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Hill and Furrow

Corn Disease Update - Monmouth, July 14

What are we seeing in the field? At the Northwestern Illinois Ag Research & Demonstration Center we are beginning to see some of the residual herbicide activity break, with weeds like giant foxtail and morning glory popping up in some fields. We are also seeing quite a bit of Japanese beetle feeding damage and very high populations in some studies. One researcher commented about just how much Japanese beetles seem to prefer edamame soybean varieties over those grown for grain.

In 2015, with the many rain events and cloudy days in May and June leading to long-periods of leaf wetness, symptoms of northern leaf blight were observed on all but the most resistant hybrids. When compared to 2015, scouting for corn disease at the Northwestern in 2016 has been uneventful. I have seen a handful of leaves exhibiting symptoms of Physoderma brown spot and one leaf with common rust symptoms.

Scouting at 55 mph doesn't count. Scouting is an essential component to making an informed decision regarding a foliar fungicide. So is gathering information about each aspect of the disease triangle: the pathogen, the plant and the environment. Many economically important fungal pathogens survive in residue from previously-infected crops. This is why disease risk tends to be higher when planting into a field with a lot of corn residue and/or a history of disease. Seed companies typically provide a numeric rating of the level of genetic disease resistance in each hybrid. Foliar fungicides are generally not recommended for use on resistant hybrids. In addition to monitoring weather conditions, scouting fields for disease provides valuable information about the fungal disease pressure in any given field.

Foliar fungicide research. Before leaving the University of Illinois for the University of Kentucky last summer, Plant Pathology Extension Specialist Dr. Carl Bradley and his team conducted uniform corn fungicide trials from 2008 through 2014 in many locations throughout the state. The data collected in these 45 location-year environments and in similar studies throughout the North Central region can provide indications as to when foliar fungicides are most likely to be profitable.

Profitable fungicide applications target disease. While many production costs remain high, corn prices have since fallen and are projected to average below $4 per bushel for the 2016 crop marketing year. All input decisions become more urgent during leaner years – to be profitable, each input must increase yield enough to at least pay for itself. A profitable fungicide application is one that increases yield enough to at least pay for itself, and includes the costs of both fungicide active ingredient(s) and application.

Dr. Bradley's work over the many years and locations suggested that regardless of the yield response needed to break even with a foliar fungicide, the chance of doing so is higher in moderate to high disease pressure environments.

While many foliar fungicides are marketed to improve standability and stalk strength, research shows that this is more likely to occur when fungicides target foliar disease. Foliar disease takes away photosynthetic leaf area. With enough compromised leaf area, corn plants can redistribute sugars from the stalk into filling kernels thereby weakening the stalk.

Fungicides in the absence of disease and pesticide resistance. There are instances in which foliar fungicides can increase yield in the absence of disease, however these physiological responses are both difficult to predict and inconsistent. From a resistance management perspective, reserving fungicides for those instances that are warranted from a disease management standpoint will serve to prolong their effectiveness. Also note that due to the differences among corn pathogens and fungicide sites of action, not all fungicides will work as well as others to protect against all pathogens.

Below are links to resources that can help when scouting corn fields for disease and making fungicide selections.

Additional resources

Corn Foliar Diseases: Identification and Management Field Guide

Fungicide Efficacy for Control of Corn Diseases, Purdue Extension, BP-160-W