The most common plant pathogens on ornamental plants in Ohio are fungi, bacteria, viruses, and nematodes. Fungi are by far the most common type of plant pathogens. Plant pathogenic fungi cause many different symptoms, including leaf spots, wilts, curled leaves, dieback, enlargements or galls, and stunted or dead plants. Some fungi are visible in forms--called signs--such as rusts, mildews, and sooty molds. Fungi are usually composed of fine threadlike structures (hyphae) that form a network or mass (mycelium) growing on or through their host. Fungi can spread through rhizomorphs, sclerotia, or by microscopic spores. Rhizomorphs are rootlike or cordlike masses of hyphae that can contaminate soil and plant parts. Sclerotia are compact masses of hyphae that can persist for relatively long periods in soil.
Fungal spores (which are produced in great numbers) can be spread by wind, water, soil movement, machinery, insects, or other things with which they come in contact, including people. Sclerotia, rhizomorphs, and some types of spores may survive for long periods in or on plants or in soil. When and where conditions such as temperature, moisture, and the presence of a host plant are suitable for growth, they produce new fungal hyphae and infect the plant. If they are large enough to be seen with the naked eye or a hand lens, signs such as mycelium, masses of spores, and spore-forming structures such as mushrooms, help in identifying fungi.
Bacteria are microscopic, one-celled organisms that obtain their food from living plants or other organic matter. Common symptoms of disease resulting from bacterial infection are shoot blight, leaf spots, soft rots, scabs, wilts, cankers, and galls on branches, twigs, stems, and roots. Unlike fungi, plant pathogenic bacteria generally do not produce spores that can survive adverse environmental conditions; they must usually remain in contact with a host plant or plant debris to survive. Plant-infecting bacteria generally require warmth and moisture to multiply, and are usually not a problem during dry summer weather, except where there is overhead irrigation. Bacteria are commonly spread by splashing water, but may also be dispersed by insects or by moving infested plants, soil, or equipment. In some instances, such as diseases of herbaceous annual flowers, bacteria may be seed-borne.
Viruses are submicroscopic particles that can infect plants and lead to stunting, discoloring, deformation, or death of leaves, stems, fruit, or entire plants. Viruses rarely kill woody plants, and some infected plants exhibit no symptoms. Viruses require a living host cell in which to reproduce, and generally do not survive for very long outside of living tissue. Many viruses are spread by aphids, leafhoppers, or other plant-feeding insects. Some viruses are spread by nematodes, budding or grafting, or the movement of infected seeds, plants, plant parts, or infested equipment. Once a plant is infected by a virus, it usually remains infected during its entire life.
Nematodes are tiny (usually microscopic) roundworms that feed on a wide diversity of organisms. Nematode species that damage plants may feed on or in roots, tubers, bulbs, leaves, or stems. With the exception of the pinewood nematode, which feeds inside branches and trunks, most species that are pests of ornamental plants feed on roots. These include root knot nematodes, cyst nematodes, root lesion nematodes, and dagger, pin, ring and stunt nematodes.