Overwintering pathogens

Over the years we have developed databases of winter temperatures followed by scouting to indicate
starting pathogen populations for Ohio, according to Dr. Kelly Tilmon, OSU entomologist.
Frogeye leaf spot – We have documented early infections and overwintering ability of the fungus,
Cercospora sojina, that causes frogeye leaf spot. It appears that when there are less than 10 days
during the months of December, January and February of less than 17 degrees F, we have had reports of
outbreaks of frogeye leaf spot.
This occurred in fields where there was a high level of inoculum at the end of the season the same or
similar moderately to highly susceptible cultivar was planted into the same field again which then
initiated the epidemic that much sooner. Losses of greater than 35% in yield or very early fungicide
applications were necessary.
Expecting continued warmer winter temperatures, for fields with a history of frogeye leaf spot, and
no-till production systems, the first thing for farmers is to do now to mitigate losses in 2020:
Rotate fields with high levels of frogeye leaf spot into corn or another crop.
If it is still targeted for soybean, look at their soybean varieties frogeye leaf spot resistance scores.
Your seed dealer will have more information. Plan now for what fields they will go into.
Scout the susceptible cultivars much earlier than what we have called for in the past and monitor levels.

Another pathogen that may be more prevalent after a warm winter is Stewart’s bacterial wilt. This disease
is transmitted to corn by corn flea beetle which survives in greater numbers in warm winters. This is a
greater problem in popcorn and sweet corn as most field corn has high levels of resistance to the
bacterium.
Most other field crop insect pests in Ohio are not highly influenced by winter conditions as they are
well-adapted to withstand cold overwintering conditions. One exception is Mexican bean beetle, an
occasional pest of soybean (especially in central Ohio). Warm winter conditions may cause higher
populations of this insect the following field season.