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Diseases that can affect yield and quality of field crops in Illinois are numerous. For plant diseases to develop, certain components of the disease triangle must be present. These components are a susceptible host crop, a plant pathogen able to infect the host crop, and an environment that favors disease development.

In general, plant diseases of field crops in Illinois are caused by biotic pathogens belonging to one of four groups: bacteria, fungi, nematodes, and viruses. Examples of important diseases that cause losses in Illinois field crops can be taken from each of these pathogen groups. Tactics used to manage these pathogens can vary, so it is essential to know the cause of the problem.

Management practices designed to reduce plant diseases affect specific components of the disease triangle. Multiple practices need to be deployed to limit more than a single component, an approach known as integrated disease management. Integrating different management practices often results in better disease reduction and helps reduce selection pressures. Pathogens are affected by selection pressures when certain individual management practices are used (i.e., some host-resistant genes and some fungicides), and this can result in new "races" of the pathogen or fungicide-resistant strains of the pathogen being selected.

The first step in managing a plant disease is to diagnose the problem. Diagnosing a disease from symptoms alone is not always possible, and some pathogens can cause similar symptoms. Misidentification can lead to inappropriate control recommendations (e.g., applying a fungicide to control a bacterial disease), so properly identifying the problem is critical. Magnification with a hand lens or microscope may help in observing spores or fruiting bodies of some plant-pathogen fungi. When diagnosis is not possible with the tools and resources you have available, collect and send affected plant samples to a plant diagnostics lab. The University of Illinois Plant Clinic serves Illinois producers during the growing season.

Copied from Chapter 14 of the Illinois Agronomy Handbook
Carl Bradley, Author

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