1 Department of Plant Pathology and Microbiology, Iowa State University2 Escuela de Agronomía, Universidad de Costa Ricasallymallowa@gmail.com
History of corn production in the U. S. Corn Belt
Teosinte (Zea mexicana) is believed to be the progenitor of corn (Zea mays). Corn has a rich, dynamic, over-5000-year history originating from the Mayan and Aztec tribes in Mexico. There are several kernel types from different plant races. In the early 1800s, the late-flowering Virginia Gourdseed and the early-flowering Northeastern Flint types were first crossed, and the superiority of the hybrid was recognized and described. From several generations of these crosses emerged the highly productive yellow U. S. Corn Belt dent hybrids (Brown et al. 1984).Yields of dent hybrids have progressively increased from 36.9 bu/acre in 1951 to about 113 bu/acre in 1982 and are currently at a national average close to 170 bu/acre. Production practices have also changed considerably in the past 50 years, primarily in the interest of soil and water conservation. Minimum tillage is a conservation practice in which growers usually work the fields in the spring and incorporate crop residue into the top 2 to 6 inches of soil. Many pathogens have been shown to survive in the debris for up to at least one year (Sturz et al. 1997). This production practice could lead to increased residue-borne foliar diseases in corn fields (Wise and Mueller 2011).
Management of fungal diseases on corn (IPM)
Several foliar diseases predominate in the U. S. Corn Belt, including gray leaf spot (Cercospora zeae-maydis), northern corn leaf blight (Setosphaera turcica), anthracnose leaf blight (Colletotrichum graminicola), common rust (Puccinia sorghi), and eyespot (Kabatiella zeae), (Wegulo 1997). Integrated pest management (IPM) principles include: prevention (using resistant hybrids), avoidance (using rotation or tillage to reduce and avoid the presence of pathogens in fields), monitoring (using scouting to focus on presence of disease at yield loss causing thresholds), and suppression (using fungicides or biocontrol products to control disease). However, the current practice of minimum or no tillage and the higher prices for corn, means farmers are not using the basic tenets of IPM, such as rotation [growing different hybrids (rotate susceptible and resistant) or different types of crops in the same area in sequential seasons] and removing or deeply plowing debris. This calls for improved disease management, since these practices may well lead to higher fungal foliar diseases that could reduce photosynthetic leaf area and result in yield loss (Wise and Mueller 2011).
The use of fungicides to manage corn diseases
It is important to scout to observe the presence and amount of disease at critical crop development stages. In corn, scouting usually begins just prior to tasseling, and the presence of disease on the ear leaf, the canopy below, and also above the ear leaf are observed. Noting where on the plant the disease is present and the amount of leaf (or canopy) affected helps formulate thresholds for the best time to spray (Bosch and Gilligan 2008). Numerous foliar fungicide products with several modes of action are available (Wise and Mueller 2011), and are marketed for use beyond just disease control. Paul et al. (2011) summarized yield response results from fungicide trials in 14 states, and reported more consistent profitability associated with fungicide applications when disease was present.
Plant health benefits of foliar fungicides
Although the main role of foliar fungicides is disease control, one fungicide product on the market has a label for plant health. In addition to enhanced growth efficiency, stress tolerance, and plants that stay greener longer, some fungicides can potentially lead to an increase in yield of up to 12-16 bu/acre. Therefore, fungicides are marketed and used also by producers beyond just disease management and as a means of plant health insurance to increase yields.
Policy influencing price of corn
The last decade has been characterized by record high corn prices. This has been attributed to the expanding global market, especially in Asia, and a U. S. government incentive that gives tax breaks for petroleum producers who blend with renewable fuels (largely from corn) (Larson et al., 2010). In the short term, this demand has made corn production more profitable (increased corn price), which has motivated farmers to plant more acres and more frequently (Cassman 2006).
Farming is a business. Thus, it is imperative that economically reliable fungicide application choices that take into consideration the cost and benefit of using foliar fungicides are made. Consider how much yield is needed at the current prices of $4-5 per bushel to compensate for the product and application costs (estimated at about $25/acre). A small increase ofapproximately 5 bu/acre might perhaps be sufficient to offset application costs and ensure yield is protected from unforeseen disease; although yield increases of up to 20 bu/acre have been reported (P. Esker, personal communication).