Disease Management Issues

The risk of some corn diseases is greater when corn follows corn, especially when some form of reduced tillage is practiced that leaves greater amounts of non-decomposed, inoculum-bearing residue on the soil surface. Two such diseases that can devastate susceptible hybrids are gray leaf spot (Cercospora zeae-maydis) and, as some experienced in 2004 and 2005, northern corn leaf blight (Exserohilum turcicum). Other diseases that may become more prevalent in corn following corn are stalk and ear rots, including those caused by Colletotrichum graminicola (anthracnose), Fusarium verticillioides, Gibberella zeae and Diplodia maydis.

Over the past two years there has been a lot of talk about substantial yield increases in field corn sprayed with strobilurin fungicides. Experimental data from repeated, replicated university trials suggest that economically beneficial responses to fungicide applications in commercial hybrid corn may occur approximately 60% of the time, but are linked closely with the actual occurrence of significant levels of disease. Economic yield responses to fungicides in the absence of disease are not well documented. On-farm tests in which single strips of untreated corn are used to evaluate the efficacy of treatment on most of the field can be misleading.

The decision to use a foliar fungicide should be based on known susceptibility of the hybrid to gray leaf spot or northern corn leaf blight and the likelihood that disease will develop. Disease risk depends – in addition to the abundance of corn residue in the field and the hybrid’s susceptibility – on weather during the summer. Frequent, well-spaced rain (not necessarily heavy), high relative humidity and dew that persists into the morning favor leaf blights. In the absence of good data to support the economic return of fungicides, it is a good idea to leave some check strips – preferably more than one – assigned to random strips across the field (i.e., don’t use portions of a field that have historically yielded less as your untreated check strips).

In the absence of research-based disease severity thresholds for fungicide application timing, many growers have opted to treat fields at or just before tassel emergence (VT). Stage VT typically occurs about three days before silks emerge (R1). If disease will become a problem in a field, treatment at this time will protect leaves during early grain fill and may reduce secondary inoculum that can cause more disease later. Therefore, it is a good idea to scout fields as they near the VT growth stage. If there is little or no leaf disease evident at this time, application of a fungicide at this time may not be economically justified. Some fungicides can be applied after silking. Check labels for preharvest intervals for each product.

Mitigate the disease risk in second-year corn by careful hybrid selection with emphasis on resistance to specific diseases, as well as on overall good plant health characteristics (Thomison, et. al., 2004; Vincelli, 2004b; Vincelli, 2005). Where practical, consider burying the stalk residues with tillage to reduce the abundance of disease inoculum for next year. The use of fungicides is often not considered economical for disease control in commercial feed-grain corn production (Vincelli, 2004c), although the experience of some farmers suggests otherwise. For more information on fungicide use in corn, see Nielsen (2007).

Insect Management Issues

The major insect threat to corn following corn in Indiana is western corn rootworm (Diabrotica virgifera virgifera). The yield and production cost consequences for corn following corn is particularly meaningful for growers in areas of the state where crop rotation remains a viable control option for this insect pest (i.e., areas where the variant rootworm has not yet appeared, primarily the southern and eastern parts of Indiana [Obermeyer, et. al., 2005b]).

There are other notable below-ground corn pests, however, particularly early in the growing season. As indicated earlier, greater levels of surface corn residues in corn following corn can delay corn emergence and growth. This results in a lengthier exposure of corn seedlings to secondary soil pests (e.g., wireworms, seedcorn maggots, white grubs and slugs) that in turn may result in weakened plants and/or stand reductions. A combination of surface corn residues and live winter annual weeds in the spring can attract cutworm and armyworm moths for egg laying, leading to corn seedling damage/death from subsequent larval feeding on plant tissue. Given all of these factors, pressure levels from these pests could potentially increase in corn following corn.

On the other hand, second-year corn should not experience greater populations or damage from European corn borer (Ostrinia nubilalis) or southwestern corn borer (Diatraea grandiosella Dyar). In both cases, adult female moths find and fly into cornfields each year to lay eggs. The use of a continuous corn cropping system over a wide area over several years may increase the risk of elevated corn borer pressure and potential yield/harvest losses, simply because of the increase in potential food sources and associated increased pest populations.

Mitigate the insect risk in second-year corn by the judicious use of soil-applied insecticides, insecticide seed treatments (high-rate formulations) or transgenic resistance (Bt-rootworm) for rootworm (Obermeyer, et.al., 2006). Scout fields during seedling emergence for cutworm and armyworm damage to leaves and stems to determine the possible need for rescue treatments of foliar insecticides. Consider using hybrids with Bt-corn borer traits where appropriate.

Hybrid Selection Issues

Good hybrids for rotation corn tend to be good hybrids for continuous corn. Therefore, growers should first seek out hybrids that demonstrate consistent high yield performance across multiple environments (years and/or locations). Consistent performance across multiple sites is important because multiple sites represent possible weather patterns your farm may experience in the future. Consult closely with your seed sales representative and check out the latest corn hybrid performance results from non-biased sources such as Purdue’s Crop Performance Program Web site.

Once you have identified otherwise good-yielding hybrids, then further filter among that group for hybrid characteristics important for a continuous-corn cropping system. Such characteristics include hybrid traits for disease resistance, stalk strength, stalk and root health, seedling vigor and overall stress tolerance. While always important, these traits take on extra meaning when adopting continuous-corn strategies because of the increased risk of diseases and often-greater risk of early season stress during the stand establishment period.