If scouting indicates that you have GLS or NCLB on either of 2 leaves below the ear leaf, then a fungicide application at VT/R1 will provide the most benefit in terms of efficacy and economic returns. This does not guarantee the fungicide application will pay, but the likelihood of it doing so will increase.
Other things to keep in mind are diseases that need to blow in such as Southern and Common Rust. We see both of these to some extent every year, but often they do not arrive until late in the season and therefore likely have a minimal impact on yield. Late planting and a wet spring in the southern states may result in us seeing some elevated levels of these diseases, but only time will tell. As a result is important to scout and pay attention to information in the WCU, Field Crop Disease Management Blog, and other regional extension resources to see if there may be a risk for these diseases. A good source of information on common and Southern rust can be found here
Lastly, remember that a new disease, tar spot, was detected in corn from Indiana and Illinois last year, and was reported in Florida corn 2 weeks ago. We have not seen the severe form of this disease where it has been detected. However, keep your eyes peeled and contact myself, your county extension agent, or the Plant Diagnostic Clinic if you suspect you may have tar spot in your fields. More information on tar spot can be found here can be found by searching this blog.
Source:udel.edu