In the last two years, disease has cost Iowa growers thousands of dollars in lost yield. Northern corn leaf blight and gray leaf spot have been the main culprits. So going into the summer of 2016, what should we expect?
Over the last two years, northern corn leaf blight (NCLB) has been the biggest yield robber in Central and South-Central Iowa. In many cases last year, NCLB robbed anywhere from 10-40 bu/acre. The pathogen will survive the winter on corn residue. Even if the residue was buried, enough inoculum is present in the neighboring fields that it can easily blow in with a wind-and-rain event. Gray leaf spot (GLS) will also survive over the winter on residue. Although NCLB will be the predominant inoculum to be the genesis of disease for 2016, GLS was present last year as well. If the right environment shows up, it will be the predominant disease.
Northern corn leaf blight thrives in temperatures from 64-80 degrees Fahrenheit and spreads in wet environments. So when there are wet and dewy mornings with temperatures in the right range, NCLB can spread quickly. If the disease gets established on the plant, any morning or environment that is conducive for disease will spread it quickly. High humidity? Dew on the corn? Morning temperatures? They all set the stage for disease.
Gray leaf spot thrives in temperatures from 75-85 degrees Fahrenheit with high humidity around 90 percent. Long periods of heavy dew will lengthen infection periods. Note that it can take up to two weeks for leaf symptoms to appear, damaging internal leaf tissue and silently robbing yield.
There have been many studies from industry professionals and universities alike on what the proper timing is for a fungicide application. The best scenario is to apply the fungicide right before the disease infects the plant, but when and how quickly the disease spreads is anybody’s guess. Many studies have focused on two timings, V5 and VT, for corn. The V5 application is early, and many times a fungicide application is done along with herbicide application. This timing helps protect the plant from early season diseases, and there are cost savings from an application standpoint. However, NCLB and GLS are typically mid-to-late season diseases, and each fungicide has a different length of residual protection and control. Because of when NCLB and GLS infect, year-on-year the most profitable timing is the VT application, which returns the most yield per dollar spent.
Since disease is knocking at our door, scouting is very important. Identify the disease early or plan a disease-protection application to protect yield and profitability.