Quantitative assay of resting spores of Polymyxa graminis on barley roots

A procedure was developed to examine amounts of Polymyxa graminis on eleven barley cultivars from a field experiment on a site infested with barley yellow mosaic virus (BaYMV) and which differed in field response to the virus. Powder produced from dried barley roots infected with P. graminis was soaked overnight at 4°C in a solution of 1 % sodium metaphosphate and 0.25% Tween 20. This was followed by high speed homogenisation, filtering, ultrasonic treatment of the residue and differential centrifugation. A suspension of individual resting spores free from other recognisable fungi was obtained, which ranged in concentration from 0.4 to 7.3 × 10⁷spores per g root. Repeated extraction of the residues suggested that most spores were liberated by the first cycle of treatment. The cultivar with the greatest incidence of BaYMV also had the most P. graminis; some cultivars resistant to BaYMV had less P. graminis but there was no general correlation between the incidences of virus and vector.