Each generation increases the number of nematodes in the soil by 100 fold. Dissemination - spread of pathogen inoculum. Almost all dissemination of pathogens that is responsible for plant disease outbreaks, and even for disease occurrences of minor economic importance, is carried out passively by such agents as air, water, insects, certain other animals, and humans. Air-most of these spores do not contact a susceptible hosts. They have a better chance in monculture.
Rusts occur at several thousand meters above infected fields and can be carried for miles.
Bacteria, nematodes, and spores, sclerotia and mycelial fragments of fungi present in the soil are disseminated by rain or irrigation water that moves on the surface re through the soil. All bacteria and the spores of many fungi are exuded in a sticky liquid and depend for their dissemination on rain or irrigation water, which either washes them downward or splashes them in all directions. Raindrops or drops from overhead irrigation pick up the fungal spores and any bacteria present in the air and wash them downward where some of them may land on susceptible plants. Water dissemination is more efficient in that the pathogens land on an already wet surface and can move or germinate immediately. Aphids and leafhoppers are primary vectors for viruses.