These types of disease are caused by microscopic living organisms called pathogens. Pathogens are broken down into four groups:

• Fungi - Several kinds of molds, mildews, and mushrooms play an important role in breaking down organic matter in compost and soil. However, there is a considerable number of fungi that also attack living plants. These plant pathogenic fungi are the most common causes of garden diseases. Some examples are powdery mildew, damping-off, anthracnose, apple scab, club root, late blight, and black spot.
• Bacteria - Bacteria are similar to fungi in the sense that both are specialized organisms with both beneficial and pathogenic species. Beneficial bacteria recycles soil organic matter and nutrients, as well as help to control insect pests such as caterpillars and beetles. Plant pathogenic bacteria, however, can infect plants and cause detrimental diseases like crown gall, bacterial wilt, and fire blight.
  Bacterial pathogenic cells travel to plants in the same way as would fungal spores. Once it has reached a susceptible plant, bacteria move into and infect plants through wounds as well as natural opening in leaves, stems, and roots.
• Viruses - Viruses are the smallest of disease agents. Once inside a living cell, they multiply by making their host plant produce even more virus particles, upsetting the plant's normal metabolism and causing apparent disease symptoms. Virus particles are carried to healthy plants by pests like insects, mites, and nematodes. They can also be spread by taking cuttings, grafts, layers or divisions from already infected plants. Compost that contains infected plants and is not properly decomposed can also spread viruses.
  Viral diseases can be the most difficult to properly diagnose. Some common indications of a viral affliction include mottled or discolored leaves, although symptoms can vary from one plant to the next. Some infected plants may show no signs of infection whatsoever. If your plants have suffered previously from viral infections, your best course of action is to become familiar with the common viruses in your locale as well as their symptoms so that you will know when to take control measures.
• Nematodes - Nematodes are microscopic roundworms, found free-living in soil and as parasites on plants and animals. Beneficial nematodes are vital to the soil community, as they feed on decaying materials and pests like cut-worms and grubs. Pest nematodes instead damage plants by puncturing cell walls with their needlelike mouthparts and drawing out the cell's contents. This causes obvious disease-like symptoms including yellowing, wilting, stunting, and reduced yields, which are very difficult to distinguish from other causes.
  Plant parasitic nematodes travel to healthy plants in water and in infested soil carried on tools, boots, and animals. If you suspect nematodes are to blame for your plant damage, you can submit root and soil sample to a diagnostic laboratory or your Cooperative Extension Service for positive identification.

Home | Library | Gardens | Tips | Garden Centers | Feedback
Garden Links | Insect Guide | Landscaping | Products