Northern Sweden

Abstract

The population dynamics of the rust Puccinia minussensis and its host plant Lactuca sibirica were studied in four localities. Three of the localities were subjected to different degrees of erosion and/or deposition (Kukkola, Aspliden and Hummelholm) during flooding, while one locality (La˙nged) was situated above the flood level. At Kukkola and Hummelholm both the densities of the host plant L. sibirica and the frequency of the rust P. minussensis varied markedly within transects. At the other two localities, La˙nged which is not disturbed and Aspliden which is dry and subjected to erosion, densities and disease frequencies remained constantly low between the years. There was a positive correlation between host densities and frequency of aecia-producing shoots at Hummelholm. However, it is suggested that this correlation is caused by similar responses to variation in environmental conditions and not by a response of the pathogen to host plant densities. Furthermore we found that systemically diseased shoots (aecia) at all localities died before August, while uredial and telial stages did not cause death of shoots. It is suggested that (1) the host densities are primarily regulated by stochastic disturbances and (2) the direct and close fit in the response of the fungus is due to resource limitation