Infection process and resistance in the weed pathosystem Senecio vulgaris-Puccinia lagenophorae and implications for biological control

The infection process and the level and type of resistance in the pathosyst em of a native weed and a naturalized rust fungus, Senecio vulgaris L. - Pu ccinia lagenophorae Cooke, were examined. Four inbred plant lines from Swit zerland (2), The Netherlands, and the United Kingdom, each at two stages of devolpment, were exposed to corresponding rust lines under controlled cond itions. Fluorescence and light microscopy were used to assess the infection process and to quantify genotype effects. Component analysis was used to p artition disease development and define the infection process. Germinating aeciospores of P. lagenophorae showed all the characteristics of the monoka ryotic parasitic stage of rust infection with direct penetration and monoka ryotic haustoria formation. Haustoria formed between 3 and 6 days after ino culation at low frequency. The highest level of resistance, for which diffe rences between plant lines were detected, occurred at penetration peg forma tion. All the studied host-pathogen interactions were compatible. The Dutch plant line was most susceptible to all rust lines, and the Dutch rust line was the most aggressive. Susceptibility of leaves increased with the leaf developmental stage. There was a continuous range of variation in susceptib ility without differential genetic interactions, thus indicating race-nonsp ecific quantitative resistance. The use of the more aggressive Dutch rust l ine may increase the level of disease, thus stimulating epidemics for biolo gical control. Over the long term, less susceptible genotypes of the weed m ay be selected, although differences in susceptibility among plant lines we re relatively small.