Development of a bioassay for Verticillium dahliae in potted Chrysanthemum

Bioassays for Verticillium dahliae infection are often aggressive and hence not representative for a field situation. The present study describes a more realistic bioassay for assessment/estimation of infestation of potted chrysanthemum by V. dahliae in a field situation. Microsclerotia were used as inoculum source and an indirect infection of potted chrysanthemums under field conditions was simulated. In total, 250 plants were inoculated with V. dahliae by placing potted chrysanthemums on a soil artificially infested with 5 microsclerotia g^-1 soil. Another 175 plants placed on Verticillium-free soil served as a control. Eleven plant parameters were measured of which eight correlated with the presence of microsclerotia in the soil. Disease incidence, disease severity and wilting showed the highest correlation and were easy to measure. For a selection of 24 plants, V. dahliae presence in the plants was assessed at the end of the experiment using plating and qPCR. A significant relationship was observed between disease incidence/severity and amount of V. dahliae DNA in the plant as assessed with qPCR. In conclusion the bioassay described here is a feasible. non-invasive method to study chrysanthemums infected with V. dahliae.