They are often spread to healthy plants by insects or on one's hands during normal gardening practices. Bacterial cells are much larger than virus particles, but they are still too small to see with the naked eye. Bacterial cells move in the water film on the leaf surface or in the water surrounding plant roots or soil particles. They are most often spread by splashing water. Fungal spores are larger than bacterial cells but are not visible without a microscope.
Fungi are like small plants.
Most plant diseases caused by fungi are most severe during periods of moderate temperatures and when water is retained on the leaves or fruit for an extended period of time. Many fungi are spread by wind, splashing rain and equipment. Nematodes are small, worm-like animals which live in the soil. They feed on plant roots, causing stunted plants. Root knot is the most damaging nematode in the home garden.