The vast majority of fungi are saprophytes, which means they live by decomposing and recycling nutrients from dead plant material and make them available for plants to use again. There is a wide range of symbiotic interactions between plants and fungi that most often include mycorrhizae and endophytic fungi. Fewer fungal species have evolved towards pathogenicity with adverse effects on plants and they are classified as biotrophs, necrotrophs, and hemibiotrophs. The plant is protected by a cell wall that makes it difficult for the fungus to penetrate the cell so wounds or stomata are common places through which pathogens enter the plant. Cell wall enhancements such as lignification, suberinization, deposition of hydroxyproline-rich glycoproteins, callose as well as cross-linking of cell wall proteins that strengthen plant mechanical barriers and limit pathogens in development are changes that occur at pathogen entry sites. Fungi secrete cell wall degrading enzymes (CWDE) that break down the cell wall and they possess specific infectious structures which vary widely depending on the type of fungus. Furthermore, they secrete effectors to interfere with the reactions of the basal plant defense mechanisms, but plants have also developed mechanisms to recognize such molecules. The recognition of these effectors in plants triggers defense reactions known as effector-triggered immunity (ETI) which results with a hypersensitive response in host plant (HR) and biosynthesis of pathogenesis-related proteins (PR). The rapid production of large quantities of reactive oxygen species (ROS), the so-called oxidative burst, is inducing the production of a large number of PR proteins. Accumulation of PR proteins at the site of infection is usually associated with the systemic acquired resistance (SAR), which is based on the increased degree of resistance of distal tissues in the event of subsequent infection with a pathogenic organism.