When a pathogen has successfully established itself in host tissue its subsequent spread is influenced by many factors including its aggressiveness, susceptibility of the tissue, and environmental conditions. For example, the lesions caused by a pathogen may be smaller, develop more slowly and produce fewer spores in a resistant than a susceptible plant. Some viruses become systemic in certain species of hosts but remain localized in others, and temperature may affect this. The spread of some leaf pathogens is limited by veinlets, resulting in typically angular lesions, or by anatomical or chemical barriers developed by the host as a response to infection (defensive reactions).
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