No. 2 Article 4/April 6, 2007

Corn on Corn: What Are the Disease Risks?

With the anticipated increase of corn to be planted in Illinois this season, a lot of acres will have to go back onto last year's corn ground. One of the key tactics to help manage plant diseases is crop rotation. Although high yields can be achieved with corn following corn, there are disease risks associated with this practice.

All plant diseases rely on the disease triangle (Figure 1) for disease development to occur. The disease triangle is composed of three elements: the pathogen, the susceptible host, and a conducive environment. Producers have full control over one of these components, the susceptible host, since they choose what they want to grow in a particular field. Producers also have some control over the pathogen. Some plant pathogens (especially leaf and ear pathogens) can survive in the previous year's crop debris, and when the same crop is planted right back into last year's debris, the pathogen and the susceptible host may be in the same place together.

 


Figure 1. Disease triangle.

Producers have no control over the third component, environment, except under irrigated conditions (and even then, control is limited). All of these components must be in place for a disease to occur. For instance, corn could be planted into corn debris that is infested with the gray leaf spot fungus, which would meet two thirds of the criteria needed for disease; however, disease will occur only if the environment is conducive for the fungus to produce spores and infect leaves.

With corn on corn, the option of managing diseases with crop rotation is gone, so other management tactics are relied upon. The first and one of the most important is hybrid selection. Choosing a hybrid with a high level of resistance to diseases is always a good idea, and the benefits of a hybrid with a good disease resistance package are likely to be even greater when planting back into corn.

Fungicides are another way to manage foliar fungal diseases of corn. Fungicides are likely to benefit susceptible hybrids more than hybrids that have high levels of resistance to foliar diseases, and even more so when the previous year's corn debris is left on the soil surface. However, a fungicide may not be able to overcome all of the detrimental characteristics of a susceptible hybrid--i.e., if the best fungicide was sprayed on the worst hybrid, there would likely still be yield losses under high disease pressure. Therefore, it is important to plant the best resistance available, scout fields for disease as they near tassel emergence, and apply a fungicide if necessary. Also, only foliar diseases can be controlled with currently available foliar fungicides for corn, according to the fungicide labels.

There is potential for different types of disease to increase in a corn-on-corn situation. Foliar fungal diseases are generally the first diseases that come to mind, but an increase of other pathogens may also occur. In Illinois, these foliar fungal diseases are primarily gray leaf spot, northern corn leaf blight, and northern corn leaf spot, which harbor in infected corn debris. The risk of ear and stalk rot diseases, such as Diplodia ear rot and anthracnose stalk rot, may also increase in corn-on-corn.

Below the soil, diseases caused by root rot pathogens and nematodes can also be on the increase in a corn-on-corn situation. Most of the nematodes that can damage corn are specific to corn and are usually managed fairly well with rotation to soybean or another nonhost. With corn-on-corn, however, there is no "check" on the buildup of these nematodes, and populations can get high enough in the first year to cause significant yield loss in the second year. This problem is compounded because most of us no longer use insecticidal chemicals, some of which had the added side benefit of suppressing corn nematode populations. Nematode damage to corn roots is often associated with root rotting due to infection by opportunistic soil fungi, and root rots are often a tip-off that nematode problems may be present. Diagnosing corn nematode problems is difficult because the symptoms can look like so many other problems, so soil sampling is an absolute requirement. See the Plant Clinic web site (plantclinic.cropsci.uiuc.edu/submit.html) for information on how to take and submit soil samples.

Root rotting, caused by opportunistic pathogens, increases when nematode populations are high, even if the nematodes themselves are not pathogenic. If you observe rotting roots, be sure to send in a soil sample for nematode analysis.

In summary, the risk of disease does increase when going to corn-on-corn. However, all three pieces of the disease triangle must be in place for disease to occur. To reduce disease risk in corn-on-corn: 1) plant the best disease-resistant hybrids available; 2) scout fields for disease; and 3) if needed, apply a fungicide to control leaf diseases. We emphasize that there is no way to determine if a foliar fungicide will be warranted in a particular field before the crop is even planted.--Carl A. Bradley and Terry Niblack

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