Cause and Dis ease Development
Septoria glycines overwinters in infested crop
residue and the debris of certain common
weeds, such as velvetleaf. The brown spot
fungus also survives both in and on seed,
but to a limited extent. Infections take place
when fungal spores produced in debris are
splashed onto wet foliage. Infection and
disease development can occur over a range
of temperatures (60o to 85oF), but is greatest
at 77oF; increasing leaf wetness (up to 36
hours) is associated with increased disease
infection and severity. Disease development
is severely hindered during hot, dry weather,
but will resume when conditions again favor
the disease and/or the crop approaches
maturity. Infections early in the season
are frequently the source of late-season
infections. As a consequence, the disease
almost always exists first in the lower canopy
and then moves into the upper canopy as
the season progresses.