This is followed by the application of radioactively labeled anti-antibody, which is allowed to react and then washed off. (Figure 2). Some clues may lead down blind alleys while others will lead down the correct path. Agric. However, when new leaves form, they will generally be free of symptoms, indicating a lack of symptom progression. A major problem in identification of biotic causal agents is the inability of some infectious pathogens to grow on artificial media. Variations in symptoms expressed by diseased plants may lead to an improper diagnosis.
Academic Press, New York, NY. If symptoms develop over time on one side of a tree or plant then damage of the roots associated with one side may be suspected such as root rots due to Phytophthora spp. Common names should not be relied upon since some distinctly different plant species may have the same common name, and the common name used in one area may be used for a completely different species in another area. Adjustment of soil moisture is another cultural practice of widespread usefulness. These lists are very helpful in suggesting possible pathogenic agents. Because of this, it is often difficult, if not impossible, to determine what is wrong with a plant when a person is describing symptoms over the phone. Another environmental factor that can be brought under control is the storage and in-transit environment. Signs of plant disease agents can often be overlooked unless careful observations are conducted. Knowing the identity of the plant species affected allows the pathologist to utilize various resources that contain lists of plant diseases associated with specific plants. This is commonly seen in multiple infections due to viruses. Div. Plant Clinic Handbook. Damping-off - This term describes the rapid death and collapse of young seedlings.
Cultural and maintenance activities can be significant. Leaves may be elongated, smaller size, or thickened. These variations can result from a couple of factors.
Are they distributed uniformly across an area or are they localized? This distribution can be especially important in looking at the possibility of non-infectious problems, such as improper herbicide use or various soil factors11. Infectious problems generally occur over time and there is a progression of symptoms. --epub.jpg)
A solution of antibody is applied to the same plate; if the antibody is specific to the antigen, it will combine with it. Burning, deep plowing of plant debris, and fall spraying are used against such diseases as leaf blights of tomato, Dutch elm disease, and apple scab. Avoidance of bruises and cuts while digging, grading, and packing potatoes, sweet potatoes, and bulb crops also reduces disease incidence. A strand of DNA from a known species (the probe) is radioactively labeled and mixed with DNA from an unidentified species. The plant disease clinic and field diagnosis of abiotic diseases.
When all of the information is successfully collected, literature sources should be consulted to determine what is already known about diseases and disease-causing agents associated with the identified plant. Have any unusual occurrences or weather patterns been noted? This can be accomplished by placing a leaf in a moist chamber11,13. 
Viral identification is often accomplished utilizing ELISA (enzyme-linked immunosorbent assay) which is based on the binding of an antibody produced to a specific virus with the virus in the infected plant material1. Is the problem occurring in only one plant species or are different plant species affected?
Review the cultural practices and growing environment. Knowledge concerning the use of microscopes and a hand lens is vitally important to the diagnostician. Late blight on potato tubers can be controlled by delaying harvest until the foliage has been killed by frost, chemicals, or mechanical beaters. 2000. When he removes plants from their pots, the roots appear to be rotten. Diseases can result in primary and secondary symptoms. Other techniques used for the identification of viruses include negative staining and electron microscopy to view the viral particles in plant tissue or suspensions. Information pertaining to the growing environment to which the affected plant has been exposed is a vital piece of the puzzle. An example from forestry is "cedar" which is used to describe eastern red cedar (Juniperus), western red cedar (Thuja), Port Orford cedar (Chamaecyparis), incense cedar (Libocedrus), and Atlas cedar (Cedrus). Is only one limb or side of a plant involved? Plastic bags or boxes may be necessary for larger specimens. Disease control starts with the best variety, seed, or planting stock available and continues throughout the life of the plant. Shoot dieback or blights - Sudden dieback of a shoot usually indicates climatic or chemical injury rather than parasitic problems. Identification of affected plants is one of the first steps in diagnosing a plant disease. Other resources are available which include the APS Press Compendium series on diseases and disorders for specific plant species, such as roses8 or diseases for specific regions, such as Florida, USA2. The infected plant tissue may contain one or more saprophytes which have moved into the infected tissue. Be able to identify a disease and disease-causing agent, Be able to narrow the problem down to several possibilities which will require further study in the laboratory before he can make a final diagnosis, or. The steps all require careful observations and questions. An example of this is shown in Figure 1 which shows peach seedlings infected with single or multiple viruses. Many times careful investigation by the diagnostician is required because, in some cases, someone may have done something improperly and may be unwilling to admit their error. Fla. Dep. Another highly sensitive immunoassay is the enzyme-linked immunosorbent assay (ELISA). Check for host specificity. Diagnostic tests for identification of abiotic plant disease causal agents. Academic Press, New York, NY. The DNA hybridization technique is an example. Many pathogens, however, are relatively unaffected by rotation because they become established as saprotrophs in the soil (e.g., Fusarium and Pythium species; Rhizoctonia solani; and the potato scab actinomycete, Streptomyces scabies) or their propagative structures remain dormant but viable for many years (e.g., cysts of cyst nematodes, sporangia of the cabbage clubroot fungus, and onion smut spores). The loss is most often caused by a blocking of the water flow through the xylem. Look for signs of biotic causal agents. A moist chamber can be a sterile petri dish containing a wet filter paper in the bottom of the dish and a triangle of glass tubing on which the sample is placed so that the sample is not directly on the wet filter paper but is exposed to humid conditions. Root galls can result from fungal and fungallike agents (Plasmodiophora brassicae), nematodes (Meloidogyne spp. Are there symptoms on the roots, leaves, stems, flowers, or fruit? 6th ed. Such measures include crop rotation, destruction of the diseased plants, elimination of alternate host plants, pruning, disinfection, and heat treatments. These plants are highly prized for this coloration; however, if an individual does not know that this coloration is the normal appearance of the plant, s/he may think that the plant is diseased. Control measures depend on proper identification of diseases and of the causal agents. Several problems can occur when trying to isolate the plant pathogenic agent. These bumps are spore-producing structures of the fungal causal agent. When no information is available on the specific plant, information on diseases and disease causing-agents of similar plants may be useful. He also needs to consider various environmental and cultural factors. The presence of drain tiles in poorly drained fields and the use of ridges or beds for plants are often beneficial. It is also important to note normal events, such as leaf drop, that may occur in a healthy plant. This may be due to problems in replicating the conditions through which the host was inoculated and also in reproducing the environmental conditions present when the host became infected. A great variation in susceptibility to a specific disease may occur within different cultivars of a plant species. American Phytopathological Society, St Paul, MN. Kluwer Academic Publishers, Boston, MA.
APS Press has published a list of fungal diseases and hosts4. The figures that follow illustrate some of the common symptoms that may be produced by different types of problems. Tomato cultivars having the "Better Boy" genetic background are generally resistant to root-knot nematodes while those with the genetic background of the variety "Rutgers" are susceptible, so knowing the genetic background of a cultivar can be important. Inspection and certification of seed and other planting stock help ensure freedom from disease. 1993. Westcott's Plant Disease Handbook is useful because specific symptoms are associated with each disease7. Has the highway department been working along the roadway, possibly applying herbicides? Often the seedlings will appear to be almost broken at the soil line (Figure 27). It is especially important to document changes in the environment. The use of modern telecommunications in diagnosing plant diseases. The process may vary with different diseases and conditions but the overall process is relatively consistent. Chemical spray or air pollutant injury - Spots associated with injury are relatively uniform in color and the interface between the affected and healthy area is usually sharp. Click on any image for a more detailed view. There are many examples in which losses by bacteria, viruses, and mycoplasma-like disease agents can be reduced by controlling aphids, leafhoppers, thrips, beetles, and other carriers of these agents. Incubation of plant material. The variety of symptoms, the internal and external expressions of disease, that result from any disease form the symptom complex, which, together with the accompanying signs, makes up the syndrome of the disease. In many cases, overall stunting of a plant may be due to problems associated with the root system (Figure 25). All of these abiotic factors can be important to the problem. Focus Publishing, Newburyport, MA. In principle this assay is similar to the RIA except that an enzyme system, instead of radioactivity, is used as an indicator of an antigen-antibody combination. Recognize healthy plant appearance. Cankers can result from mechanical injury (e.g. Therefore, diagnosis is one of the most important aspects of a plant pathologist's training. Does the plant normally have new foliage that is yellow or red and becomes darker green as the foliage ages? Masses of different spores such as rust spores (Figure 4) on leaves can also be important in disease diagnosis. Inoculation of a healthy host and obtaining the symptoms originally observed in the field may be difficult. Control of viral diseases of potato, for example, can be accomplished by growing the seed crop in northern regions where low temperatures are unfavourable for the aphid carriers. What has been the progression of symptoms on plants in the affected area? - root-knot), and bacteria (Agrobacterium sp.). Drought and freezing may have a similar effect. One of the most important things is for Dr. Shoe to use his powers of observation. These hypotheses are generated through observations of the plant, environment, and information from the grower. 1998. Selection of the best time and depth of seeding and planting is an effective cultural practice that reduces disease impact. For example, decayed roots on a tree may be a primary symptom while the toppling over of the tree or windthrow is a secondary symptom. The symptoms of some diseases are most commonly seen on specific plant parts and this observation can be important in diagnosis. Symptoms can often be grouped as follows; for definitions of terms, see the APSnet Education Center Illustrated Glossary: Diseases also involve a progression of symptoms that can vary significantly. Is the entire plant involved? It is also important to remember that appearance can vary with different cultivars. Herbicides, such as 2,4-D, can cause leaf distortion which may be confused with viral diseases. How prevalent is the problem? This can be accomplished by regulation of the environment, cultural and handling practices, control of insect carriers, and application of chemical pesticides. Do the descriptions vary significantly? Hansen, M. A. and R. L. Wick. This discoloration may be similar to mosaic and ringspot symptoms observed on leaves (Figure 20). 14. It is important to know what the normal appearance of a plant is before you decide there is a problem. 2000. There are estimated to be some 1.6 million fungal species3,9, most of which are not infectious pathogens.
A variety of postharvest diseases of potato, sweet potato, onion, cabbage, apple, pear, and other crops are controlled in storage and shipment by keeping humidity and temperature low and by reducing the quantity of ethylene and other natural gases in storage houses. As a test of this you may want to take a plant exhibiting symptoms and have three different individuals describe the symptoms that they observe on a sheet of paper. This book followed a previous publication by USDA12, but the APS Press publication only includes fungal pathogens. Once you select the plant of interest, you will see a list of bacterial, fungal, nematode, parasitic plants and viral diseases associated with the specific plant. pectinase, isozyme patterns)5.
It is important to keep an open mind until all of the facts related to the problem can be collected. Proper disease diagnosis is therefore vital. Overall Stunting or Decline - These symptoms can be caused by several very different factors.
Selection of outdoor growing areas where weather is unfavourable for disease is a method of controlling disease by regulating the environment. 3rd edition. Color is usually uniform and no signs of plant pathogen are evident. This article presents the various steps/activities which are associated with accurate plant disease diagnosis. The possibility of multiple causal factors must also be considered. Needle drop in conifers - Conifers normally retain their needles for several years but these needles will eventually be lost. Some plant cultivars have naturally yellow to pale green leaves (e. g. new hosta cultivars, herbs like golden oregano, and coleus varieties) which at first glance appear to have symptoms of under-fertilization, root stress or soil pH problems. Systemic symptoms are those involving the reaction of a greater part or all of the plant, such as wilting, yellowing, and dwarfing. Signs are not visible when taking a quick ride by plants looking through the windshield of a truck and may not even be visible to the naked eye. Abiotic factors such as nutritional deficiencies, soil compaction and herbicide residues can also result in overall stunting or decline. In some cases these lists of plant diseases may suggest potential disease possibilities or they may lead the diagnostician to rule out other diseases. A uniform pattern on an individual plant and uniform damage patterns over a large area are generally not associated with biotic agents, but are usually due to abiotic agents. Spots are not limited by leaf veins (Figure 6).
Destruction of weed hosts also helps control such viral diseases as cucumber mosaic and curly top. How does a plant pathologist or a plant pathology student go about diagnosing plant problems? Answers to these questions can assist in the identification of the problem. Signs are either structures formed by the pathogen or the result of interaction between pathogen and hoste.g., ooze of fire blight bacteria, slime flux from wetwood of elm, odour of tissues affected with bacterial soft rot. Unfavorable growing conditions, such as drought, may cause an acceleration of needle drop. Bacteria are often isolated by chopping up infected tissue in a small amount of sterile water. If the symptoms all appeared at the same time and there has been no further development of symptoms, this would indicate a possible episodic event such as a change in temperature or possible improper chemical usage. One of the first steps when getting back to the laboratory may be to place a sample of the diseased tissue under conditions that will allow an infectious agent to grow and possibly induce sporulation. Identify characteristic symptoms. What can Dr. Shoe do for Ms. Green now? Bound to DNA from the unknown species, the probe acts as a marker and identifies the bacteria. Brown, and G. Ruhl. Symptoms associated with these infected plants may be significantly different from the symptoms expressed in response to each of the different pathogens acting separately. Fungi on Plants and Plant Products in the United States. For example, Dr. Shoe is asked by Ms. Green to examine azaleas in her nursery. Microscopic disease symptoms are expressions of disease in cell structure or cell arrangement seen under a microscope.
Alfieri, S. A., Jr., K. R. Langdon, J. W. Kimbrough, N. E. El-Gholl, and C. Wehlburg. Leaf Distortion - Leaves of the infected plant may be distorted from their normal shape and size. Biotic agents can also include insects and mammals, such as voles, which may be feeding on plants in an area. Dying branches of trees and shrubs - If scattered branches in a tree or shrub start to decline and eventually die, a canker disease or a shoot blight should be suspected (Figure 23). Is there a definite pattern to the distribution?
American Phytopathological Society, St. Paul, MN. Relatively few diseases are controlled by a single method; the majority require several approaches. Fungal leaf spots - spots usually vary in size. Carlile, M. J., S. C. Watkinson, and G. W. Gooday.
Secondary symptoms result from the physiological effects of disease on distant tissues and uninvaded organs (e.g., wilting and drooping of cabbage leaves in hot weather resulting from clubroot or root knot). What equipment was used in its application?
Waller, J. M., B. J. Ritchie, and M. Holderness. If dieback is somewhat more gradual and there is a cracking or splitting of the bark and wood, cold injury should be suspected along with bacterial blights caused by Pseudomonas or Erwinia. It is the compilation of the information and clues that will ultimately lead to the most accurate diagnosis. Has someone been mowing in the area? For example, does it occur only along the edges of a greenhouse near open windows, next to roadways or driveways, in low spots of a field, along a planted row, or is it affecting plants at random in a field? The radioactivity that remains on the plate is a measure of the amount of antibody that combined with the known fixed antigen. The seedling on the left is infected with both Prune dwarf virus and Prunus necrotic ringspot virus.
It is often impossible to replicate in the laboratory the original conditions present during the disease development. The student should keep in mind that s/he is a detective. phytoplasmas), require a living host in order to grow. Environmental factors affecting disease development, Technological advances in the identification of pathogenic agents, Regulation of fertility level and nutrient balance, The use of genetic engineering in developing disease-resistant plants, Classification of plant diseases by causal agent. Elimination of citrus canker in the southeastern United States has been one of the few successful eradication programs in history. 2002. Isolation of fungi usually requires that pieces of infected plant tissue be placed on various nutrient media11.
165. Once an organism is isolated, is that organism the true cause of the problem? More than 150 countries now have established quarantine regulations. A plant pathologist or a student taking plant pathology is often asked by friends or associates the following questions. For example, seed decay, damping-off (the destruction of seedlings at the soil line), and other seedling diseases are favoured by excessively wet soils. Bull. Identify symptom variability. BIOLOG test), and enzyme activity testing (i.e. Needles infected by foliar fungal diseases are generally more scattered and rarely are all needles of a particular growth period killed (Figure 15). The decline may be gradual and may eventually affect the entire tree, but in some cases the death may occur on one side of the plant initially. 84:1256-1265. Once the "normal" appearance of the specific plant is determined, several comparisons can be made between the problem plants and healthy plants. The principle of exclusion and avoidance is to keep the pathogen away from the growing host plant. Viruses, as well as some fungi (e.g. The diagnostician must have very good observation skills, and s/he also needs to be a good detective. trees which have been damaged by collisions with cars or lawnmowers), and various fungi or bacteria. If the decline is sudden, a toxic chemical in soil or weather extremes such as freezing or drought should be suspected. Successful disease control requires thorough knowledge of the causal agent and the disease cycle, host-pathogen interactions in relation to environmental factors, and cost. Plant Ind. Plant disease diagnosis: present and future prospects. Environmental factors to consider include: extreme temperatures (freezing and heat), rainfall, hail, lightning, prolonged drought, temperature inversions (important in possible air pollutant damage and pesticide drift) and prevailing winds. For example, when we look at the susceptibility of wheat to wheat stem rust caused by Puccinia graminis f. sp. Developments in microscopy, serology and immunology, molecular biology, and laboratory instrumentation have resulted in many new and sophisticated laboratory procedures for the identification of plant pathogens, particularly bacteria, viruses, and viroids. It may be too late to help the specific plant when the question is asked, but proper diagnosis may be extremely important in preventing the problem on other plants or in preventing the problem in the future. Index of Plant Diseases in the United States. 1994. These often need to be integrated into a broad program of biological, cultural, and chemical methods to control as many different pestsincluding insects, mites, rodents, and weedson a given crop as possible. It may also be important to get samples of plant tissue analyzed for nutrient content to determine if there are macro- or micronutrient deficiencies or toxicities. Agricultural Handbook No. Macroscopic symptoms are expressions of disease that can be seen with the unaided eye. Some examples of this are rusts which are often recognized by the rusty brown to black spores (Figure 4) and smuts which are identified by the black spores which often replace the seed structure (Figure 12).
In such cases, it may be necessary to bring a sample back to the laboratory for further tests to isolate and identify the causal agent. If you do not know what to expect of the plant you cannot recognize when something is wrong. Introductory Plant Pathology. Dissecting and compound microscopes are useful for the observation of specific spores and spore structures, and can lead to further identification of possible disease agents. Cauliflower roots infected with clubroot. Compendium of Rose Diseases. Generally are round and occasionally elongate on stems. If branches die suddenly and, especially if affected branches are concentrated on one side of tree, weather conditions should be suspected (wind, snow, etc.) The use of a hand lens and a knife can be valuable for a diagnostician in the field. tritici, we know that all wheat cultivars are not susceptible to all races of P. graminis. An example of such a list is found on the American Phytopathological Society (APS) web site as a part of its the online resources. Koch's postulates are seldom conducted for routine diagnoses, but may be extremely important for new diseases and for research. Many fungi and bacteria have never been isolated and identified.
It is possible that there is more than one problem present, and in some cases there may be more than one pathogen infecting a plant. In this procedure a known antigen is overlayed on a plastic plate to which antigen molecules adhere. Needles diseased by infectious agents are generally affected over varying lengths and are often straw yellow or light tan. Zones of different color or texture may develop giving the spot a "bull's eye" effect. The major control measure for this disease is based on planting wheat cultivars each year that are resistant to the pathogen races that are predicted to be present during the growing season.
Wilts - Wilts are characterized by a general loss of turgidity of leaves or possibly entire plants due to the loss of water (Figure 21). What pesticides or other chemicals have been applied? Each plant species has special growth habits, colors and growth rates. Distribution on plant may be associated with where spray or pollutant comes in contact with the plant. or animal or mechanical damage at the trunk base; however, this is not always the case. Farr, D. F., G. F. Bills, G. P. Chamuris, and A. Y. Rossman. Site factors such as soil type, possible drainage problems, and soil pH should also be evaluated. For example, is a wilt observed correlated with a disruption of the vascular system which may be indicated by browning of the vascular system or are the roots of the plants abnormal including rots, decreased feeder roots, etc. However, Phytophthora and Pythium root rots can cause problems on many different plant species; therefore, the fact that more than one plant species is affected does not completely eliminate infectious agents. ; are necrotic lesions observed strictly on younger leaves? The problem may not be due to anything that the grower has done; the problem could be related to what his/her neighbor has done. Fruit Decays and Rots - Various fungi and bacteria can cause rots of fruit.
