Soil contains both beneficial organisms and pathogenic organisms, such as fungi, nematodes, bacteria and viruses. Pathogenic fungi and nematodes (tiny worms) are responsible for the majority of soilborne diseases. Plants grow well when populations of beneficial and harmful organisms are well balanced. However, when environmental conditions change and populations become unbalanced, plants become diseased. The success of treatments for soilborne plant diseases depends on environmental conditions, the nature of the pathogenic organism and the sensitivity of the plant.
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Adding compost to soil increases the number of beneficial organisms, which will out-compete the pathogenic organisms for nutrients and inhibit the germination of fungal spores. Mixing compost with soil also improves the structure and drainage of soil; overly moist soil is conducive to the growth of many pathogens, or harmful organisms.
Preventing nutrient deficiencies and adjusting the soil pH are other methods to treat soil diseases. For example, wilting and root rot of seedlings caused by damping-off disease due to Pythium species may be prevented by adding calcium. Adjust the soil pH to grow healthier and stronger plants and to prevent the survival of pH-sensitive pathogens such as clubroot (Plasmodiophora brassicae), which causes swelling and root rot of vegetable roots. Applying lime to raise the pH to 7.3 or higher prevents clubroot.
When vegetables or annual flowers are repeatedly grown in the same location, there may be a buildup of harmful organisms that favor these plants. One way to avoid soil pathogen buildup is to rotate vegetables or other plants each year. Once a pathogenic organism has established itself in a given soil location, you may have to avoid planting there for two, three or more years, depending on the type of specific disease organism. Also, don't plant the garden bed with closely related plants, because the same pathogen may attack them. Many home gardeners don't have the luxury of using multiple garden plots for crop rotation and need to resort to other methods.
Solarization uses the sun's energy to heat the soil and kill harmful organisms. The heat treatment also kills most beneficial soil organisms and weed seeds, an extra bonus. Addition of compost after solarization replenishes beneficial organisms. Solarization takes several weeks to months to work. Long summer days are best, while cool rainy days will not do. Place a clear, 0.5 to 4 mil-thick plastic sheet or tarp over moist soil -- dry soil is not effective. You must bury the edges of the plastic sheet in the soil. The main drawback to this method is that the garden plot cannot be used for a whole season.
Chemical or biological fungicides kill either a specific type of pathogen or a large range of soil pathogens. The chemical fungicide mefenoxam, for example, kills the pathogenic fungi Pythium and Phytophthora, but is not effective against other soil pathogens. PCNB, also called quintozene, is effective against a number of fungi, including Sclerotium rolfsii, which causes southern wilt.
Biological fungicides are soilborne fungi that control harmful fungi. Gliocladium virens controls the disease-causing fungi Pythium and Rhizoctonia, while the Trichoderma harzianum fungus kills Rhizoctonia.