Source: Ohio State University Extension
By Pierce Paul, Plant Pathologist; Andy Michel, Entomologist
Foliar diseases continue to spread up the corn plant in some fields, so this may be the year to apply a foliar fungicide to minimize losses due to diseases such as Northern Corn Leaf Blight (NCLB) and Gray Leaf Spot (GLS).
Both GLS and NCLB may cause yield losses as high as 50% if they become established before silking (R1) and substantial damage to the ear leaves and the leaves above the ear before grain fill is complete. The current weather forecast suggests that conditions (wet and humid with moderate to warm temperatures) will continue to favor the spread of GLS and NCLB as fields begin to tassel.
This would be the time to protect the upper leaves with a fungicide, particularly if your hybrid is susceptible and your field is in an area with a history of foliar disease problems. More of the available fungicides are effective against GLS and NCLB, so follow the labels and keep your eyes on the fungicide price and application cost when making a decision.
Unlike diseases, we are not seeing any major problems with insects, so we do not see any reason why you should tank-mix an insecticide with the fungicide. This is not something that we would ever recommend without inspecting your field for the presence of insects.
There may be some silk clipping occurring by Japanese beetles or western corn rootworms, but any impact on kernel set or yield would be rare. Clipping would have to occur:
1) During the first week of silking;
2) With less than 50% pollination;
3) Silks less than ½ inch;
4) Japanese beetles reaching a count of 5 per ear, or with rootworm adults reaching 3 per ear, for damage to be economic.
These beetles tend to be on the edge, so make sure you scout the entire field before considering insecticides. Not only will this save you money, but also help prevent insecticide resistance.