Relatively little is known of the biochemistry of recognition of a susceptible host by a pathogen. Fungal and bacterial inoculum (infectious material) is spread at random to both hosts and nonhosts. Mucilagenous substances on the surface of the inoculum facilitates adherence to host surfaces. Some host chemicals that serve as signals leading to penetration are known, and some pathogen chemicals that serve as elicitors in disease development have been identified. In some cases penetration occurs but growth in the host is limited, and disease does not develop. Either the agent does not produce the elicitors that lead to infection, or the host produces chemicals that prevent infection. Since viruses and phytoplasmas are brought to hosts by insect vectors, disease may result from an adaptive sequence in which the vector feeds preferentially on the hosts that are susceptible to the pathogen.
Fungi usually penetrate leaves by production of a specialized structure called an appressorium. As a fungal hypha grows over the surface of a leaf, the hyphal tip mounds up and becomes cemented to the leaf, forming an appressorium. A specialized hypha, called a penetration peg, grows from the appressorium and penetrates the leaf, largely by mechanical pressure. The penetration peg also may produce cutinase and cellulose enzymes that soften the tissue. The leaf epidermis is covered by a cuticle made primarily of a waxy substance called cutin, and the epidermal cell walls have a high cellulose content. Sometimes a fungus penetrates through a stoma, a hole in the lower epidermis of the leaf formed by two guard cells. Even when a fungus penetrates through a stoma, an appressorium is usually produced. Of course, the penetration peg meets no resistance.
Inside the leaf, fungal hyphae grow between cells (intercellular) and through cells (intracellular) to obtain nutrients. When the leaf dies, the fungus is able to obtain nutrients from the dead cells. The fungi that are obligate parasites (downy mildews, powdery mildews, and rusts) grow inter-cellularly and produce haustoria (specialized hyphal structures) that penetrate the host cells. The haustoria produce enzymes and obtain nutrients from the host cells. Eventually the cells die, and these fungi are no longer able to obtain nutrients.
Bacteria that attack leaves are disseminated in splashing rain and penetrate through stomata or wounds. The bacteria are found between cells in the host and never penetrate the living cells. Nutrients leaking from the host cells provide sufficient food for the bacteria. After the death of leaves, the bacteria continue to obtain nutrients from the dead cells.
Viruses and phytoplasmas are usually transmitted by insects. Feeding by the insects deposits these agents into the phloem or parenchyma tissues. Some viruses can be transmitted by workers in the field. Handling plants causes small wounds and transmits small amounts of contaminated sap. Many viruses attack crops that are propagated vegetatively (by bulbs, corms, bud ding, etc.), and the diseases are transmitted through use of infected planting stock. Both viruses and phytoplasmas are obligate parasites and cannot obtain nutrients from tissues that have died.
Nematodes penetrate roots mechanically by use of the stylet mouth part. Once inside they insert the stylet into parenchyma cells of the cortex and obtain nutrients. Some nematodes have a long stylet and feed on plant roots while the body is outside the root. Plant pathogenic nematodes are all obligate parasites, capable of obtaining nutrients only from living host cells.
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