Scouting for corn diseases is important regardless of the time of year. Most foliar diseases occur at specific times of the year. However, in some cases, diseases such as southern corn rust can occur earlier in the corn growing season depending on the environment encountered during the season. For example, southern corn rust was observed in June during 2010 but not until late July during 2012. In addition, foliar diseases such as gray leaf spot can occur earlier than normal if excessive corn stubble remains in the field over several successive seasons. But, with that in mind, I have only seen GLS on vegetative stage corn in one particular, no-till field setting where GLS has been the predominant disease at that location.
The majority of the diseases contained in the provided calendar are of the foliar variety. I chose to leave a few minor diseases off the calendar as I have only observed them in extremely limited situations (e.g., eyespot, diplodia leaf streak). Some diseases, such as charcoal rot, anthracnose or some of the virus diseases encountered in our production system are likely the result of infection that could have occurred at earlier growth stages. Note that some diseases can show up in more than one growth stage area but will ultimately depend on the environment encountered throughout the season and to a greater extent planting date. But, in some cases, previous crop will also dictate the foliar disease pressure. For example, corn following corn will likely have additional inoculum pressure for such diseases as gray leaf spot and northern corn leaf blight. However, the inoculum to produce rust infection (common and southern) will have to blow into our production area from an overwintering source of inoculum.