Potential of Using Culture Filtrates from Verticillium albo-atrum to Evaluate Alfalfa Germ Plasm for Resistance to Verticillium Wilt

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Abstract
The potential was assessed for using cellfree filtrates from potato-dextrose broth cultures of Verticillium albo-atrum in a fast, effective screening procedure for Verticillium wilt resistance in alfalfa [medicago sativa]. In one assay, cut stems were allowed to imbibe filtrate and were rated for symptoms 7 days later. The symptoms that developed in susceptible cuttings after this treatment were typical of those in field-infected alfalfa. Six percent of the cuttings from susceptible Saranac were rated resistant by this procedure, compared with 32 and 56% from resistant Maris Kabul and NAPB-34, respectively. Control cuttings treated with uninoculated potato-dextrose broth or autoclaved water did not develop symptoms. Similar results were obtained with susceptible cuttings after uptake of filtrate from V. albo-atrum cultured in alfalfa decoction medium. In the second assay, filtrate was perfused by pressure into the leaflets. Susceptible leaflets became chlorotic after 4-6 days. Seventeen percent of Saranac and 70% of NAPB-34 plants were rated resistant by this method. Control treatments of uninoculated culture medium or autoclaved water did not cause symptoms. The activity of the filtrate remained stable after autoclaving or freezing.