Phytopathogenic microorganisms cause plant diseases. They are: phytopathogenic bacteria, phytopathogenic fungi and plant viruses. They cause various changes in function, shape and integrity of the plant. Phytopathogenic bacteria cause diseases on plants. The most common causes of diseases are bacterial species Erwinia carotovora, Erwinia amylovora, pseudomonas syringae, Pseudomonas solanacearum, Agrobacterium tumefaciens, Xanthomonas campestris, Streptomyces scabies. Mycoses are diseases caused by phytopathogenic fungi. Common causes of diseases are these types of fungi: Peronospora tabacina, Pythium ultimum, Alternaria brassicicola, Alternaria brassicae, Ustilago tritici, Fusarium subglutinans and Botrytinia fuckeliana. Plant viruses are transfered from the affected plants to healthy plants. They enter the host cell passively through wounds in the epidermis of the plant cells. Viral species on plants are: Cucumber mosaic virus, Plum pox virus, Prunus necrotic ringspot virus, Grapevine fanleaf virus, Tobacco mosaic virus, TMV and Potato leaf roll virus. Cultural practices are conducted in order to protect the plants from the phytopathogenic microorganisms. Mechanical injuries are avoided during the cultivation and harvesting of the plants, infected parts of the plant are removed or destroyed, adequate biopesticides are applied, healthy seeds are used and planted into crop rotation, and copper based solutions are effective for protection of some plants.