   Plant Diseases - May 23, 2012 Jeff Schalau, Agent, Agriculture & Natural Resources University of Arizona Cooperative Extension, Yavapai County The study of plant diseases is called plant pathology or phytopathology. Plant diseases can occur if a pathogen (disease causing organism) is present on a susceptible host plant under suitable environmental conditions for the necessary period of time. These four elements (pathogen, host, environment, and time) are the points of the “plant disease tetrahedron”. Remove or alter one of these elements, and the plant disease will not occur. This relationship is also the rationale for treatment of 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, herbicide injury, poor drainage, etc. These are called abiotic plant diseases and no infectious pathogen is involved. Biotic and abiotic diseases are often differentiated by determining if the disease is spreading within or between plants. When the disease spreads, we look for living pathogens. When the disease is stationary, it is likely abiotic. Diagnosing plant diseases is like being a plant detective. You begin by looking for clues: signs and symptoms. Signs are visible evidence of the disease organism itself and could be fungal fruiting bodies (mushrooms or conks) or bacterial ooze. Symptoms are how the plant reacts to the pathogen. Examples of symptoms are cankers, blight, wilt, necrosis, and other types of abnormal growth. Plant disease diagnosis requires some experience in recognizing normal vs. abnormal growth. 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 fungal 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, Cotton 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 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) and occur as single cells. Many bacteria are beneficial to individual organisms and ecosystems (such as those present in our digestive systems). A few species of bacteria are plant pathogenic. These bacterial plant diseases are mostly managed through prevention, sanitation, and somethimes 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. The root knot nematode is the most common plant pathogenic nematode encountered by home gardeners. 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 practical 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 water, mineral nutrients, and carbohydrates. Management strategies vary based on the host and the specific pathogen. Phytoplasmas are spiroplasmas are relatives of bacteria, vectored by insects, and uncommon plant pathogens outside the tropics. In many cases, plant diseases are “complexes” consisting of multiple causal agents. For instance, a tree may have been damaged by a string trimmer at the base and that wound could become an entry point for a pathogen. Being a plant disease detective requires knowledge of plants and diseases, field experience, and a broad view of the entire picture. Follow the Backyard Gardener on Twitter – use the link on the BYG website. If you have other gardening questions, call the Master Gardener help line in the Camp Verde office at 928-554-8999 Ext. 3 or e-mail us at cottonwoodmg@yahoo.com and be sure to include your name, address and phone number. 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