   Plant Diseases - July 19, 2006 Jeff Schalau, County Director, Associate Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County The study of plant diseases is called plant pathology or phytopathology. Plant diseases can occur when a pathogen (disease causing organism) is present on a susceptible host plant under suitable environmental conditions. These three elements are the basis of the plant disease triangle: pathogen, host, and environment. Alter one of these three elements, and the plant disease will not occur. This relationship is the basis for treatment for plant diseases. Living organisms that cause plant diseases (biotic pathogens) include fungi, bacteria, viruses, nematodes, parasitic plants, phytoplasmas, and spiroplasmas. Plant diseases can also be caused by non-living factors such as wind, drought, air pollution, extreme temperatures, poor drainage, etc. These are called abiotic plant diseases. While there is no pathogen involved, treatments for abiotic plant disease use similar approaches. Fungi are similar to plants, but lack chlorophyll and must find energy through associations with other living or dead organisms. They are the single most important cause of plant disease. Of the 100,000 known species, 10,000 can cause diseases in plants. All plants are attacked by some form of fungi and many fungi can cause disease on a wide variety of plant species. For example, Texas Root Rot (Phymatotrichum omnivorum) has been recorded on over 2,300 species of broadleaf plants. However, monocots (grasses, lilies, palms, bamboos, and many others) are resistant. Management of most fungal plant diseases is difficult. The most common approaches are prevention and sanitation (cleaning tools, sterilizing soil, etc.). Fungicides are most effective as a preventative treatments. Once the fungal pathogen is present, it is much more difficult to manage. Bacteria are the most abundant of all organisms and they occur mainly in soil and water, but can also be airborne. They are generally very small (0.5-5 microns), occur as single cells, and some are associated with other organisms through symbiotic relationships. An example of a symbiotic relationship is the bacteria that live in our gut and help us breakdown and digest food. Many other bacteria are beneficial to individual organisms and ecosystems. Bacterial plant diseases are managed through sanitation and through the use of antibacterial sprays. Fire blight is an example of a bacterial disease that affects some plants in the rose family. Viruses are much smaller than bacteria and made up of only genetic material (DNA or RNA) surrounded by a protein coat. They are commonly introduced to plants through an insect vector (often aphids, whiteflies, and leafhoppers). Once inside a plant, they reproduce and cause various symptoms. Prevention and sanitation are the only management strategies available. Curly top virus is a common disease in our area that affects beets, tomatoes, peppers, beans, potatoes, spinach, and cucurbits. It is vectored by the beet leafhopper. Nematodes are microscopic roundworms. Many nematode species are considered beneficial because they parasitize pest insects. Root knot nematode is the most common plant pathogen in our area. Root knot nematodes feed on plant roots causing swelling of root tissue and impairment of water and nutrient uptake. Treatments for root knot nematodes are limited. Since it is not a native organism, it must be introduced through soil importation or infected plant material. Here again, prevention and sanitation are the only management strategies. Common parasitic plants in Arizona are true mistletoes, dwarf mistletoes, and dodder (a stringy, straw-colored mass growing atop other plants). These are seed plants that infect a host and use its resources. True mistletoes are green, contain chlorophyll, and conduct photosynthesis to produce carbohydrates. They rely on the host for water and mineral nutrients. Dwarf mistletoes and dodder lack chlorophyll and rely on the host for everything. Management strategies vary based on the host and the pathogen. Phytoplasmas are spiroplasmas are relatives of bacteria and uncommon plant pathogens. They are vectored by insects and not fully characterized by researchers. Plant diseases require skill to diagnose and can be difficult to treat. If you suspect a plant disease, call the University of Arizona Cooperative Extension for assistance. Master Gardener volunteers can be reached at the Cottonwood office at 646-9113 ext. 14 or E-mail at cottonwoodmg@yahoo.com. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/. |