All viruses that spread within host tissues (systemically) can be transmitted by grafting branches or buds of diseased plants to healthy plants. Natural grafting and transmission are possible through root grafting and with parasitic fodder (Cuscuta species). It is important to remember that within each of the three components (host, pathogen and environment) there are numerous variables that can affect both the incidence and the severity of the disease. These variables include genetic diversity, the biology and life cycle of the host plant and pathogen, and environmental conditions.
Fungi and microorganisms, such as bacteria and viruses, are the main pathogens in plants as in humans, and employ some of the same infection mechanisms. For example, spores produced by fungi can be carried by air and carried from one plant to another by wind or insects. Moisture from rain and irrigation can transmit microorganisms between susceptible garden plants, and many diseases can be spread by direct contact with contaminated garden tools. While the use of masks, rapid tests and hand sanitizers are not options, there are practical measures to control infections in the garden.
Since a certain plant disease usually affects only one species, the lack of spread to other types of garden plants also suggests the existence of a disease. Non-infectious diseases are caused by unfavorable growing conditions; they are not transmitted from a sick plant to a healthy one. On the other hand, although the bacteria that cause fire blight are not visible, the typical appearance of brown, scorched leaves on pear branches is a symptom of the disease.