Selecting Corn Hybrids to Minimize Diseases in 2014 - UNL CropWatch, Oct. 3, 2013
October 3, 2013
Carefully planning for the next crop season can help minimize many corn diseases and the losses they cause. Selecting more disease-resistant corn hybrids and placing them strategically in fields with histories of those diseases is one of the most economical ways to manage diseases. There are several things to consider related to disease management and seed selection.
Field Conditions and Disease History
Figure 1. Freckles caused by the bacteria causing Goss’s bacterial wilt and blight can often be visible near the edges of lesions.
Fields can be vastly different as far as disease history, growing conditions, availability of irrigation, tillage regime, rotation sequence, and other factors that influence the incidence and severity of crop diseases. Familiarity with the cropping practices and conditions that favor diseases can help you anticipate which disease(s) may be problematic next year in a field and strategically plan to avoid and/or manage them.
Most of the diseases in Nebraska corn are caused by pathogens that overwinter (survive) from year to year. Under favorable weather conditions, you can expect to see these diseases develop whenever susceptible corn hybrids are planted. In fact, all of the major diseases, except rust diseases (including common and southern rust), are caused by pathogens that overwinter in Nebraska fields. Rust fungi produce airborne spores that must be blown in from other areas of the country, often from states south of us, making it difficult to anticipate whether rust will be a problem with any certainty. However, increased corn production in states south of Nebraska and potentially in northern Mexico could provide for more rust spore production and increase the chances of rust diseases developing in our area annually.
Most of the remaining corn diseases are caused by pathogens that survive in infected corn residue from the previous season(s). Fields with continuous corn and/or minimum tillage can be higher risk for some diseases because they provide a constant source of residue for overwintering pathogens. In fields with higher risk, it is especially important to plant resistant or tolerant hybrids.
Examples of common residue-borne diseases are:
- Goss’s bacterial wilt and blight. Goss’s wilt has become increasingly common across Nebraska and the Midwest during recent years, often causing severe disease and yield loss, especially following widespread crop damage, such as that caused by hail. Diagnostic symptoms of the more common leaf blight phase of Goss’s wilt include the dark colored freckles and glossy bacterial exudate on the surface of lesions (Figure 1). Research comparing the importance of cropping practices and other field conditions identified the corn hybrid’s rating for Goss’s wilt as the most important factor affecting disease development (Langemeier, 2012). Although tolerant hybrids are not immune to the disease, they can substantially reduce Goss’s wilt severity and subsequent yield loss.
Figure 2. Gray leaf spot lesions are rectangular gray lesions that develop on lower leaves first, and on higher leaves as favorable weather conditions persist. (Photo by C. Schleicher, UNL)
- Gray leaf spot. In contrast, the pathogen of gray leaf spot (Figure 2) requires high relative humidity of 96% or more for at least 11 hours and warm temperatures. Gray leaf spot lesions are rectangular, gray to brown and develop on the lower leaves first and continue to develop higher up the plant as long as favorable weather conditions persist. Tolerant hybrids can reduce disease lesions and slow their development and spread up the plant. Foliar fungicides are also very effective at managing gray leaf spot.
Many seed companies will rate how their hybrids react to these common diseases and these hybrid reactions can vary greatly. Many companies also rate their hybrids’ reactions to other diseases, such as eyespot, northern corn leaf blight, rusts, and some stalk rots.
It is important to note that resistance or tolerance to a disease does NOT imply resistance to other diseases. That’s why it’s important to familiarize yourself with the diseases in your fields and position tolerant hybrids appropriately for the best effects.
In addition, it is common for competing seed companies to utilize opposing rating scales in seed catalogs. For instance, a 1-9 scale used by one company may indicate “1” as best while the same scale used by another company may indicate “9” as best. Numerous other rating scales exist. Pay close attention to rating scales and their exact meaning to avoid misinterpretation and contact your local company representative for clarification or if you have any questions.
If you need assistance identifying a disease, consider submitting a sample to the UNL Plant and Pest Clinic for a diagnosis.
Improved Understanding of Factors Influencing the Re-emergence of Goss's Bacterial Wilt and Blight of Corn, Craig B. Langemeier, Thesis, 2012, University of Nebraska-Lincoln.
Extension Plant Pathologist