Date of this Version
Published in Soil Management: Building a Stable Base for Agriculture (2011) 265-273. DOI:10.2136/2011.soilmanagement.c17
Plants can be attacked by a variety of parasitic microorganisms, primarily fungi, bacteria, nematodes, and viruses. From 2001 to 2003, an average of 7% to 15% of the major world crops (wheat, rice, potato, maize, and soybean) were lost due to fungi and bacteria (Oerke, 2005). Along with weeds and insects, plant pathogens are the major biotic limitation to crop health and yield. Many of these pathogens are foliar and attack aboveground parts of plants, with inoculum spread by wind and rain. Examples are rust, powdery mildew, and foliar leaf pathogens such as the fungus Septoria. However, some of the most severe, intractable, and difficult to control pathogens are soil-borne pathogens, which live in the soil for part or all of their life cycle and interact with the soil biota and the edaphic environment. These pathogens can survive in the soil and infect the root systems of plants. Fungi, fungus-like Stramenopiles (Oomycetes) and nematodes are probably the most important of the soil-borne pathogens. Fungi are eukaryotic organisms that form threadlike filamentous hyphae that can spread through the soil and form resistant structures such as oospores or sclerotia. These structures allow the fungus to survive in the absence of the host, or during unfavorable environments such as heat, cold, or dry soils. When these resting structures encounter a seed or root in the soil, they are stimulated to germinate, chemotactically grow toward the root, and infect the epidermal cells. Some fungi can also destroy seedlings before they emerge from the soil. Once the root is infected, fungi can spread inside the root, rotting the root by producing enzymes and toxins. Fungi also destroy lateral roots, feeder roots, and root hairs. As a result, the plant loses its ability to absorb water and nutrients. Above ground, plants are stunted and show nutrient deficiencies, and yields are reduced. Some pathogens can also move up the roots to the base of the plant, girdling the base or infecting the lower stem. Finally, another group of fungal pathogens can induce wilt by colonizing the xylem system, restricting the conduction of water to the leaves.