Wheat yellow mosaic virus resistance in wheat cultivar Madsen acts in roots but not in leaves

Wheat yellow mosaic virus (WYMV), a member of Bymovirus, is a plant pathogenic virus transmitted by the soil-borne plasmodiophorid Polymyxa graminis. The in planta titer of the virus must be quantified to elucidate the dynamics of WYMV infection and the mechanism(s) underlying host resistance. Here, we describe a robust and sensitive TaqMan-based real time PCR assay based on the sequence of the WYMV coat protein gene. WYMV was undetectable in either the leaves or roots of plants of WYMV-resistant cultivar Madsen grown in infected soil. However, when Madsen leaves were mechanically inoculated with the virus, viral multiplication was not inhibited, showing that the cultivar's resistance was ineffective in the leaf. The virus was able to move from the virus-inoculated leaf to the stem but not into the roots of Madsen, indicating that Madsen's resistance was effective only in the roots.