Suppose you're walking along the edge of your cornfield and you notice lesions on leaves. Some of them are fairly large. Maybe the lesion is shiny. You're concerned that if it spreads quickly, it could dry up enough of the light-catching factory of the plant to affect yield.
What's your first instinct? Go hire either an aerial applicator or a custom ground applicator to apply a fungicide as quickly as possible, right? Unless you take time to identify the disease, and it may take help from trained experts to do that, you're basically rolling the dice on whether the application would help your corn at all or not.
That's because not every foliar disease that affects corn, or for that matter soybeans, is a fungal disease. Some of them are caused by bacteria. Applying a fungicide designed to tell fungi has no effect on a plant that is showing signs of a bacterial disease.
One of the confusing ones showing up in the upper areas of the Corn Belt this summer is Goss's wilt. This disease is caused by a bacteria. Not normally a huge problem, Purdue plant pathologist, Kiersten Wise, speculates that because of storms that damaged many plants, wither with hail or high winds, that the bacteria that causes the wilt had an opportunity to enter the plant that it normally doesn't get.
As a result, Goss' wilt is showing up in some places. In Indiana, she's finding it in more northern counties. The problem with a field diagnosis is that it looks a lot like a fungal disease, which could be hit yet with a fungicide, or that it may not be caused by a disease agent at all. She notes that it can be confused with symptoms of nutrient deficiencies, such as potash deficiency.
The solution is to first scout your fields, or make sure an agronomist or pest management scout is checking fields for you. If they find suspicious samples, instead of assuming what they are, send them to the plant identification service in your state. Wait for results before seceding if a pesticide will even help the situation.