The colonization ratio: a measure of pathogen invasiveness and host resistance in Verticillium wilt of alfalfa

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Abstract
Seeds of four alfalfa cultivars were grown and the plants cloned by rooting cuttings. Individual plants were root-dip inoculated with Verticillium albo-atrum Reinke and Berthold, and scored for relative pathogen resistance by symptom evaluation and reisolation of the fungus during a 7-month period. Nine individual plants were selected: two were resistant to colonization and symptom expression, two remained colonized but were resistant to symptom expression, and five were susceptible. Disease-free cuttings of each selected plant were infused with a mixture of conidia and red vinyl particles which mark primary spore trapping sites. At 168-h postinoculation the colonization ratio was determined for each sample. The colonization ratio is a reliable quantitative measure of invasiveness which is calculated by scoring a section of tissue for the number of secondary colonization sites established per primary trapping site in a given unit of time. The colonization ratio values of the nine selected plants were correlated with the level of disease resistance as determined by two methods: (i) symptom evaluation and pathogen resiolation, and (ii) genetic S1 analysis. The colonization ratio, a new measure of fungal invasiveness and host resistance, might be useful in the breeding and assessment of alfalfa cultivars with resistance to Verticillium wilt.