Delayed planting coupled with the wet spring may mean this is a good year to make a fungicide application on corn. Wet conditions have favored the development of some foliar diseases within the canopy, and many fields are delayed developmentally due to later planting. In the past two growing seasons, most of the corn in Iowa has been flowering by mid-July. This year, it will be another 10 days or more before widespread tasseling occurs in the state. When diseases start showing up during early grain fill, it increases the risk of reduced yield.
Under what conditions will a foliar fungicide application most likely pay off? Alison Robertson, Iowa State University Extension plant pathologist, says research has shown that greater yield responses due to a fungicide application consistently occur in the presence of disease. In Iowa trials the mean yield response was greatest when disease severity in a field was high at the R5 stage of corn growth. If disease severity on the ear leaf at R5 was less than 5%, the mean yield response was 4.83 bushels per acre. However, when disease severity on the ear leaf at R5 was greater than 5%, the mean yield response was 9.46 bushels per acre.
Therefore, to increase the chance of getting a return on investment by making a fungicide application, farmers should target fields that are at high risk for disease development during grain fill.
Greater yield response from fungicide application is more likely to occur if disease is present
To identify fields that are at an increased risk for disease, and consequently have an increased chance they will benefit from a fungicide application, consider the following three factors:
* Hybrid: Corn hybrids vary in their tolerance to various diseases. "Race horse" type hybrids are often susceptible to one or more diseases. Fields planted to susceptible hybrids may require a fungicide application to protect yield.~~~PAGE_BREAK_HERE~~~
* Presence of disease: Gray leaf spot (shown in the photo) and northern leaf blight often develop in the lower canopy and then move up the plant. Common and southern rust are more often found in the mid-to-upper canopy. Presence of leaf spots in the canopy means the weather has been favorable for disease development and inoculum is present in the field.
* Weather conditions during grain fill: For disease to happen, or continue to develop, certain environmental conditions need to occur. Those are described in the accompanying table.
Table 1. Conditions that favor foliar disease development in corn
Early symptoms of gray leaf spot are small tan spots that are bordered by leaf veins and occur in the lower canopy.
There are several fungicides registered for use on corn, and new products continue to be introduced. The Corn Disease Working Group, a group of corn pathologists from across the U.S., developed a Fungicide Efficacy Table that is available online here. The pre-harvest interval or PHI of the products ranges from 7 to 45 days--so always remember to check the label and consider days to harvest before applying a product.
Pre-tassel applications of fungicides may result in arrested ear syndrome. Research at Purdue University showed spray additives (NIS surfactants and crop oil) added to pesticides applied between V10 and VT growth stage of corn increased the risk of this disorder.
Another consideration: Wet spring conditions across Iowa this year resulted in uneven stands that range widely in crop growth stage. "In this situation you should try to wait to apply a fungicide until the field has completely tasseled," says Robertson. "Some fungicides do not require the addition of a surfactant – check the label. However, when fungicides are applied aerially, crop oil is used to ensure the fungicide product does not evaporate and is deposited within the corn canopy."~~~PAGE_BREAK_HERE~~~
Consider the following factors when planning an application of foliar fungicide
DuPont Pioneer and university research across 475 on-farm trials shows corn yield increases an average of 7 bushels per acre in response to a foliar fungicide application. The average yield response is generally greater in fields with large amounts of crop residue on the soil surface, such as corn-following-corn, and no-till or strip-till. Later-maturing fields can also be at greater risk to foliar disease and are more likely to benefit from a fungicide application.
Corn fungicide decision guidelines: DuPont Pioneer corn disease specialists recommend scouting for foliar diseases in corn just before tassel emergence. Consider the following factors when planning an application of foliar fungicide.
* Previous crop— Many foliar pathogens survive in corn residue. The risk of foliar diseases, such as gray leaf spot and northern leaf blight, increases when corn is planted into a field that was corn the previous year.
* Weather conditions— Rainy and/or humid weather is most favorable to foliar diseases. In growing seasons when these conditions prevail, the risk for disease development increases.
* History of disease— Some field locations may have a history of greater foliar disease severity. Fields in river bottoms, low areas or surrounded by trees may be more prone to foliar corn diseases.
* Hybrid resistance— If the disease resistance rating is a 6 or greater, a fungicide application may not provide a yield benefit. For susceptible corn hybrids with disease rating less than 4, spray if disease symptoms are present on the third leaf below the ear or above on 50% of the plants examined.
For intermediate corn hybrids with disease rating of 5, determine if disease symptoms are present on the third leaf below the ear or above on 50% of the plants examined. Also consider the above factors, spraying if the field is in an area with history of foliar disease, a corn-on-corn planting, contains 35% or more surface residue, and weather is warm and humid. For more information on foliar fungicide use, visit pioneer.com.