Research has shown that when applied at labeled rates, fungicides can and do significantly increase crop productivity in disease favorable environments. Rates provided on labels are determined by chemical companies following years of research and huge financial investments. Not only are the rates chosen to give the grower adequate control of listed pathogens, but they also reduce fungicide resistance risk.
By applying fungicides at levels lower than the rates listed on the label you not only significantly reduce any benefit in terms of disease suppression, but also increase the chance that pathogens with resistant characters will continue to grow and reproduce.
Once resistant individuals dominate a population disease control with that product and also products with active ingredients belonging to the same group lose efficacy. In some cases, such as with the strobilurins, efficacy may be completely lost. This is why products such as Quadris and Headline can no longer be used to control Frogeye Leaf spot in soybeans in many states in the South and Midwest.
I encourage you to follow the labels and ensure that these valuable disease management tools remain effective for years to come. There are many products available that have excellent efficacy on fungal diseases in Corn and Soybean and these products vary in price. I suggest selecting a product that fits your budget and can be applied at the labeled rate as opposed to purchasing a more expensive product and cutting the rate to make it fit your budget.