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Managing corn diseases

By Jerron Schmoll, Agronomy Research Manager, Pioneer

The practice of managing diseases with fungicides in corn has increased significantly in the last few years as commodity prices have made this practice more economically viable. As we approach tasseling, many growers will be considering whether or not to spray their acres. Let’s review what we have learned from previous seasons, and what is different about 2011 that might help guide our decisions.

Before we dig into the data, let’s review the primary diseases that we are managing with fungicides: gray leaf spot (GLS) and northern corn leaf blight (NCLB). GLS causes rectangular shaped lesions that turn gray as the disease progresses, while NCLB causes gray-green elliptical or cigar-shaped lesions. Both diseases are favored by prolonged periods of leaf wetness in the form of prolonged dews and high humidity, and both can produce substantial yield losses, particularly on susceptible hybrids. Increased stalk lodging can occur with both diseases as the plant “cannibalizes” carbohydrates in the stalk to fill the ear instead of relying on green tissue to photosynthesize carbohydrates. Scouting for these diseases and applying fungicides as needed is the most sound economic approach. Not surprisingly, data indicates that when disease pressure is low, yield response to fungicides is also low. As either of these diseases starts to infect the ear leaf or the leaves above the ear leaf, it may be time to consider spraying.

Data from the last couple of growing seasons has reinforced the mantra that the environment is key in terms of determining yield response. Both diseases overwinter on corn leaf tissue, so any management practice that reduces the amount of residue on the soil surface will reduce the incidence of disease. Crop rotation helps with this. Pioneer agronomy science data from the 2009-2010 seasons indicated that fungicides provided around 4 bushel per ace yield response in a corn-corn rotation relative to a corn-soybean rotation. Tillage also impacted yield results. Fungicides applied in a no-till/strip till environment provided around 9 bushel-per-acre yield response relative to conventional tillage. However, both of these diseases can be carried on wind currents, so while crop rotation and tillage reduce the probability of a high level of disease infection, these diseases can still occur in tilled ground in rotation, particularly on susceptible hybrids. Most seed companies rate their hybrids for GLS and NCLB and company representatives should be able to tell you which hybrids are more likely to respond positively to a fungicide application under disease pressure.

Timing is another important management consideration. Tasseling is the typical recommended application timing as this if often the time that disease begins to develop. Early and late vegetative stage applications have been studied as a method to improve season-long disease control, and while there is on average a positive yield response associated with vegetative stage applications, the response is typically not economically positive. There is also an increased risk of arrested ear development with late vegetative applications. Adjuvants used to improve fungicide efficacy seem to be the primary cause of arrested ear development, so leaving them out of early applications may be warranted, however the safest bet is to wait until the reproductive stages of development. Uneven stands due to tough spring conditions may make crop staging difficult as some plants in the field may be at a late vegetative stage while others may be at tasseling to silking stage. Recent research has shown that erring a little on the late side of application (R1) can produce essentially the same result as a VT application and may help avoid potential injury from a late vegetative stage application.

Finally, what may be different about 2011? With the late planted crop, stalk quality may be a little more challenged as growers elect to let the crop stand to dry in the field. While fungicides can add a little moisture to the grain relative to an untreated field, they may also help improve standability into the fall by controlling diseases that contribute to stalk cannibalization, giving the crop more potential drying time to shed the moisture.

So consider disease presence, your agronomic practices, your hybrid, and the weather to help make an informed decision around fungicide application in corn. Good luck scouting this summer.

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