Polymyxa graminis, a fungus-like organism, carries
wheat soilborne mosaic virus. The virus particles are carried
on or in specialized spores (zoospores) of this organism
that are able to swim in the thin film of water surrounding
particles of saturated soils. The organism invades root hairs
of the young wheat in the fall during periods of high soil
moisture. Apparently spring infections are possible, but
they are unlikely to cause severe disease problems in wheat.
Polymyxa forms dark clusters of resting spores in the wheat
roots, which are released to the soil as the roots decay.
Since the organism survives in the soil as resting spores,
the disease is always associated with infested soil. Soils may
remain infested for many years. Wheat soilborne mosaic
virus is not spread by insects or in the seed produced by
infected plants.