Culture-based Determination of VerticilliumDensities in Soil Overestimates Disease Pressure of Verticillium Wilt of Potato in Manitoba

Verticillium wilt affects potato fields in Manitoba where 80% of the fields are planted to the moderately susceptible cultivar, Russet Burbank. An accurate determination of Verticillium dahliaeinoculum in soil is critical for disease management. In this study, we investigated the presence of microsclerotia-producing Verticilliumspecies in potato fields in Manitoba and compared published quantitative real-time PCR assays for V. dahliae, V. tricorpusand V. longisporumagainst a traditional plating method. Selected real-time PCR assays could differentiate and quantify the major microsclerotia-producing species, V. dahliae, V. tricorpusand V. longisporum. Results showed that the presence of V. tricorpuscaused an overestimation of V. dahliaepropagule density when using the plating method. As a result, Verticillium wilt severity was negatively related to cfu from traditional plating, while positive with the amounts of V. dahliaegenomic DNA in soils.