The best time to apply a fungicide treatment is between tassel emergence (VT) and blister stage (R2). Even without the presence of significant disease pressure, this time frame consistently provides the best opportunity to control disease and increase yields. Most fungicides used on corn include both curative and preventative chemistry to fight fungal diseases. Even so, fungicides are far more effective at preventing disease than controlling disease after plants have been infected and disease is spreading in the canopy. Aside from disease prevention and treatment, fungicides also mitigate plant stress including heat and drought that occurs from pollination through grain fill.
Hybrids that respond positively to fungicide treatment, or historically have been sensitive to disease pressure should be treated now. With hybrids that have a lower response to fungicide, and/or strong genetics against disease, you may want to continue to evaluate until disease pressure reaches threshold levels. If you elect to wait on fungicide treatment, a later treatment is still effective at stopping disease progression and can improve harvest standability, but positive yield responses to offset treatment costs will be more difficult to obtain. Any corn hybrid that sustained wind or hail injury through the early grain fill stages usually benefit from a fungicide treatment, offsetting yield reductions from defoliation and advantageous stalk rots.