Plant Pathology

2+1 Credits

Azarbaijan Shahid Madani University-

M. PAZHOUHANDEH Fall 1391 (Sep 2012)

Course is available on : http://agri.azaruniv.ac.ir/?PageID=7

Plant Protectionحفظ نباتات یا گیاىپزشکی

افشایش جوعیت، هحدّد بْدى ططح سیز کشت :مشکل

: افشایش هیشاى تْلید در ّاحد ططح :راه حل

بَ سراعی، بَ ًژادی، جلْگیزی اس ضایع شدى هحصْالت کشاّرسی چگونو؟

...کاشت، داشت، بزداشت، سهاى کاشت، سهیي کاشت، آبیاری، : بَ سراعی

اطتفادٍ اس ارقام پزهحصْل، هقاّم بَ آفات ّ بیواریِا ّ شزایط سیظتی ًاهٌاطب: بَ ًژادی

گیاىپزشکی: جلْگیزی اس ضایع شدى هحصْالت کشاّرسی

اس ٌُگام گذاشتي داًَ در خاک تا تبدیل بَ گیاٍ کاهل ّ تْلید هحصْل ّ حتی پض اس بزداشت در حیي حول ّ ًقل ّ

اًبارداری

.طاالًَ یک طْم هحصْالت کشاّرسی در اثز خظارت آفات ّ بیواریِا اس بیي هیزّد

.ُدف علن گیاُپششکی آشٌایی با آفات ّ بیواریِا ّ در ًِایت کٌتزل آًِاطت

: ّاحدی 3تقظین بٌدی ایي درص

هقدهَ ای بز بیواری شٌاطی، تعاریف، بیواریِای اًگلی ّ بیواریِای غیز اًگلی

ّیزّطِا، باکتزیِا، ًواتدُا، قارچِا ّ گیاُاى اًگل: چزخَ بیواري، پاتْژًِا

Plant Pathologyبیماری شناسی گیاىي

What is Plant Pathology or Phytopathology?

Plant Pathology or Phytopathology is the study of plant diseases.

plants have diseases and get sick just like people and animals

Controlling plant diseases and keeping plants healthy is the main objective of a plant pathologist,

Controlling plant disease requires an understanding of the agents that cause disease as well as an understanding of how plants are affected by disease.

Plant diseases cause an estimated loss of 8 billion dollars per year in USA.

All of our food, much of our clothing, building materials, and much of the beauty in nature is provided by plants. Plant diseases are a constant threat to the world’s food and fiber crops, forests, and landscape plants.

Throughout history, plant diseases have been responsible for the death and suffering of millions of people and countless animals.

History:Leeuwenhoek first saw microbes with the microscope he invented in 1674

in 1861–1863, Anton deBary proved that potato late blight was caused by a fungus

Louis Pasteur In 1864 proved that microorganisms were produced from preexisting microorganisms and that most infectious diseases were caused by germs. “germ theory of disease,” Pasteur invented pasteurization in 1880

Significant impetus to this progress was added by Robert Petri, who developed artificial nutrient media for culturing the microorganisms (Petri dishes),

and by Robert Koch who established that for proving that a certain microorganism was the cause of a particular infectious disease, certain necessary steps (Koch’s postulates) must be carried out and certain conditions must be satisfied.

Ref:

PLANT PATHOLOGY

GEORGE N. AGRIOS

American Phytopathological Society (APS)

“Koch’s postulates.”1. The suspected causal agent (bacterium or other microorganism) must be present in every diseased organism (e.g., a plant) examined.2. The suspected causal agent (bacterium, etc.) must be isolated from the diseased host organism (plant) and grown in pure culture.3. When a pure culture of the suspected causal agent is inoculated into a healthy susceptible host (plant), the host must reproduce the specific disease.4. The same causal agent must be recovered again from the experimentally inoculated and infected host, i.e., the recovered agent must have the same characteristics as the organism in step 2.

Symptoms

The visible changes caused by agent on plant are the symptomsof the disease. Symptoms may change continuously from the moment of their appearance until the entire plant dies or they may develop up to a point and then remain more or less unchanged for the rest of the growing season.The amount of damage caused to plants is often much greater than would be expected from the removal of nutrients by the parasite. This additional damage results from substances secreted by the parasite or produced by the host in response to stimuli originating in the parasite.Tissues affected by such substances may show increased respiration, disintegration or collapse of cells, wilting, abscission, abnormal cell division and enlargement, and degeneration of specific components such as chlorophyll.

Symptom of diseases

infection of roots may cause roots to rot and make them unable to absorb water and nutrients from the soil; infection of xylem vessels, as happens in vascular wilts and in some cankers, interferes with the translocation of water and minerals to the crown of the plant; infection of the foliage, as happens in leaf spots, blights, rusts, mildews, mosaics, and so on, interferes with photosynthesis; infection of phloem cells in the veins of leaves and in the bark of stems and shoots, as happens in cankers and in diseases caused by viruses, mollicutes, and protozoa, interferes with the downward translocation of photosynthetic products; and infection of flowers and fruits 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 a great deal more (hypertrophy) than normal cells and to produce abnormal amorphous overgrowths (tumors) or abnormal organs.

A few examples of devastating diseases are:1. Potato Late Blight Phytophthora infestans2. Ergot of Rye Claviceps purpurea3. Black Stem Rust of Wheat Puccinia graminis4. Southern Corn Leaf Blight5. Dutch Elm Disease6. Bacterial Canker on Citrus7. Plum Pox Virus on Stone Fruit

All of these are infectious diseases caused by plant pathogens.

I. Infectious, or biotic, plant diseases1. Diseases caused by fungi2. Diseases caused by prokaryotes (bacteria and mollicutes)3. Diseases caused by parasitic higher plants and green algae4. Diseases caused by viruses and viroids 5. Diseases caused by nematodes 6. Diseases caused by protozoa

II. Noninfectious, or abiotic, plant diseases1. Diseases caused by too low or too high a temperature2. Diseases caused by lack or excess of soil moisture3. Diseases caused by lack or excess of light4. Diseases caused by lack of oxygen5. Diseases caused by air pollution6. Diseases caused by nutrient deficiencies7. Diseases caused by mineral toxicities8. Diseases caused by soil acidity or alkalinity (pH)9. Diseases caused by toxicity of pesticides10. Diseases caused by improper cultural practices

Simpler Virus

Bacterium

Protozoa

Fungus

Plant/Animal

DNA

Protein

DNA

nucleus

AntibioticsDiseases

Size of pathogens

Late Blight of PotatoCaused by the fungus Phytophthora infestans

• The fungus killed most of the potatoes grown in Ireland during the mid 1800’s.

• Potatoes were the main source of food for the Irish people.

• This plant disease resulted in the Irish Potato Famine of 1845

1 million people died1.5 million people left Ireland

The puzzle of what caused blight of potato continued unanswered for 16 years after the 1845 destruction of potatoes by the blight. Finally, in 1861, Anton deBary did a simple experiment that proved that the potato blight was caused by a fungus. DeBary simply planted two sets of healthy potatoes, one of which he dusted with spores of the fungus collected from blighted potato plants. When the tubers germinated and began to produce potato plants, the healthy tubers produced healthy plants, whereas the healthy tubers dusted with the spores of the fungus produced plants that became blighted and died. No matter how many times deBary repeated the experiment, only tubers treated with the fungus became infected and produced plants that became infected. Therefore, the fungus, which, we know now, is an oomycete was named Phytophthora infestans (“infectious plant destroyer” from phyto = plant, phthora = destruction, infestans = infectious), was the cause of the potato blight.

Late Blight on Potato Tubers

Ergot of RyeCaused by the fungus Claviceps purpurea

The pathogen produces a structure called a sclerotium that grows in place of the rye kernel. This sclerotium contains poisons that are very harmful to man and animals.

The sclerotium is harvested with the rye grain. The grain is ground into flour, made into bread and is eaten by people. Eating the contaminated bread results in a disease called Ergotism.

ERGOTISM OFTEN RESULTS IN DEATH.

Ergot of Rye(note dark sclerotia)

Ergot of Rye

The sclerotia also contains hallucinogenic

compounds. People that eat ergot often see Fire.

In the middle ages, people called the vision of fire induced by Ergot

Holy Fire or St. Anthony’s Fire

Ergotism can also cause abortions and gangrene.

THIS PLANT DISEASE IS STILL KILLING PEOPLE IN THE WORLD TODAY

Black Stem Rust of Wheat1916 - Destroyed 300 million

bushels of wheat in the United States and Canada

1935 - Destroyed 135 million

bushels in Dakotas and Minnesota

SPORATIC EPIDEMICS STILL COST NORTH

AMERICAN FARMERS BILLIONS OF DOLLARS

Black Stem Rust of Wheat

Southern Corn Leaf Blight

SusceptiblePlants

ResistantPlants

Plum Pox Virus

Chapter2: PARASITISM and DISEASE DEVELOPMENT

Infectious diseases are those that result from infection of a plant by a pathogen. In such diseases, the pathogen can grow and multiply rapidly on diseased plants, it can spread from diseased to healthy plants, thereby leading to the development of a small or large epidemic.Parasite: An organism that lives on or in some other organism and obtains its food from the latter is called a parasite. The removal of food by a parasite from its host is called parasitism. A plant parasite is an organism that becomes intimately associated with a plant and multiplies or grows at the expense of the plant. The removal by the parasite of nutrients and water from the host plant usually reduces efficiency in the normal growth of the plant and becomes detrimental to the further development and reproduction of the plant. In many cases, parasitism is intimately associated with Pathogenicity: the ability of a pathogen to cause disease, as the ability of the parasite to invade and become established in the host generally results in the development of a diseased condition in the host. In some cases of parasitism, as with the root nodule bacteria of legume plants and the mycorrhizal infection of feeder roots of most flowering plants, both the plant and the microorganism benefit from the association. This phenomenon is known as symbiosis.Pathogenicity: is the ability of the parasite to interfere with one or more of the essential functions of the plant, thereby causing disease.

Some parasites, including viruses, viroids, mollicutes, some fastidious bacteria, nematodes, protozoa, and fungi causing downy mildews, powdery mildews, and rusts, are biotrophs, i.e., they can grow and reproduce in nature only in living hosts, and they are called obligate parasites. Other parasites (most fungi and bacteria) can live on either living or dead hosts and on various nutrient media, and they are therefore called nonobligate parasites. Some nonobligate parasites live most of the time or most of their life cycles as parasites, but, under certain conditions, may grow saprophytically on dead organic matter; such parasites are semibiotrophs and are called facultative saprophytes.

Others live most of the time and thrive well on dead organic matter (necrotrophs) but, under certain circumstances, may attack living plants and become parasitic; these parasites are called facultative parasites. Usually no correlation exists between the degree of parasitism of a pathogen and the severity of disease it can cause, as many diseases caused by weakly parasitic pathogens are much more damaging to a plant than others caused even by obligate parasites. Moreover, certain pathogens, e.g., slime molds and those causing sooty molds, can cause disease by just covering the surface of the plant without parasitizing the plant.Obligate and nonobligate parasites generally differ in the ways in which they attack their host plants and procure their nutrients from the host. Many nonobligate parasites secrete enzymes that bring about the disintegration of the cell components of plants, and these alone or with the toxins secreted by the pathogen result in the death and degradation of the cells.

HOST RANGE OF PATHOGENSPathogens differ with respect to the kinds of plants that they can attack, with respect to the organs and tissues that they can infect, and with respect to the age of the organ or tissue of the plant on which they can grow. Some pathogens are restricted to a single species, othersto one genus of plants, and still others have a wide range of hosts, belonging to many families of higher plants. Some pathogens grow especially on roots, others on stems, and some mainly on the leaves or on fleshy fruits or vegetables. Some pathogens, e.g., vascular parasites, attack specifically certain kinds of tissues, such as phloem or xylem. Others may produce different effects on different parts of the same plant. With regard to the age of plants, some pathogens attack seedlings or the young tender parts of plants, whereas others attack only mature tissues. Many obligate parasites are quite specific as to the kind of host they attack, possibly because they have evolved in parallel with their host and require certain nutrients that are produced or become available to the pathogen only in these hosts.

The disease Triangle

at least two components (plant and pathogen) must come in contact and must interact. If at the time of contact of a pathogen with a plant, and for some time afterward, conditions are too cold, too hot, too dry, or some other extreme, the pathogen may be unable to attack or the plant may be able to resist the attack, and therefore, despite the two being in contact, no disease develops. Apparently then, a third component, namely a set of environmental conditions within a favorable range, must also occur for disease to develop. Each of the three components can display considerable variability; however, as one component changes it affects the degree of disease severity within an individual plant and within a plant population.

THE DISEASE CYCLEIn every infectious disease a series of more or less distinct events occurs in succession and leads to the development and perpetuation of the disease and the pathogen. This chain of events is called a disease cycle. A disease cycle sometimes corresponds fairly closely to the life cycle of the pathogen, but The events in a disease cycle are:Inoculation, penetration, establishment of infection, colonization (invasion), growth and reproduction of the pathogen, dissemination of the pathogen, and survival of the pathogen in the absence of the host, i.e., overwintering or oversummering overseasoning) of the pathogen

InoculationInoculation is the initial contact of a pathogen with a site of plant where infection is possible. The pathogen(s) that lands on or is otherwise brought into contact with the plant is called the inoculum. The inoculum is any part of the pathogen that can initiate infection. Thus, in fungi the inoculum may be spores,sclerotia (i.e., a compact mass of mycelium), or fragments of mycelium. In bacteria, mollicutes, protozoa, viruses, and viroids, the inoculum is always whole individualsIn nematodes, the inoculum may be adult nematodes, nematode juveniles, or eggs.In parasitic higher plants, the inoculum may be plant fragments or seeds. One unit of inoculum of any pathogen is called a propagule.Types of InoculumAn inoculum that survives dormant in the winter or summer and causes the original infections in the spring or in the autumn is called a primary inoculum, and the infections it causes are called primary infections. An inoculum produced from primary infections is called asecondary inoculum and it, in turn, causes secondary infections. Sources of Inoculum In some fungal and bacterial diseases of perennial plants, such as shrubs and trees, the inoculum is produced on the branches, trunks, or roots of the plants.The inoculum sometimes is present right in the plant debris or soil in the field where the crop is grown; other times it comes into the field with the seed, transplants,tubers, or other propagative organs or it may come fromsources outside the field. Outside sources of inoculummay be nearby plants or fields or fields many miles away. In many plant diseases, especially those of annual crops, the inoculum survives in perennial weeds or alternate hosts, and every season it is carried from them to the annual and other plants.

InoculationFungi, bacteria, parasitic higher plants, and nematodes either produce their inoculumon the surface of infected plants or their inoculum reaches the plant surface when the infected tissue breaks down. Viruses, viroids, mollicutes, fastidious bacteria, and protozoa produce their inoculum within the plants; therefore, it can be transmitted from one plant to another almost entirely by some kind of vector, such as an insect.

Landing or Arrival of InoculumThe inoculum of most pathogens is carried to hostplants passively by wind, water, and insects. Airborne and Soilborn inoculum

Prepenetration Phenomena: Attachment of Pathogen to HostPathogens such as mollicutes, fastidious bacteria, protozoa, and most viruses are placed directly into cells of plants by their vectors and, in most cases, they are probablyimmediately surrounded by cytoplasm, cytoplasmic membranes, and cell walls. However, almost all fungi, bacteria, and parasitic higher plants are first brought into contact with the external surface of plant organs. Before they can penetrate and colonize the host, they must first become attached to the host surface. Attachment takes place through the adhesionof spores, bacteria, and seeds through adhesive materials that vary significantly in composition and in the environmental factors they need to become adhesive. The propagules of these pathogens have on their surface or at their tips mucilaginous substances consisting of mixtures of water-insoluble polysaccharides, glycoproteins, lipids, and fibrillar materials, which, when moistened, become sticky and help the pathogen adhere to the plant. Spore Germination and Perception of the Host SurfaceIt is not clear what exactly triggers spore germination, but stimulation by the contact with the host surface, hydration and absorption of low molecular weight ionic material from the host surface, and availability of nutrients play a role. Spores also have mechanisms that prevent their germination until they sense such stimulations or when there are too many spores in their vicinity.Once the stimulation for germination has been received by the spore, the latter mobilizes its stored food reserves, such as lipids, polyoles, and carbohydrates, and directs them toward the rapid synthesis of cell membrane and cell wall toward the germ tube formation and extension. The germ tube is a specialized structure distinct from the fungal mycelium,often growing for a very short distance before it differentiates into an appressorium. The germ tube is also the structure and site that perceives the host surface and, if it does not receive the appropriate external stimuli, the germ tube remains undifferentiated and, when the nutrients are exhausted, it stops growing. The perception of signals from plant surfaces by pathogenic fungi seems to be the result of signaling pathways mediated by cyclic adenosine monophosphate (cAMP) and mitogen-activated protein kinase (MAPK).

The size of the turgor pressure inside an appressorium is 40 times greater than the pressure of a typical car tire. The turgor pressure of an appressorium is due to the enormous accumulation of glycerol in the appressorium, which, due to its high osmotic pressure, draws water into the cell and generates hydrostatic pressure that pushes the thin hypha (appressorial penetration peg) outward through the host cuticle.