6 Pathogensis - Copy

6 Pathogensis - Copy - PP 315 / 590J Lecture 6 Pathogenesis...

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PP 315 / 590J Lecture 6 Pathogenesis Objectives Understand the process of disease development (pathogenesis) within a host, including different mechanisms of host penetration. Learn the basic concept of enzymatic degredation of the epidermis and cell wall, which aids the inoculation process as well as the role of toxins in disease formation. pathogenesis - stages in disease development The first step in disease development is inoculation . In all but a few cases this is a passive phenomenon. The pathogen is deposited by the wind or rain on a host or the host contacts the pathogen as it grows through the soil. Tropisms (oriented movement) may be involved in initiation of a host-parasite interaction: Before inoculation - chemotaxis - attraction to exudates (nutrients), such around a root tip or at wounds on the root; e.g. by zoospores (See laboratory 2) Masses of zoospores of Phytophthora cinnamomi accumulating in the zone of elongation on a blueberry root. Exudation of nutrients primarily occurs in this region of the root. electrotaxis - attraction to electrical fields; e.g. around roots After inoculation - (Rust fungi) oriented growth across leaf to stomates Environmental conditions and host infection After inoculation occurs, specific conditions must be present that allow for germination and/or growth of the pathogen. Unless the pathogen enters the host through a wound or is placed inside the host by a vector, there must be a film of water on the surface on the leaf or root for a specific amount of time before the pathogen can penetrate the host. This is most critical for leaf pathogens, and is typically referred to as leaf wetness duration . The duration required for infection to occur is also dependent on the temperature. The film of water allows the fragile hyphae of fungi or bacterial cells time to breach the host's barriers before dessication of the pathogen inoculum occurs.
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Leaf wetness Rose leaves with symptoms of black spot. Note the free water on the leaves in the form of dew in early morning. Extended periods of leaf wetness are very conducive to development of new infections. Host penetration - how pathogens gain access to host tissues 1) INDIRECT - penetration of host barriers through natural openings or wounds. 2) DIRECT - penetration directly through the epidermis (through or between epidermal cells) The mechansism of direct penetration was debated for many years, with two main hypotheses; use of mechanical pressure and the use of enzymes. Cuticle - composed of cutin , waxes, pectin , and a small amount of cellulose with a pectin layer holding it to the cell wall. How do fungi penetrate this layer?
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This note was uploaded on 03/22/2008 for the course PP 315 taught by Professor Shew during the Spring '08 term at N.C. State.

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6 Pathogensis - Copy - PP 315 / 590J Lecture 6 Pathogenesis...

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