Organic Disease Management |
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By: R.W. Heiniger, Crop Science Extension Specialist, NCSU For disease to occur three ingredients are essential. You must have a susceptible host, the pathogen must be present, and the environmental conditions for pathogen development on the host must coincide. This concept is referred to as the disease triangle. Because disease is progressive, a fourth factor, time, distinguishes biotic diseases caused by the various organisms versus injury caused by lighting or hail. This results in the disease pyramid depicted in Figure 1. The key to managing diseases in corn is the elimination of one of these key factors. The use of resistant hybrids removes the host from the pyramid and thus eliminates disease. Where soilborne plant pathogens are concerned the pathogen may always be present and long-term management strategies must be used such as rotation to reduce the populations of these organisms. Various plant pathogens are favored by different environmental conditions. For example: corn leaf, ear, and stalk diseases generally are favored by warm, wet weather. Most root rots, such as Pythium root rot, are more severe in wet than in dry soils.
Fig. 1. Disease triangle showing the four key factors in control: environment, pathogen, time, and host. Seedling diseases are favored by cool soil temperatures that delay emergence and growth of seedlings. Therefore, diseases of corn seedlings tend to be more severe when planting time is unusually wet and cool. Symptoms of nematode damage are more pronounced in sandy soils, that can support large nematode populations. In some instances we can modify the environment to eliminate the disease. Irrigation may reduce crop stress, which will suppress the amount of disease caused by pathogens that attack stressed plants. Planting on beds places the seedlings in a warmer and drier environment. Because of the temperature and moisture requirements of different plant pathogens, the arrival of conditions favorable for plant growth may retard or end the progress of the disease in time, and the crop will recover. There are no completely effective measures for controlling all corn diseases; however, losses can be minimized by following certain recommended practices. Growers should become acquainted with disease problems and factors affecting the severity of the disease and apply the appropriate disease management tactics including:
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