- - AGRICULTURE CORE CURRICULUM - - (CLF6000) Advanced Core Cluster: ORNAMENTAL HORTICULTURE (CLF6300) Unit Title: DISEASES AND PESTS OF ORNAMENTAL PLANTS ____________________________________________________________________________ (CLF6301) Topic: PLANT PATHOLOGY Time Taught in Year(s) AND PLANT HEALTH 3 hours 3 and 4 ____________________________________________________________________________ Topic Objectives: Upon completion of this lesson the student will be able to: Learning Outcome #: (E-2) - Summarize the relationship of proper plant nutrition with improved plant health. (E-3) - Explain the effect of diseases on development, growth, and production of plants. (A) - Discuss common diseases in your area. (B) - List possible ways that diseases are spread. (C) - Describe a nematode of your choice, the type of damage it does, and the plants it infests. (D) - Differentiate between bacteria, fungi, virus, and nematode. (E) - Identify vectors that spread plant disease. Special Materials and Equipment: References: Agrios, George N. (1978). PLANT PATHOLOGY (2nd ed.). NY: Academic Press. (pp. 7, 172-191, 435-446, 549-580, 612-623) Evaluation: TOPIC PRESENTATION: PLANT PATHOLOGY INTRODUCTION A. Plant diseases are important because they cause damage to plants and plant products. B. Both the yield and quality of plants are reduced by a wide array of plant diseases. C. A plant pathologist is a person who studies plant diseases and works to diagnose and control them. D. Some plant diseases are easily controlled by one or another method. E. For other plant diseases, however, the cost of control is as high or higher than the expected value of the crop. PLANT PATHOLOGY A. There are many ways in which plant disease pathogens can affect plants. 1. They can suppress the chlorophyll content. 2. They can reduce the leaf area. 3. They can limit the movement of solutes and water through the stems. 4. They sometimes reduce the water-absorbing capacity of the root system. 5. They suppress the translocation of photosynthates away from the leaves. 6. They sometimes promote wasteful use of the products of photosynthesis (as in the formation of galls). B. Most pathogens are parasitic; they invade the host and obtain food from it. 1. Many can be seen only with a microscope, making visual identification difficult. 2. By definition, plant pathogens are capable of spreading from one host to another. 3. The most important plant pathogens are bacteria, fungi, viruses, and nematodes. 4. Most of the plant pathogens have a saprophytic (living on dead or decaying organic matter) existence apart from their host plants during most of the year. C. What are the differences between bacteria, fungi and viruses? 1. Bacteria are small, single-celled, microscopic organisms. a. Anything that moves and comes in contact with bacteria may spread them to other areas. This includes farm equipment, rain, plant material, seeds, birds, insects, nematodes, and people. b. In order to start an infection in a plant, they must first enter a natural opening or a wound. c. Under warm, moist conditions, bacteria have the ability to multiply rapidly. 1. They divide by binary fission (one bacterium divides in half and becomes two bacteria). 2. Under proper environmental conditions this division takes place every twenty minutes. 3. At this rate, one bacterium can give rise to 17 million cells within 12 hours if food, moisture, and temperature are favorable. d. Bacterial diseases cause galls, leaf spots, soft rots, scabs, and systemic disorders. Examples of bacterial plant diseases are crown gall, fireblight, walnut blight, deep phloem canker of walnuts, soft rots of vegetables, and bacterial wilt of cucurbits. e. Controls for bacterial infections include the use of antibiotics, Bordeaux mixtures, and fixed coppers. f. Plant breeding has produced many plant varieties that are resistant to bacterial infection. 2. Fungi are small, usually microscopic, plants that lack chlorophyll and conductive tissues. a. Unlike green plants, they do not photosynthesize their own food, so they depend on living or dead plant or animal tissue. b. Virtually all plants can be attacked by some type of fungi. c. Each of the parasitic fungi can attack one or many kinds of plants. d. Some fungi grow and multiply by living on their host plant during their entire life. e. Other fungi can multiply on dead organic matter as well as living on plants. f. Fungi reproduce mainly by means of spores. 1. These spores are special reproductive bodies made up of one or a few cells. 2. The spores perform the same job as seeds in higher plants. 3. Dissemination of fungi can be by wind, rain, insects, irrigation or flooding, contaminated seed, infected plants, animals, tillage equipment, and pruning shears and knives. g. The vegetative body (mycelium) of fungi is made up of very small filaments or threads called hyphae. 1. These branch and grow in all directions through their food supply. 2. They absorb food from the cells of their host plants. h. Fungi are controlled by several methods. 1. Fungicides (chemical sprays that specifically kill fungi) have been used with success for many years. 2. Soil pasteurization and the use of fumigants work well in some instances. 3. The development of resistant species and cultivars continues to show success and more promise. 4. Crop rotation, good soil drainage, proper handling of the crop, and low-temperature storage are all helpful in controlling various fungi. i. Examples of fungus diseases are stem rust of wheat, corn smut, powdery mildews, rusts, brown rot, damping-off, and Dutch elm disease. 3. Viruses are pathogenic particles that infect most higher plants and animals. a. Virus particles are extremely small (20 to 250 millimicrons). b. Viruses are not cells, nor do they consist of cells. c. They can be seen only with an electron microscope. d. They cannot grow or multiply except when they are within a host cell or insect vector cell. e. These particles move from one plant to another by vectors (carriers). 1. Aphids and leafhoppers spread 95% of viruses; thrips and mites spread the remaining 5%. 2. Also, viruses are spread to new plants when any asexual method is used for propagation; once a virus is in the mother plant, all plants started from it will likely have the virus. NOTE: Virus-free propagation materials are available from the universities and other research organizations for most major crop plants that are asexually propagated. f. Viruses can often be identified by their symptoms: 1. mosaic patterns on leaves 2. yellowing (leaves lacking chlorophyll) 3. stunting 4. ringspots 5. flower break 6. vein clearing (veins are chlorotic) g. These symptoms may be accompanied by other symptoms elsewhere on the same plant. h. The best way to control a virus is to keep it out of an area through quarantine, inspection, and certification programs. i. Other methods of control include: 1. eradication of diseased plants 2. controlling insect vectors 3. removal of weeds which serve as virus hosts 4. use of virus-free tubers, budwood, scionwood, and other propagating materials 5. breeding resistant varieties j. As yet, no chemical substances (viricides) are available for controlling virus diseases. k. Examples of virus diseases are tobacco mosaic, curly top of sugar beets, barley yellow dwarf, necrotic ring spot of stone fruit, tristeza disease of citrus, and blackline of walnuts. D. Nutrition affects the rate of growth and the state of readiness of plants to defend themselves against attack by pathogens. 1. High nitrogen fertilization increases the suceptibility of some plants to bacterial and fungal diseases. 2. Proper nutrition of phosphorous, potassium, calcium, and micronutrients have been shown to aid a plant's resistance to certain diseases. 3. In general, plants receiving a balanced nutrition, are more capable of protecting themselves than plants with either excessive or deficient amounts of nutrients. 4. Nutrient imbalances can themselves cause (abiotic) disease. __________________________________________________________ ACTIVITY: 1. Rot soft fruits in class (e.g., peaches, cherries, plums). a. Leave some fruit intact. b. Injure others to simulate insect or bird injury. c. Observe and record results. 2. Visit local fields, orchards, or vineyards infected with plant disease pathogens. a. Compare results of different pest management programs. b. Was the cost of the program warranted? __________________________________________________________ 1/4/91 MT/clh #%&C