Introduction to Plant Pathology PDF

Introduction to Plant Pathology  What is a plant disease? A plant disease is any abnormal condition that alters the appearance or function of a plant. It is a physiological process that affects some or all plant functions. Disease may also reduce yield and quality of harvested pro...

Introduction to Plant Pathology  What is a plant disease? A plant disease is any abnormal condition that alters the appearance or function of a plant. It is a physiological process that affects some or all plant functions. Disease may also reduce yield and quality of harvested product. Disease is a process or a change that occurs over time. It does not occur instantly like injury.  What is a plant disease? Visible effects of disease on plants are called symptoms. Any detectable changes in color, shape, and/or functions of the plant in response to a pathogen or disease-causing agent is a symptom. Signs of plant disease are physical evidence of the pathogen, for example, fungal fruiting bodies, bacterial ooze, or nematode cysts. Signs also can help with plant disease identification.  What causes plant disease? Infectious plant diseases are caused by living organisms that attack and obtain their nutrition from the plant they infect. The parasitic organism that causes a disease is a pathogen. Numerous fungi, bacteria, viruses, and nematodes are pathogens of corn and soybean in Iowa. The plant invaded by the pathogen and serving as its food source is referred to as a host.  Types of pathogens Fungi Bacteria Viruses Nematodes  Role of the environment A favorable environment is critically important for disease development – even the most susceptible plants exposed to huge amounts of a pathogen will not develop disease unless environmental conditions are favorable. The Disease Triangle Host The Disease Triangle Soybean rust  Groups of plant pathogens - fungi Vast majority are beneficial Can cause plant, human, and livestock diseases Most cannot be seen without a microscope Lack chlorophyll Composed of growing structure of delicate, threadlike filaments called hyphae Reproduce by forming spores  Groups of plant pathogens - bacteria Extremely small organism requiring microscope to be seen Bacteria population can increase in number in short time period Cells clump together in masses called colonies Obtain food from dead or decaying organic matter or living www.foodmate.net tissue Spread plant to plant by wind-driven rain Gain entrance through natural plant openings or injuries  Groups of plant pathogens - viruses Most familiar because they cause human and animal diseases such as influenza, polio, rabies, smallpox, and warts Cause some destructive plant University of Florida diseases Measure only about one- millionth of an inch in size Are not complete living systems Survive only in living cells Transmitted by insects which are called vectors Groups of plant pathogens - nematodes Round, slender, threadlike worms Some are parasites on animals, insects, fungi, other nematodes, and plants Plant-parasitic nematodes have a stylet Most live in the soil and feed in or on plant roots  Disease cycle Survival Inoculum produced Production of survival Dispersal structures Symptoms Colonization Infection Adapted from P. Vincelli, 2005  Comparison of disease cycles Fungi Bacteria Viruses Nematodes Survival Crop residue Crop residue - Crop residue Soil Soil - Soil Alt. hosts Alt. hosts Alt. hosts - - Insect vectors Insect vectors - Dispersal Wind Wind - Tillage Rain Rain - Equipment Insects Insects Insects Water run-off Infection Directly - - Directly Wounds Wounds - - Insect feeding Insect feeding Insect feeding -  Inoculum Source of inoculum varies for each disease May be produced on residues left in the field Present in the soil Present in weeds or other crops in the area Present in or on the seed Present in soil sticking to equipment or tools Carried by wind or water Carried by insect vectors Carried in by animals, birds, and people  Spread of inoculum Two ways 1. Plant placed in soil that contains a pathogen 2. Inoculum moves from its source to host plant Keith Weller, U.S. Department of Agriculture  Mode of pathogenesis There are many ways in which plant disease pathogens can affect plants 1. By utilizing host cell contents 2. By killing host or by interfering with its metabolic processes through their enzymes, toxins etc. 3. By weakening the host due to continuous loss of the nutrients. 4. By interfering with the translocation of the food, minerals land water. Routes of entry of pathogens are as follows First contact of pathogen with host –on plant surface Aerial plant part surfaces consist primarily of cuticle and/or cellulose Root cell wall surfaces that consist only of cellulose. Cuticle consists primarily of cutin Lower layer consisting predominantly of pectic substances Polysaccharides of various types are often found in cell walls. epidermal cell walls may also contain suberin and lignin. Pathogen attack the plant by degrading these substances by several enzymes  Mode of pathogenesis Mode of pathogenesis of organisms are Enzymes Toxins Growth regulators polysaccharides Enzymes Enzyme Degrading Pectin Eg. Brown rot (Monilinia fructicola) in plum– Bacterium Erwinia sp.– Pseudomonas. Enzyme Degrading Cellulose Eg.Corn stalk rot fungus-Fusarium Enzyme Degrading Lignin Ceratocystis paradoxa (stem bleeding in coconut)  Toxins Fungal toxins: Mycotoxin – Aflotoxin, fumonisin Mushroom toxin- Amanitia muscaria, A. phallaoid Bacterial toxin Syringomycin- Pseudomonas syringae pv. Phaseolotoxin- Pseudomonas phaseolicola Fungal Diseases  RUST OF WHEAT Wheat leaf rust is a common fungal disease in wheat, rye and barley. This disease affects, the stems, leaves and grains of these plants. The pathogen that causes the wheat leaf rust is the Puccinia rust fungus.Leaf rust, caused by Puccinia triticina, is the most common rust disease of wheat. Stem rust (also known as black stem rust) is caused by Puccinia graminis.It is primarily a disease on wheat, though it can also cause minor infections on certain cultivars of barley and rye. Mode of Transmission Leaf rust spores are spread by wind and splashing water. Optimal environmental conditions for development of infection are temperatures ranging from 59 to 68 degrees F and at least six hours of moisture on the leaf surface.  RUST OF WHEAT Continued… Symptoms dusty, reddish-orange to reddish-brown fruiting bodies that appear on the leaf surface Treatment Select rust-resistant plant varieties when available. Pick off and destroy infected leaves and frequently rake under plants to remove all fallen debris. Water in the early morning hours — avoiding overhead sprinklers — to give plants time to dry out during the day.  Late Blight of Potato Late blight is caused by the fungus pathogen Phytophthora infestans. This potentially devastating disease can infect potato foliage and tubers at any stage of crop development. Symptoms The first symptoms of late blight in the field are small, light to dark green, circular to irregular-shaped water-soaked spots. These lesions usually appear first on the lower leaves. Lesions often begin to develop near the leaf tips or edges, where dew is retained the longest. Late blight, also called potato blight, disease of potato and tomato plants that is caused by the water mold Phytophthora infestans. The disease occurs in humid regions with temperatures ranging between 4 and 29 °C (40 and 80 °F) Treatment and Control Use potato tubers for seed from disease- free areas to ensure that the pathogen is not carried through seed tuber. The infected plant material in the field should be properly destroyed. Grow resistant varieties like Kufri Navtal. Fungicidal sprays on the appearance of initial symptoms  Red rot of sugarcane Red rot is a very serious disease of sugarcane. The symptom of the disease is the reddening of the internal internodal tissues with crossbars of white patches in the reddened area. This red colour is caused by a dye which is secreted by the host and is antagonistic to the red rot fungus. Symptoms Third leaf from top start - yellowing Later infected cane – shriveled Rind shrink Cane light in weight & easily broken Reddening of pith with clear area (white) running across the width of the cane Causative organism :- Colletrotrichum falcatum Perfect stage:- Glomerella tucumanensis  Red rot of sugarcane The fungi causing the disease occur in two stages. Asexual stage, which is called anamorph or imperfect stage. Perfect stage :-which is called teleomorph or the sexual stage. Treatment and control Use Always Disease-free seed treatment – Carbendazim @ 2.5 gm/lit. of water for 30 minutes Hot water treatment – 52° C for 8 hrs or 54° C for 2 hrs Hot air treatment - 54° C for 6 hrs Removal of infected stools Crop rotation 2-3 yrs Use disease Resistant. Varieties.- Cos -767  BACTERIAL DISEASES OF PLANT Plant pathogenic bacteria cause many different kinds of symptoms that include galls and overgrowths, wilts, leaf spots, specks and blights, soft rots, as well as scabs and cankers. In contrast to viruses, which are inside host cells, walled bacteria grow in the spaces between cells and do not invade them.  CITRUS CANKER Citrus canker it is a disease affecting Citrus species caused by the bacterium Xanthomonas sps. Infection causes lesions on the leaves, stems, and fruit of citrus trees, including lime, oranges, and grapefruit. Causative organism -Citrus canker is caused by Xanthomonas citri. It is a gram negative, monotrichous, capsulated bacillus, non-endospore forming which is strictly aerobic in nature. Symptoms Citrus canker causes premature leaf and fruit drop, twig dieback, general decline, and blemished fruit Blister-like lesions on leaves and fruit start small and expand as the disease progresses. These lesions may darken to tan or black and develop a water-soaked margin with a yellow halo surrounding it.  CITRUS CANKER The center of the lesion on leaves as well as on stems and twigs can appear raised and corky or scabby surrounded by a water-soaked margin. Mature lesions on older symptomatic leaves may have a shot-hole look and these lesions eventually die and fall out. Treatment and control 1. Complete destruction of diseased plant by burning them. 2. Use of disease free nursery stock for planting. 3. Spraying the plant before planting with 1% Bordeaux mixture. 4. Antibiotic spray with streptomycin, phytomycin. 5. Spraying leaves with neem-cake (160 lbs/acre). 6. Disease resistant varieties.  BLIGHT OF RICE Rice bacterial blight, also called bacterial blight of rice. It is a deadly bacterial disease that is among the most destructive afflictions of cultivated rice (Oryza sativa and O. glaberrima). Major rice cultivating countries are: China, India, Indonesia, Bangladesh,Vietnam, Thailand,Myanmar and Philippines. Xanthomonas oryzae pv. Oryzae is the Causative organism and it causes wilting of seedlings, yellowing and drying of leaves. Transmission The disease usually occurs during the early stage of planting from maximum tillering to panicle initiation Older plants are more resistant to the disease. The disease is transmitted through seeds Infection occur through wounds and stomata Infected seeds present in the water or those surviving in the debris left after  BLIGHT OF RICE harvest, are also sources of inoculums in the next planting season. The disease also disperse and spread by wind and rain water. High dose of nitrogenous fertilizer, excessive shade and close planting favour the disease Symptoms On seedling, the infection appears tiny water soaked spot at the margin on the leaves. On leaf blade, the infection begins at the margin as water socked stripe. Resulting in the wavy margin and yellow leaf within few days. The lesion can cover the whole blade, turn white and later grayish contaminated with various saprophytic fungi. A turbid ooze of the bacterium, streaming from the vascular bundle can be observed on dipping the cut end of affected leaves in clear water. Symptoms observed at the tillering stage. Seedling wilt can be observed in 1-3 weeks after transplanting.  BLIGHT OF RICE Treatment and control Planting resistant varieties is efficient way to control bacterial blight. Use balanced amounts of plant nutrients, especially nitrogen. Ensure good drainage of fields and nurseries. Keep fields clean. Remove weed hosts and plow under rice stubble , straw which can serve as hosts of bacteria. Allow fallow fields to dry to suppress disease agents in the soil and plant residues. Seed treatment with bleaching powder (100g/ml) and zinc sulphate (2 per cent) reduce bacterial blight. Copper compounds and antibiotics. Application of streptocycline @.6% for spray  VIRAL DISEASES OF PLANTS Plant viruses consist of a nucleoprotein that multiplies only in the living cells of a host. viruses are generally specific, what infects a plant does not cause disease in an animal, and vice versa. The first record of a disease that was later found to be caused by a plant virus was on tulips in the 17th century in the Netherlands. First experimental demonstration of the infectious nature of viral disease was recorded by Lawrence, who described the transmission of a disease of jasmine by grafting. Properties of plant viruses Noncellular, ultramicroscopic particles, that multiply only in living cells. very, very small! (size measured in nanometers). Most plant viruses consist of protein shells surrounded by a core of positive- stranded nucleic acid (normally ssRNA - nucleotides , but sometimes these viruses contain dsRNA or dsDNA.  VIRAL DISEASES OF PLANTS 5-40% of virus is nucleic acid 60-95% is protein. Protein coats or shells can be different shapes, but are normally rod, filamentous, isometric, quasi- isometric/bacilliform or variants of these structures. Transmission Mechanical transmission through sap by plants touching one another Through root grafts, and manhandling. Vegetative propagation and grafting. Seed, pollen, mites, nematodes, dodder, Fungi (carried by zoospores and mycelium) Insects (aphids, leafhoppers, scale insects, thrips, grasshoppers, beetles, whiteflies).  LEAF CURL OF PAPAYA Papaya leaf curl disease is caused by Papaya leaf curl virus (PaLCuV), a begomovirus naturally transmitted through whitefly (Bemisia tabaci). Papaya leaf curl was first reported in Tamil Nadu in 1939. Causative organism- Papaya leaf curl virus (PaLCuV), is a DNA virus from the genus Begomovirus and the family Geminiviridae. PaLCuV causes severe disease in papaya (Carica papaya), but can sometimes infect other crops such as tobacco or tomato. Symptoms The most prominent symptoms are the rolling of the leaves downward and inward in the form of an inverted cup and the thickening of veins.  LEAF CURL OF PAPAYA Sometimes all the leaves at the top of the plant are affected by these symptoms. In advanced stages of the disease, defoliation takes place and the growth of the plant is arrested. The production of flower and fruits are compromised. Fruits will be detorated in shape and fall immaturely. Transmission Vector: Silver leaf whitefly (Bemisia tabaci). Treatment and control Immediately after the disease is noticed, the plant must be uprooted and destroyed. Avoid growing alternative hosts such as tomato and tobacco near papaya. Spray insecticides for vector control.  VEIN CLEARING OF LADY’S FINGER This is a viral disease occurring on bhendi (Okra/Lady's Finger). Yellowing of the entire network of veins in the leaf blade is the characteristic symptom. The veins of the leaves will be cleared by the virus and interveinal area becomes completely yellow or white. The veins become considerably thickened. Causative organism This disease is caused by a complex Begomovirus Bhendi yellow vein mosaic virus (BYVMV, family: Geminiviridae) and a small satellite DNA beta component. BYVMV can systemically infect bhendi upon agroinoculation but produces only mild leaf curling in this host. Bhendi yellow vein mosaic was first reported in okra plants in 1924 in India and Sri Lanka.  VEIN CLEARING OF LADY’S FINGER Symptom Vein clearing and vein chlorosis of leaves. The complex network of interwoven yellow, thickened veins and veinlets encloses islands of green tissue. In severe cases, the chlorosis may extend to the interveinal area and leaves may turn completely yellow. Fruits are dwarfed, malformed and yellow-green. Transmission The virus is transmitted by the whitely Bemisia tabaci Treatment and control Use resistant cultivar Sow disease free certified seeds Follow crop rotation Keep the field free from weeds Control vector with suitable insecticide  Potato Spindle Tuber Disease Potato Spindle Tuber Viroid (PSTVd) is an EU listed quarantine pathogen of potatoes which causes stunting of the plant and malformation and cracking of tubers. The main host is potatoes but the disease also affects tomatoes and solanaceous ornamentals. Symptoms Mild strains generally cause no obvious symptoms in potatoes. However severe strains in sensitive cultivars may cause foliage to be spindly, very upright, with overlapping leaflets and sometimes with upward rolling of terminal leaflets. Plants will be stunted. Tubers may show the following deformities; small, elongated, cylindrical, spindle or dumb-bell-shaped, with prominent eyes evenly distributed over the tuber, and cracking. Sprouting is slower than in healthy tubers  Potato Spindle Tuber Disease Transmission The disease is mechanically transmitted by contact between healthy and diseased plants, tractor wheels, tools, etc. Within potato plants, it is found most readily in the upper leaves and tubers. Transmission in true seed of potato depends upon the cultivar. PSTVd can pass through both the pollen and ovule. Long Range dispersal mainly occurs through the movement of infected tubers and true seed. PSTVd can also be spread by aphids. But this only occurs in the presence of Potato leaf roll virus (PLRV). Control There are no chemical or biological controls available to control PSTVd within infected plants.  Potato Spindle Tuber Disease Therefore control is essentially through the use of healthy virus-free seeds or planting material, and good crop sanitation practices. If infection does occur, destruction of infected plants and strict hygiene measures to prevent infection of subsequent crops are the only courses of action.  Nematodes as Pathogen A number of genera and species of nematodes are highly damaging to a great range of hosts, including foliage plants, agronomic and vegetable crops, fruit and nut trees, turfgrass, and forest trees. Some of the most damaging nematodes are: Root knot (Meloidogyne spp.); Cyst (Heterodera and Globodera spp.); Root lesion (Pratylenchus spp.); Spiral (Helicotylenchus spp.); Burrowing (Radopholus similis); Bulb and stem (Ditylenchus dipsaci); Reniform (Rotylenchulus reniformis); Dagger (Xiphinema spp.); and Bud and leaf (Aphelenchoides spp.). Nematodes as Pathogen Symptoms of Fungal Disease Symptoms of Fungal Disease Symptoms of Fungal Disease Major Biological Control Strategies for Plant Pathogens Biological Control of Plant Pathogens Kamal Krishna Pal*, Visiting Scholar, Department of Plant Pathology, Ohio State University, OARDC, Wooster, OH Brian McSpadden Gardener, Department of Plant Pathology, Ohio State University, OARDC, Wooster, OH *Permanent address: National Research Centre for Groundnut, Ivnagar Road, PB No. 5, Juangadh-362 001, Gujarat, India Penetration of inoculum and infection Infection occurs when a pathogen successfully enters a plant and grows, reproduces, and spreads within the plant Pathogens enter a host through natural openings, wounds on plant surfaces, or by penetrating directly into the plant Spore Mycelial Pustule Penetration Sporulation germination growth formation Syngenta  Secondary cycles Some diseases have only one cycle during the growing season (often root rots) Some diseases develop secondary or repeating cycles during the growing season (often foliar diseases) Number of cycles depends on the pathogen, susceptibility of the host, and environmental conditions  Pathogen survival Pathogens survive season to season in: Soil Crop residue Weed or noncrop hosts Seed or vegetative plant parts Insects Mild climates  Summary Understanding the difference between a sign and a symptom is key in identifying a plant disease A plant disease cannot develop if a susceptible host, pathogen, and favorable environment do not occur simultaneously The major plant pathogens responsible for disease development in plants are fungi, bacteria, viruses, and nematodes The disease cycle describes the interaction of the pathogen with the host