Principle Of Plant Protection (606101) PDF

Principle of Plant Protection (606101) Plant Pathology 1 Introduction Plant pathology is the science that studies plant diseases. It is concerned with how pathogens cause disease and how plants resist pathogens. It describes how diseases spread in time and place. It a...

Principle of Plant Protection (606101) Plant Pathology 1 Introduction Plant pathology is the science that studies plant diseases. It is concerned with how pathogens cause disease and how plants resist pathogens. It describes how diseases spread in time and place. It also seeks to find methods to protect plants from the impacts of disease. Plant pathology is the study of; the organisms that cause disease in plants, the environmental factors that cause disease in plants, the mechanisms by which these factors induce disease in plants, the methods of preventing or controlling disease and reducing the damage it causes. Plant pathology for plants is what medicine is for humans and veterinary medicine is for animals. Each discipline studies the causes, mechanisms, and control of diseases affecting the organisms with which it deals, i.e., plants, humans, and animals, respectively. Plant pathology is an applied science that has benefitted from input from a wide range of fundamental and applied disciplines. Much of the current plant pathology research is rooted in microbiology, biochemistry, genetics, cell biology, and ecology. It interfaces with chemistry and physics. As an applied science, it collaborates with engineering, economics, agronomy, weed science, and entomology. It has also been the forum in which fundamental knowledge was developed that advanced other sciences. The use of pure chemicals to cure diseases was first carried out in plants. The concept that germs caused disease 2 was first worked out in plants and later confirmed in studies on animal diseases. Most of the major advances in virology were first made using plant viruses. Healthy plants (cells) carry out its physiological functions. They divide and differentiate as needed, and different types of specialized cells absorb water and nutrients from the soil; translocate these to all plant parts; carry on photosynthesis, translocate, metabolize, or store the photosynthetic products; and produce seed or other reproductive organs for survival and multiplication. When the ability of the plant cells or plant parts to carry out one or more of the essential physiological functions is interfered by either a pathogenic organism or an adverse environmental factor, the activities of the cells are disrupted, altered, or inhibited, the cells malfunction or die, and the plant becomes diseased. At first, the affliction is localized to one or a few cells and is invisible. Soon, the reaction becomes more widespread and affected plant parts develop changes visible to the naked eye. These visible changes are the symptoms of the disease. Disease in plants, then, can be defined as the series of invisible and visible r e s p o n s e s of plant cells and tissues to a pathogenic organism or environmental factor that result in adverse changes in the form, function, or integrity of the plant and may lead to partial impairment or death of plant parts or of the entire plant. 3 The kinds of cells and tissues that become affected by disease determine the type of physiological function that will be disrupted first. For examples: ⁃ Root infection causes root rots and make them unable to absorb water and minerals from the soil. ⁃ Xylem infection causes vascular wilts and some cankers that interfere with the translocation of water and minerals to the crown of the plant. ⁃ Phloem infection causes cankers that interfere with the downward translocation of photosynthetic products. ⁃ Foliage infection causes leaf spots, blights, rusts, mildews, mosaics, and so on, that interfere with photosynthesis. ⁃ Flowers and fruits infection interferes with reproduction. Although infected cells in most diseases are weakened or die, in some diseases, e.g., in crown gall, infected cells are induced to divide much faster (hyperplasia) or to enlarge (hypertrophy) than normal cells, therefore it produces abnormal amorphous overgrowths (tumors) or abnormal organs. Each kind of crop plant can be affected by a hundred or more plant diseases. Pathogens vary in the range of hosts they can infect, some pathogens affect only one variety of a plant (one host), Other pathogens affect several dozen or even hundreds of species of plants (multiple host). 4 Fig. 1: Schematic representation of the basic functions in a plant (left) and of the kinds of interference with these functions (right) caused by some common types of plant diseases. 5 Classification (Grouping) of plant diseases can be based on: ⁃ The symptoms they cause (root rots, wilts, leaf spots, blights, rusts, smuts). ⁃ The plant organ they affect (root diseases, stem diseases, foliage diseases). ⁃ The types of plants affected (field crop diseases, vegetable diseases, turf diseases, etc.). ⁃ The causal agent that causes the disease (the most useful one). The advantage of this grouping is that, it indicates the cause of the disease, which immediately suggests the probable development and spread of the disease and also possible control measures. Plant diseases based on the causal agents Living organism (biotic) = Infectious = Pathogen Include diseases caused by: ⁃ Fungi ⁃ Prokaryotes (Bacteria and Mollicutes) ⁃ Viruses and viroids ⁃ Nematodes ⁃ Protozoa ⁃ Parasitic higher plants ⁃ Green algae 6 Fig. 2: Schematic diagram of the shapes and sizes of certain plant pathogens in relation to a plant cell. Bacteria, mollicutes, and protozoa are not found in living plant cells. 7 Fig. 3: Morphology and ways of multiplication of some of the groups of plant pathogens. 8 Environmental factors (abiotic) = Noninfectious Include diseases caused by: ⁃ Too low or too high temperature ⁃ Lack or excess of soil moisture ⁃ Lack or excess of light ⁃ Lack of oxygen ⁃ Air pollution ⁃ Mineral deficiencies ⁃ Mineral toxicities ⁃ Soil acidity or alkalinity (pH) ⁃ Toxicity of pesticides ⁃ Improper cultural practices 9 Fig. 4: Various types of symptoms caused by different environmental factors. 10 History of Plant Pathology It is probable that every year, and in some years more than in other years, part of the crop was lost due to plant diseases. In such years’ food supplies were insufficient and hunger was common. In years when wet weather favored the development of plant diseases, most or all of the crop was destroyed and famines resulted, causing immense suffering and probably the death of many humans and animals from starvation. Therefore, it is not surprising that plant diseases are mentioned in some of the oldest books such as Homer (c. 1000 B.C.) and Old Testament (c. 750 B.C.). Plant diseases were feared as much as human diseases and war were feared. Efforts to control plant diseases were hampered by the lack of information on the causes of disease and by the belief that diseases were manifestations of the wrath of God. Some ancient writers, e.g., Homer (c. 1000 B.C.), mention the therapeutic properties of sulfur on plant diseases. Democritus (c. 470 B.C.) recommended controlling plant blights by sprinkling plants with the olive grounds left after extraction of the olive oil. Most ancient reports, however, dealt with festivals and sacrifices to thank, please, or appease a god and to keep the god from sending the dreaded rusts, mildews, blasts, or other crop scourges. Very little information on controlling plant diseases was written anywhere for almost 2000 years. 11 Until about A.D.1200 that a higher plant, the mistletoe, was proposed as a parasite that obtains its food from the host plant, which it makes diseased. It was also noted that the host plant can be cured by pruning out the part carrying the mistletoe. Nobody, however, followed up on this important observation. Fig. 5: Mistletoe plant parasite on forest trees. In the mid-1600s, a group of French farmers noted that wheat rust was always more severe on wheat near barberry bushes than away from them. The farmers thought that the rust was produced by the barberry plants from which it moved to wheat. They asked the French government to pass the first plant disease regulatory legislation that would force towns to cut and destroy the barberry bushes to protect the wheat crop. 12  Fig. 6: Wheat rust disease severe on wheat near barberry bushes Expanding Role of Fungi as Causes of Plant Disease In the early 1700s, independently, Connecticut (American State) farmers observed that wheat rust was worse near barberry bushes, the farmers came to believe that barberry fathered the rust, which then moved to Wheat. The farmers request for legislation to force towns to eradicate barberries to protect the wheat plants from rust. At the same time, (Hooke, 1667) observed spores of the rust fungus with the compound microscope for the first time in England. In Italy (1729), Micheli: described many new genera of fungi, illustrated their reproductive structures. In France (1755), Tillet showed that wheat smut is a contagious plant disease, but even he believed that it was a poisonous substance contained in the smut dust, rather than a living microorganism, that caused the disease. In northern Europe, particularly Ireland (1840s), the devastating epidemics of late blight of potato not only dramatized the effect of plant diseases on human suffering and survival, but also greatly stimulated interest in their 13 causes and control. The German Kühn (1870s) contributed significantly to the studies of infection and development of smut in wheat plants and promoted the development and application of control measures, particularly seed treatment for cereals. Kühn wrote the first book on plant pathology, “Diseases of Cultivated Crops, Their Causes and Their Control,” in which he recognized that plant diseases are caused by an unfavorable environment and can be caused by parasitic organisms such as insects, fungi, and parasitic plants. In 1887, Koch formulated his rules of disease diagnosis that became known as “Koch’s postulates.” These rules became the standard procedure for proving that a disease is caused by a bacterium or any other kind of pathogen. 14 The Discovery of Other Causes of Infectious Diseases 1- Nematodes The first report of nematodes associated with a plant disease was made in England by Needham in 1743. He observed nematodes within small, abnormally rounded wheat kernels (wheat galls); however, he did not show or suggest that they were the cause of the disease. Fig. 7: Left; Wheat gall nematode. Right; infected wheat seeds with wheat gall nematode. In 1855, the second nematode (Root knot nematode) was observed in cucumber root galls. Fig. 8: Symptoms of Root knot nematode. 15  2- Bacteria Burrill in Illinois (in 1878), showed that bacteria cause the fire blight disease of pear and apple. Fig. 9: Symptoms of fire blight disease caused by Erwinia amylovora. Studies on how Agrobacterium tumefaciens (the causal agent of crown gall), cause tumors in plants led to the discovery, that whenever the bacterium infects plant it transfers part of its DNA to the plant and that the DNA is expressed by the plant as if it were plant DNA. Fig. 10: Symptoms of crown gall disease caused by Agrobacterium tumefaciens. 16 Fig. 11: Utilization of Agrobacterium tumefacies in plant transformation. The discovery that Agrobacterium tumefaciens acts as a natural genetic engineer of plants led to the development of Agrobacterium tumefaciens could be loaded with DNA segments coding for desirable characteristics, and then transfer to plants. This development is formed the basis of Plant Biotechnology. Fig. 12: Method of Agrobacterium-mediated gene transfer. 17  3- Viruses The Dutchman Adolph Mayer (in 1886) injected juice obtained from tobacco plant leaves showing various patterns of greenish yellow mosaic into healthy tobacco plants, then the healthy tobacco plants developed similar mosaic patterns. Because no fungus was present on the plant or in filtered juice, Mayer concluded that the disease was probably caused by bacteria. In 1898, Beijerinck, concluded that the tobacco mosaic disease was caused not by a microorganism, but by a “contagious living fluid” that he called a virus. Fig. 13: Left: Particles of tobacco mosaic virus. Right; Tobacco leaf showing symptoms of tobacco mosaic. 18  4- Protozoa In 1931, Stahel found flagellates infecting the phloem of coffee trees, causing abnormal phloem formation and wilting of the trees. Fig. 14: Left; different type of flagellates. Right; forest tree infected with Myxomycota spp. In recent years, the Myxomycota and the Plasmodiophoromycota, which were previously thought to be fungi, have been transferred to the kingdom protozoa. 5- Mollicutes For nearly 70 years after viruses were discovered, many plant diseases were described that showed symptoms of general yellowing or reddening of the plant or of shoots proliferating and forming structures that resembled witches’ brooms. These diseases were thought to be caused by viruses, but no viruses could be found in such plants. 19 Fig. 15: Reddening symptoms caused by Mycoplasma-like on Kalanchoe plant. Fig. 16: Witches’ brooms symptoms caused by Mycoplasma-like on rose plant. In 1967, Doi and colleagues in Japan observed wall-less bodies in the phloem of plants exhibiting yellows and witches’ broom symptoms, he called Mycoplasma-like (prokaryotic cell that lack a cell wall around their cell membranes). 20 Then, mycoplasma-like organisms (MLOs) that infect plants have been reclassified and named as Mollicutes. The circular shaped cells known as phytoplasmas, and some of them that have helical bodies are known as spiroplasmas. Fig. 17: Left; Mycoplasma-like cells. Middle; Phytoplasmas. Right; Spiroplasmas. 6- Viroids In 1971, studies of the potato spindle tuber disease showed that it was caused by a small, naked, single stranded, circular molecule of infectious RNA, which was called a viroid. Viroids have been found to be the cause of several plant diseases. Viroids seem to be the smallest infectious nucleic acid molecules. No viroids have been found that infect animals or humans. Fig. 18: Potatoes infected with potato spindle tuber viroid. 21 Plant Pathology in The 20th Century 1- The Main areas of progress was chemical control of plant diseases. In 1885, Millardet noticed that vines sprayed with a bluish-white mixture of copper sulfate (CuSO4) and lime (CaCO3) retained their leaves, whereas the leaves of untreated vines were killed by the disease. After trying several combinations, Millardet concluded in that same year that a mixture of copper sulfate and hydrated lime could effectively control the downy mildew of grape. This mixture, which became known as Bordeaux mixture. Fig. 19: Downy mildew of grape, left; on leaves and right on grape cluster. For more than 100 years, Bordeaux mixture was used more than any other fungicide against a wide variety of plant diseases in all parts of the world, and even today it is one of the most widely used fungicides worldwide. 22 Fig. 20: The University of Jordan campus, demonstrates the current use of Bordeaux mixture to control plant pests. In 1913, organic mercury compounds were introduced as seed treatments, and such treatments were routine until the 1960s when all mercury-containing pesticides were banned because of their toxicity. 2- Alternative Controls for Plant Diseases Concern over the potential toxicity of pesticides and over the continuing loss of appropriate, effective pesticides available for plant disease control has continued to increase since the 1970s. This has led to the re-examination and improvement of many old practices and to the development of some new cultural practices for use in controlling plant diseases. Proper cultural practices include: ⁃ Removal of plant debris and infected plant parts. ⁃ Use of seed free of pathogens. 23 ⁃ Crop rotation. ⁃ Soil fallow. ⁃ Adjusting the time and rate of sowing and date of harvest. Economic Importance of Plant Diseases Losses Caused by Plant Diseases Plant diseases are importance to humans because they damage plants and plant products on which humans depend for food, clothing, furniture, the environment, and, in many cases, housing. Losses caused by plant diseases could be related to: 1. Reduction in the quality and quantity of plant products The kinds and amounts of losses caused by plant diseases vary with the plant or plant product, the pathogen, the locality, the environment, the control measures practiced, and combinations of these factors. The quantity of loss may range from slight to 100%. Plants or plant products may be reduced in quantity by disease in the field, as indeed is the case with most plant diseases, or by disease during storage, as is the case of the rots of stored fruits, vegetables, grains, and fibers. Sometimes, destruction by the disease of some plants or fruits is compensated by greater growth and yield of the remaining plants or fruits as a result of reduced competition. Frequently, severe losses may be incurred by reduction in the quality of plant products. For instance, whereas spots, scabs, blemishes, and blotches on fruit, vegetables, or ornamental plants may have little effect on the quantity produced, the inferior quality of the product may 24 reduce the market value so much that production is unprofitable or a total loss. For example, with apples infected with apple scab, even as little as 5% disease may cut the price in half. Fig. 21: Apple scab disease on fruits. With others, e.g., potatoes infected with potato scab, there may be no effect on price in a market with slight scarcity, but there may be a considerable price reduction in years of even minor of produce Fig. 22: Potato scab disease on potato tuber. 25 2. Limit the kinds of plants and industries in an area The American chestnut was annihilated in North America as a timber tree by the chestnut blight disease. The American elm is being eliminated as a shade tree by Dutch elm disease. The lethal yellowing disease was first found Florida in 1971, by 1975, 75% of the coconut palm trees were dead. Fig. 23: Lethal yellowing of coconut palm trees. Left; Coconut palms at different stages of the disease, with the disease advancing from the lower fronds upward until the apical bud is killed. Right; Telephone pole-like trunks of coconut palms left after trees were killed by the lethal yellows phytoplasma. Outbreak of Xylella fastidiosa in olive trees in southern Italy resulted in replacement of olive trees with carob and argan trees. Fig. 24: Olive quick decline syndrome on olive trees caused by Xylella fastidiosa. 26 Plant diseases may also determine the kinds of agricultural industries and the level of employment in an area by affecting the amount and kind of produce available for local canning or processing. Such as what has happened in southern Italy when Xylella fastidiosa outbreaks in olive trees, resulted in closing the olive mills. Plant diseases are also responsible for the creation of new industries that develop chemicals, machinery, and methods to control plant diseases; the annual expenditures to this end amount to billions of dollars in the United States alone. 3. May make plants poisonous to humans and animals. Some diseases, such as ergot of rye and wheat, make plant products unfit for human or animal consumption by contaminating them with poisonous fruiting structures. Fig. 25: Ergot of cereals. Left; Ergot sclerotia replacing the kernels in the heads of wheat. Right; Ergot sclerotia from barley mixed with healthy barley kernels. Many grains and sometimes other seeds and also plant products such as bread, hay, and rotting fruit are often infected or contaminated with one or more fungi that produce toxic compounds known as mycotoxins. 27 Animals or humans consuming such products may develop severe diseases of internal organs, the nervous system, and the circulatory system and may die. 4. Cause financial losses In addition to direct losses in yield and quality, financial losses from plant diseases can arise in many ways. ⁃ Farmers may have to plant varieties or species of plants that are resistant to disease but are less productive, more costly, or commercially less profitable than other varieties. ⁃ They may have to spray or otherwise control a disease, thus incurring expenses for chemicals, machinery, storage space, and labor. ⁃ Shippers may have to provide refrigerated warehouses and transportation vehicles, thereby increasing expenses. ⁃ Plant diseases may limit the time during which products can be kept fresh and healthy, thus forcing growers to sell during a short period of time when products are abundant and prices are low. ⁃ Healthy and diseased plant products may need to be separated from one another to avoid spreading of the disease, thus increasing handling costs. ⁃ The cost of controlling plant diseases, as well as lost productivity, is a loss attributable to diseases. ⁃ Some plant diseases can be controlled almost entirely by one or another method, thus resulting in financial losses only to the amount of the cost of the control. 28 ⁃ Sometimes, the cost of control may be almost as high as, or even higher than, the return expected from the crop, as in the case of certain diseases of small grains. ⁃ For other diseases, no effective control measures are yet known, and only a combination of cultural practices and the use of somewhat resistant varieties makes it possible to raise a crop. ⁃ For most plant diseases, however, as long as we still have chemical pesticides, practical controls are available, although some losses may be incurred, despite the control measures taken. In these cases, the benefits from the control applied are generally much greater than the combined direct losses from the disease and the indirect losses due to expenses for control. Despite the variety of types and sizes of financial losses that may be caused by plant diseases, well informed farmers who use the best combinations of available resistant varieties and proper cultural, biological, and chemical control practices not only manage to produce a good crop in years of severe disease outbreaks, but may also obtain much greater economic benefits from increased prices after other farmers suffer severe crop losses. 29