EPWS 310 - Plant Pathology

EPWS 310 - Plant Pathology

Lectures - Fall 1997

Lecture 1

1. Call role

2. Introduce Karen Braun and Sue Fender

3. Go over syllabus outline of course

4. Textbook Agrios - corrections to textbook handout

I. Definition "Plant Pathology" the study of plant diseases.

A. The study of Plant Pathology includes several aspects:

1)the living entities and the environmental conditions that cause disease in plants

2)the mechanisms by which these factors produce disease in plants

3)the interactions between the diseasecausing agents and the diseased plant

4)the methods of preventing or controlling disease and alleviating the damage it causes.

B. Plant pathology involves many disciplines:

Biology, meteorology, plant physiology, soil science, microbiology (mycology, bacteriology, virology), nematology, horticulture, physics, plant anatomy, epidemiology, biochemistry, genetic engineering, computer science, mathematics and the list goes on.

C. Plant pathologists' investigate plant disorders caused by:

Fungi, bacteria, viruses, viroids, nematodes, mycoplasmas, parasitic higher plants, and protozoa. Plant pathologists' also investigate nutritional disorders of imbalances in plants that reduce yield and may increase susceptibility to pathogens.

D. What is a plant disease?

Any disruption (biotic or abiotic) of the plants normal metabolism. This is often expressed in dollar amounts.

Agrios"The malfunctioning of host cells and tissues that results from their coninuous irritation by a pathogenic agent or environmental factor and leads to the development of symptoms. Disease in a condition involving abnormal changes in the form, physiology, integrity, or behavior of the plant.

E. What is a pathogen?

An agent that causes, incites, or induces disease?

1. Incite to riot once triggered it takes off. Example Agrobacterium tumefaciens, incites crown gall.

2. Cause the host, pathogen, and environment all interact to result in a disease. A pathogen all by itself cannot cause disease.

3. Induce Probably the best. Pathogens induce disease.

F. How do pathogens cause disease?

1)Weakening the host by continually absorbing food from the host cells for their own use. Pythium, mistletoe,

2)Killing or disturbing the metablolism of host cells through toxins, enzymes, or growthregulating substances they secrete. Foolish disease of rice, Gibberella fujikuroi; Bacterial soft rots, Erwinia carotovora

3)blocking the transportation of food, mineral nutrients, and water through the conductive tissues. Verticillium wilt, Fusarium wilt.

4)consuming the contents of the host cells upon contact. nematodes.

LECTURE 2

G. Why study plant pathology?

Note: excess eating in US, yet 35,000 people die daily due to chronic starvation. In addition, even with surpluses, more efficient and less environmentally damaging production techniques are always needed.

1. Food, fiber, chemicals, and atmosphere is sustained and dependent on plants.

2. Plants and products are damaged which results in loss of life or lifestyle.

3. Plant pathogens are competing for our food source.

*8000 fungi in North America cause 80,000 different diseases.

*180 species of bacteria

*500 different viruses cause plant diseases

*150 different nematodes

4. Example Tomato is attacked by 80 fungi, 11 bacteria, 16 viruses, and a number of nematodes.

H. The beginning of Plant Pathology

In 1845, a tragedy struck Ireland. The days were warm and sunny that summer and the potato crop was growing well. THe weather turned overcast and rainy for weeks, the potato plants rotted as the Irish peasants watched. This event more than any other gave rise to the science of Plant Pathology. In the 19th century the peasant farmer became dependent on the potato. The potato originated in the highlands of South America. The Spanish conquistadors discovered this tuber and the first potato arrived in Spain around 1570. It was a well established food crop in Ireland by the 1800's. The potato provided high nutrition and caused a burst in population from 4.5 million in 1800 to 8 million in 1845. The grain crops grown by the peasants were used to pay the rent to landowners in England. Only a small number of potatoes were brought to Europe and so the genetic diversity wsa very limited. That is, the Irish potato was genetically uniform. What contributed to this epidemic? 1) A large population was dependent on a single crop. 2) Weather conditions turned cool and wet, which was favorable to the pathogen 3) the pathogen was present. In 1846 the weather was the same. One million people died of starvation and 1.5 million migrated because of the famine.

Dr. C. Montagne, a French physician in Napoleon's army, first described the fungus found on the potatoes. He shared his observations with Rev. M. J. Berkeley, who recognized that this new fungus was connected with the blight. His rival was Dr. John Lindley, a botany professor at University College in London, who did not believe that the fungus was the cause of the blight. Their arguments were published in The Gardener's Chronicles . These were intense. Read The Advance of the Fungi, by E. C. Large for more juicy details. Anton De Bary, the father of Plant Pathology, a German botanist, performed the experiments that proved the role of the fungus in the blight. The notion of causality established the science of plant pathology.

In 1860, Louis Pasteur disproved the dogma of Spontaneous Generation (Flies come from rotten meat (Redi, Italian), rats come from old rags and moldy cheese). This theory was replaced by the germ theory in which it was finally recognized that microbes were inducers of disease.

Read: THE MICROBE HUNTERS, The Advance of the Fungi,Famine on the Wind

LECTURE 3

II. Current crop losses due to plant diseases

A. Estimated production of world's major food crops.

1. Note that the crops that are high on the list have many diseases that have been investigated.

2. Estimated 1982 world crop production and preharvest losses (in millions of tons and percent of world production lost to diseases, insects and weeds.

3. Note that of the 13 crops listed diseases are the major problem on 8.

4. Estimated 1982 crop production and preharvest losses (in Millions of tons) and percent lost to diseases and other pests(insects, weeds) in developed and developing countries.

5. Note that in every crop, developing countries have a higher loss to disease, insects and weeds.

6. Percentage of all produce lost to deseases, insects and weeds by continent or region. Note percent loss in developing countries.

7. Postharvest losses in developing countries are estimated at between 1025%. In developing countries between

F. The total loss to pests in the U. S. is estimated at 40% and for the entire world to about 48% of all food crops.

III. History of Plant Pathology

A. Genetic uniformity

1. History Anthropologists tell us that humans began as foragers (nut and fruit gatherers) and nomadic hunters. In time, humans learned how to grow crops and development of societies began. As humankind has developed agriculture, the need for breeders, agriculturalists, plant scientist, and plant pathologist came around. We have moved from a society where everyone was at one time involved in growing crops or domestic animals, to a society in which only 23% of the United States population is involved in Agriculture. With our intense agricultural system, we have been able to produce enough food to feed the world. However, we have also left ourselves open to severe epidemics because of our limited gene source. That is, many of our major crops are genetically uniform within their particular species. (Page 143 Agrios).

Example: Dry beans are grown on 1.4 million acres. Total varieties are 25. 2 major varieties are grown on 60% of the acreage. (overhead)

a. Pesticide use. Note that the use of pesticides to control plant diseases and other pests has been increasing steadily at an annual rate of about 14% since the mid1950's. So far, about 35% of all pesticides are applied in the U.S. and Canada, 45% in Europe, and the remaining 20% in the rest of the world.

b. Agricultural land lost. In the United States, two million acres of land each year are converted from agricultural to nonagricultural uses, including 420,000 acres for urban development, an equal amount for reservoirs and flood control, and nearly one million acres for parks, wilderness, and wildlife areas. The amount of cropland is decreasing at an annual rate of 3 %. How long can it continue? With this decrease in farmland the use of pesticides will become greater, and the tendency to rotate crops or use long term cultural practices will decrease.

2. Genetic makeup of crops in the past.

They were not the seed of careful genetic crosses and many generations of inbreeding and backcrossing to obtain maximum yields but

the a result of many years of survivor selection by farmers. These were seeds that would grow into plants that could produce some yield despite competition from weeds, insects, pathogens, and poor soil fertility.

General resistance to a multitude of pests in plants selected under such circumstances ensured at least some harvest, even if the maximum was not possible. Heterozygous populations.

characteristics, Genetic diversity, many kinds of crops interspersed, slash and burn (nomadic ag.), Incas of Peru developed crop rotation laws (Ex. potatoes planted in the same soil every seven years).

3. Examples of epidemics as a result of genetic uniformity

a. Late blight of potato in Northern Europe (18456). Talk about it later.

b. Southern corn leaf blight(1970)the result of the widespread use of corn hybrids containing the Texas malesterile cytoplasm. Caused over $1 billion loss in the corn crop. The pathogen is Bipolaris maydis. The pathogen had been observed for several years, but it was never a serious problem. In 1970, nearly 80% of all hybrid field corn produced in the U. S. contained TMS cytoplasm. A genetic change or a population shift occurred in the B. maydis population. A new race of B. maydis was found to be particularly virulent on corn with TMS cytoplasm. The new race was called Race T to differentiate it from Race O that caused a minor leaf spot disease. Race T was much more aggressive and could reproduce a new generation of infective spores within 51 hrs. In many southern states, entire fields were destroyed and 80100% were common.

Components of an epidemic

1. Susceptible host change in susceptibility

2. Genetic change in the pathogen population.

3. Environmental factorsB. maydis likes hot, humid conditions.

4. Time

Go over the disease triangle and disease pyramid.

b. Helminthosporium blight of Victoria oats Many varieties were replaced by the rustresistant Victoria oat variety. Unfortunately, this variety was very susceptible to Helmithosporiumvictoriae, previously a minor pathogen.

c. Coffee rust in Ceylon (COFFEE or TEA) The whole island was planted to coffee for England to drink. Coffee (Coffea arabica) was grown in the DutchEnglish colonies of Ceylon(Sri lanka), Java, and Sumatra. The English loved their coffee. A rust fungus called Hemileia vastatrix, reached Ceylon in 1875. nearly 400,000 acres (160,000 hectares) were covered with coffee trees. The trees were very susceptible to the fungus.

The fungus can liberate about 150,000 spores per leaf pustule. One leaf can have 100's of pustules. No effective chemical fungicides were available. In 1870 Ceylon exported 100 million pounds of coffee. In 1889, production was down to 5 million pounds. Production of coffee ceased and the crop was switched to TEA.

Marshall K. Ward was hired by the British government to find a way to stop the rust epidemic. He didn't stop the epidemic, but he was able to warn us about monculture. He also observed that it is very important to anticipate the disease and not to wait for symptoms to appear before spraying .

LECTURE 4:

B. Great names in Plant Pathology

Earliest recorded history is found in Genesis. The blight, blast, and mildew were induced by fungi. We still use these terms to describe disease symptoms.

The greek philosopher Theophrastus (370286 B. C.) was the first to write about diseases of trees, cereals, and legumes. He made the interesting observation that plant disease was more severe in low lying areas.

For the next 2000 years little progress was made in the area of plant pathology. In 1660, Anton Van Leeuwenhoek (16321723)(a janitor), first saw bacteria. He had a hobby of grinding lenses. He made 247 different microscopes that could enlarge objects 40 to 270 times. He saw stagnant water teeming with life.

In 1729, Micheli described the spores of Rhizopus (a common bread mold). When these 'dust particles' were placed on freshly cut slices of melon, the same fungus was reproduced. He concluded that the spores were the fungal seeds and that these seeds were carried through the air.

In 1755 Tillet added black dusts from wheat to seeds and kept some seed clean. He observed that the seed that had the bunt or stinking smut dust had more disease. He showed from his experiments that stinking smut, bunt, of wheat is a contagious disease. However, Tillet believed it was a poisonous substance contained in the dust, rather than living organisms.

In 1807, Prevost proved that bunt is the cause of bunt of wheat. He was able to control the disease by dipping the seed in copper sulfate. We still use copper sulfate today. Unfortunately, most scientist of the day still believed in spontaneous generation and thought microbes were the result rather than the agent of disease.

Dr. C. Montagne, a French physician in Napoleon's army, first described the fungus found on the potatoes. He shared his observations with Rev. M. J. Berkeley, who recognized that this new fungus was connected with the blight. His rival was Dr. John Lindley, a botany professor at University College in London, who did not believe that the fungus was the cause of the blight. Their arguments were published in The Gardener's Chronicles . These were intense. Read The Advance of the Fungi, by E. C. Large for more juicy details. Anton De Bary, the father of Plant Pathology, a German botanist, performed the experiments that proved the role of the fungus in the blight.

In 1860, Louis Pasteur disproved the dogma of Spontaneous Generation (Flies come from rotten meat (Redi, Italian), rats come from old rags and moldy cheese). This theory was replaced by the germ theory in which it was finally recognized that microbes were inducers of disease.

Read: THE MICROBE HUNTERS, for a good time.

In 1880, an epidemic on grapes in France hit. Downy mildew (Plasmopara viticola) hit. The grape vines were going down and so was the wine industry (50 billion dollars). Alexis Millardet was a botany professor. One day he observed that some of the grapes were not suffering from Downy mildew. He also observed that these grapes were covered with a substance that looked like bird excrement. He talked to the farmer and the farmer said he put a mixture of Copper and lime on the plants to stop kids from eating the grapes. From this came the first fungicide, Bordeaux mixture. Bordeaux mixture is effective against many fungi and bacteria, is inexpensive, and even today, over 100 years later, is the most widely used fungicide in the world.

How to prove pathogenicity: Koch's postulates

Robert Koch, a German microbiologist, who worked on anthrax of sheep developed a method to prove pathogenicity.

Koch's postulates:

1. The symptoms and any evidence of the pathogen in the diseased host are carefullly described.

2. The suspected pathogen is isolated from the diseased host and from all other contaminating microorganisms, usually on a nutrient medium that will keep the organism alive. A description is made of the suspected pathogen.

3. A healthy host is inoculated with the suspected pathgen. It is later observed for symptoms, which must be identical to those described in Step 1.

4. The pathogen is reisolated from the inoculated host and must be identical to the organism described in Step 2.

R. J. Petri, a student of Koch first developed dishes to cut down on contaimination. They were called Petri dishes.

In 1878, Thomas Burrill, an American Plant Pathologist, implicated a bacterium as the disease agent of the fire blight disease in Northe America that was causing the death of apple and, especially, pear trees. Another American plant bacteriologist, Erwin F. Smith, contributed greatly to our understanding of bacteria. He was the first president of the American Cancer Society in the early 1900's. He had done some extensive work on Agrobacterium tumefaciens, causal agent of Crown gall. Causes tumorous growth (Hypertrophy a plant overgrowth due to abnormal cell enlargement).

In 1892, a Russian scientist named D. Ivanoski, first showed that viruses could cause disease. He worked with TMV (Tobacco mosaic virus).

Controversy with A. Mayer. a german scientist who thought it was a bacteria.

Norman Borlaug, a plant pathologist, received the Nobel Peace Prize in 1970 for his contributions to the development of high yielding wheat, which led to the media phrase Green revolution.

Roger Beachy, Steve Tankesly, Brian Stackawicz

IV. Diagnosis of Plant Disease

Diagnosis of plant disease is both an art and a science. In lab today we will discuss the differences between biotic or abiotic symptoms. That is the first step in diagnosis, to determine between environmental and organismal effects. We will also look at different reference sources that are used for diagnosis and ID. Today, I want to go over the basic symptoms of pathogens and parasites.