The Mechanisms by Which Pathogens Cause Disease

Plant diseases cause a huge loose of fruits, vegetables and crops since long time ago, historically diseases causes huge loose that leads to the dead of millions of peoples on earth. During early peoples, not know the causes of diseases, they were considering the cause of diseases some mysterious agents, much myth related to the causes of diseases.	As science advance the mysterious agents of diseases discovered, peoples come to know the real causes and agents. Scientist observed fungi, bacteria and microorganisms were the causes of different diseases, but another big question arise after the discovery of casual agents, “How fungi and microorganisms cause diseases?”	Previously it became noticeable that fungi and other microorganisms were the causes rather than the results of plant disease, efforts began to understand the mechanisms by which microorganisms cause disease.
	In 1886, deBary, working with the Sclerotinia rot disease of carrots other vegetables, noticed that host cells were killed in advance of the invading fungal hyphae and a juice from rotted tissue released that could able break down healthy host tissue, when juice from rotted tissue boiled and treat healthy tissues, had no effect on healthy tissue. DeBary concluded that the pathogen produces enzymes and toxins that degrade and kill plant cells from which the fungus can then obtain its nutrients.
After deBary, many attempted to show that most plant diseases, particularly vascular wilts and leaf spots, were caused by toxins secreted by the pathogens, but those claims could not be confirmed. A 1925 suggestion that the bacterium Pseudomonas tabaci, the cause of the wildfire disease of tobacco, produces a toxin that is responsible for the bacteria-free chlorotic zone (“halo”) surrounding the bacteria-containing necrotic leaf spots was confirmed in 1934.		The wildfire toxin was the first toxin to be isolated in pure form in the early 1950s. In 1947, a species of the fungus Helminthosporium (Bipolaris), which attacked and caused blight only on oats of the variety Victoria and its derivatives, was shown to produce a toxin named victorin. This toxin could induce the symptoms of the disease only on the varieties susceptible to the fungus.
	Many other bacterial and fungal toxins were subsequently detected and identified. The toxins exhibited several distinctive mechanisms of action, each affecting specific sites on mitochondria, chloroplasts, plasma membranes, specific enzymes, or specific cells such as guard cells. In addition, several detailed biochemical studies were carried out to elucidate the mechanisms by which toxins affect or kill plant cells or by which cells of resistant plants avoid or inactivate them.

Fire Blight

By Laura Pottorff, Colorado State University Cooperative Extension agent, horticulture/plant pathology

It's hard to even think it, but Spring sometimes can be a mixed blessing.

Just ask the homeowner who's trying to cope with a laundry list of plant diseases -- fire blight, for example.

Fire blight thrives in warmer, wet weather when the disease-causing bacterium becomes active. Apple, pear, quince, crabapple and mountain ash, but NOT GREEN ASH, commonly are affected. The bacterium oozes out of cracks and crevices in the bark, and bees or other pollinating insects pick it up on their bodies. The disease spreads as these insects pollinate other flowers.

If your trees are affected by fire blight, they will begin to show symptoms just before their flower petals fall. This is the blossom blight stage when flowers begin to turn brown and mushy and wilt. Eventually, the bacteria will move down into the branches and leaves of the tree. Leaves darken and wilt but remain attached to the tree (see above), giving it a scorched or burned look. Branch tips blacken and curl, causing a "shepherd's crook" symptom.

Fruit also can be affected. Bacteria often oozes out of the infected fruit and, as symptoms progress, fruits remain attached to the tree as shriveled "mummies" (see above). Cankers, which are sunken areas darker in color than the surrounding bark, form as the disease progresses. If present on the main trunk, cankers often are fatal, as they eventually will girdle the tree.

Canker  on trunk of mountain ash

Cankers also serve as the overwintering source of the bacterium. The following spring, bacteria will ooze from the cankers or cracks in bark. Insects may come in contact with the ooze and spread the disease to other trees.

Fire blight control can be a challenge. A variety of controls are available and success usually is greatest when we integrate all of them.

The use of resistant varieties is the first line of defense. Listed on the table below are apple and crabapple trees with certain degrees of resistance to the blight. Resistance doesn't necessarily mean immunity. A tree with resistance can become infected, but the problem won't be nearly as severe as if there were no degree of resistance.

Avoid overfertilization with nitrogen. Nitrogen stimulates new growth and the new growth is highly susceptible to fire blight infection.

Practice sanitation when pruning trees. Prune out and destroy all infected and dead plant tissue. Make pruning cuts 6 to 12 inches into healthy tissue. Always dip tools in a disinfectant solution, such as 10 percent household bleach, between each cut to minimize disease spread. Prune newly infected twigs as soon as possible in the spring. Do all other pruning during winter.

You may want to spray next spring to prevent the infection. Chemicals will have little or no effect on existing infections. Copper compounds, such as Bordeaux mixture, Kocide or lime sulfur, or streptomycin (Agristrep) are effective chemicals. Timing, however, is critical. If you use copper, you must apply during dormancy and bud break. Copper may burn leaves and fruit if it is applied later in the season. If you use streptomycin, apply when bloom is 50 percent and repeat twice at 4-to-5 day intervals.

Tree varieties with some degree of fire blight resistance*

Apple	Pear	Crabapple
Early McIntosh	Douglas	Radiant
Grimes	Golden Kieffer	Kelsey
Golden Delicious	Seckel	Red Splendor
Missouri Pippin	Dawn	Royalty
Sharon	Anjou	Snow Cloud
Red Delicious	Magness	Vanguard
Winesap	Moonglow	Dolgo
Duchess	Brandywine
Turley	Centurion
Haralson

* Resistance does not mean immunity.
Photographs courtesy of Judy Sedbrook..

Source: http://www.colostate.edu

Plant Diseases May 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. Sometimes, however, this cost 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, wellinformed 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.

Source: Plant Pathology by GEORGE N. AGRIOS

Verticillium Wilt of Tomato

by Arden Sherf Department of Plant Pathology, Cornell University,Ithaca, NY

The soilborne fungus Verticilliurn albo-atrum is present in most cool soils of the Northeast and can attach over 200 plant species. especially tomatoes, potatoes, eggplant, strawberries, and black raspberries. Other susceptible vegetables include artichoke, beet, broad bean. chicory. cucumber, dandelion, endive, horseradish, muskmellon, okra. peppers, radish, rhubarb, salsify, and watermelon. Corn, other cereals, and grasses are resistant.

Symptoms

In spite of the name verticillium wilt, a true wilt seldom occurs in tomato, at least not until late in the season. Rather, under good conditions of moisture and nutrition, yellow blotches on the lower leaves may be the first symptoms, then brown veins appear, and finally chocolate brown dead spots. The spots may be confused with alternaria early blight, but they are not definite, nor do they develop concentric bull's-eye rings.

The leaves may wilt, die and drop off. The disease symptoms progress up the stem, and the plant becomes stunted. Only the top leaves stay green. Fruits remain small, develop yellow shoulders, and may sunburn because of loss of leaves.

Infection takes place directly when the fungus threads enter the root hair. It is aided in its entrance if rootlets are broken or nematodes have fed on the root system. The fungus grows rapidly up the xylem, or sap-conducting channels. Its activity there results in interference with the normal upward movement of water and nutrients. The fungus produces a toxin that contributes to the wilting and spotting of the leaves. Diagnosis involves making a vertical slice of the main stem just above the soil line and observing a brown color in the conducting tissues under the bark. This discoloration can be traced upwards as well as downwards into the roots. In contrast to fusarium wilt, verticillium wilt discoloration seldom extends more than 10-I2 inches above the soil, even though its toxins may progress farther.

The Causal Fungus

Its wide host range permits Verticillium to persist in soils for long periods. It remains alive by means of darn resting threads, which form in great numbers on dying diseased underground plant parts. It can attack and multiply in many common weeds, including ragweed, cocklebur, and velvetleaf. One form of the fungus produces tiny black resting bodies (microsclerotia), which help it survive over winter.

Controls

Long rotations (4-5 years) with nonrelated crops, well-drained soils, and soil moisture kept at the minimum for good growth are advisable.

In greenhouses or with plastic-strip mulch, soil fumigation gives good control and is feasible on high-value crops.

By far the most feasible and economic control is the use of Verticillium-tolerant tomato cultivars of which there are many with varying maturities and excellent horticultural qualities. These include the following:

The pathogen is sensitive to soil moisture and temperature. Tomatoes and potatoes must have at least a day of saturated soil before infection occurs. Soil temperatures must be moderate or cool for infection to take place: 75° F (24° C) is optimum with 55° F (13° C) minimum and 86° F (30° C) maximum.

New Yorker (V) Springset Pic Red Jet Star Supersonic Heinz 1350 Heinz 1439 Westover Royal Flush Floramerica Veebrite Veemore Veegan Veeset Burpee VF Hyb. Starshot

Earlirouge Supersteak Campbell 1327 Fireball (V) Beefmaster Better Boy Bonus Gardener (V) Monte Carlo Nova (Paste) Crimson Vee (Paste) Veeroma (Paste) Veepick (Paste) Ramapo Moreton Hyb. Spring Giant

Basket Vee Campbell 17 Big Set Setmore Small Fry Terrific Big Girl Mainpak Early Cascade Jumbo Wonder Boy Rutgers 39 Ultra Boy Ultra Girl Rushmore Jetfire

Source:http://vegetablemdonline.ppath.cornell.edu

Establishing the Cause of Disease

Source: http://www.sciencemag.org/content/276/5313/726/F1.large.jpg

The correct diagnosis of a plant disease and its cause is not always an easy task. In the first instance symptoms may be ill defined which make their association with any organism problematic (Derrick and Timmer, 2000) and, secondly, plants grow in environments which are notably non-sterile. In particular, besides supporting a microflora on their aerial parts, the phylloplane, they are rooted in soil which may contain in excess of 1 million organisms per gram. The plant pathologist is therefore faced with trying to determine which, if any, of the organisms associated with the diseased plant is responsible for the symptoms. This is normally achieved by the application of the postulates of Robert Koch, a German bacteriologist of the 19th century, which for plant pathogens may be stated as follows:

1. The suspected causal organism must be constantly associated with symptoms of the disease.
2. The suspected causal organism must be isolated and grown in pure culture.
3. When healthy test plants are inoculated with pure cultures of the suspected causal organism they must reproduce at least some of the symptoms of the disease.
4. The suspected causal organism must be re isolated from the plant and shown to be identical with the organism originally isolated.

Clearly, these criteria can only be met with organisms that can be cultured, ruling out all obligate pathogens which include a number of important fungi, many phytoplasmas and all viruses and viroids. Establishing these organisms as causal agents of disease usually involves purification of the suspected agent rather than culture and the demonstration that these purified preparations reproduce at least some of the disease symptoms.
Reference: Introduction to Plant Pathology. Richard N. Strange. 2003. John Wiley and Sons Ltd.

Types of Plant Diseases

Tens of thousands of diseases affect cultivated and wild plants. On average, each kind of crop plant can be affected by a hundred or more plant diseases. Some pathogens affect only one variety of a plant. Other pathogens affect several dozen or even hundreds of species of plants. Plant diseases are sometimes grouped according to the symptoms they cause (root rots, wilts, leaf spots, blights, rusts, smuts), to the plant organ they affect (root diseases, stem diseases, foliage diseases), or to the types of plants affected (field crop diseases, vegetable diseases, turf diseases, etc.).

One useful criterion for grouping diseases is the type of pathogen that causes the disease. The advantage of such a 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. On this basis, plant diseases are classified as follows:

I. Infectious, or biotic, plant diseases

1. Diseases caused by fungi.

2. Diseases caused by prokaryotes (bacteria and mollicutes).

3. Diseases caused by parasitic higher plants and green algae

4. Diseases caused by viruses and viroids.

5. Diseases caused by nematodes.

6. Diseases caused by protozoa.

II. Noninfectious, or abiotic, plant diseases.

1. Diseases caused by too low or too high a temperature

2. Diseases caused by lack or excess of soil moisture

3. Diseases caused by lack or excess of light

4. Diseases caused by lack of oxygen

5. Diseases caused by air pollution

6. Diseases caused by nutrient deficiencies

7. Diseases caused by mineral toxicities

8. Diseases caused by soil acidity or alkalinity(pH)

9. Diseases caused by toxicity of pesticides

10. Diseases caused by improper cultural practices

PLANTS AND DISEASE

Plants make up the majority of the earth’s living environment as trees, grass, flowers, and so on. Directly or indirectly, plants also make up all the food on which humans and all animals depend. Even the meat, milk, and eggs that we and other carnivores eat come from animals that themselves depend on plants for their food.

Plants are the only higher organisms that can convert the energy of sunlight into stored, usable chemical energy in carbohydrates, proteins, and fats. All animals, including humans, depend on these plant substances for survival.

Plants, whether cultivated or wild, grow and produce well as long as the soil provides them with sufficient nutrients and moisture, sufficient light reaches their leaves, and the temperature remains within a certain “normal” range. Plants, however, also get sick. Sick plants grow and produce poorly, they exhibit various types of symptoms, and, often, parts of plants or whole plants die. It is not known whether diseased plants feel pain or discomfort.

The agents that cause disease in plants are the same or very similar to those causing disease in humans and animals. They include pathogenic microorganisms, such as viruses, bacteria, fungi, protozoa, and nematodes, and unfavorable environmental conditions, such as lack or excess of nutrients, moisture, and light, and the presence of toxic chemicals in air or soil. Plants also suffer from competition with other, unwanted plants (weeds), and, of course, they are often damaged by attacks of insects. Plant damage caused by insects, humans, or other animals is not usually included in the study of plant pathology.

Plant pathology is the study of the organisms and of the environmental factors that cause disease in plants; of the mechanisms by which these factors induce disease in plants; and of the methods of preventing or control-ling disease and reducing the damage it causes. Plant pathology is for plants largely 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 integrative science and profession that uses and combines the basic knowledge of botany, mycology, bacteriology, virology, nematology, plant anatomy, plant physiology, genetics, molecular biology and genetic engineering, biochemistry, horticulture, agronomy, tissue culture, soil science,  forestry, chemistry, physics, meteorology, and many other branches of science. Plant pathology profits from advances in any one of these sciences, and many advances in other sciences have been made in attempts to solve plant pathological problems.

As a science, plant pathology tries to increase our knowledge about plant diseases. At the same time, plant pathology tries to develop methods, equipment, and materials through which plant diseases can be avoided or controlled. Uncontrolled plant diseases may result in less food and higher food prices or in food of poor quality. Diseased plant produce may sometimes be poisonous and unfit for consumption. Some plant diseases may wipe out entire plant species and many affect the beauty and landscape of our environment. Controlling plant disease results in more food of better quality and a more aesthetically pleasing environment, but consumers must pay for costs of materials, equipment, and labor used to control plant diseases and, sometimes, for other less evident costs such as contamination of the environment.

In the last 100 years, the control of plant diseases  and other plant pests has depended increasingly on the extensive use of toxic chemicals (pesticides). Controlling plant diseases often necessitates the application of such toxic chemicals not only on plants and plant products that we consume, but also into the soil, where many pathogenic microorganisms live and attack the plant roots.

Many of these chemicals have been shown to be toxic to nontarget microorganisms and animals and may be toxic to humans. The short- and long-term costs of environ-mental contamination on human health and welfare caused by our efforts to control plant diseases (and other pests) are difficult to estimate. Much of modern research in plant pathology aims at finding other environmentally friendly means of controlling plant diseases. The most promising approaches include conventional breeding and genetic engineering of disease-resistant plants, application of disease-suppressing cultural practices, RNA and gene-silencing techniques, of plant defense-promoting, nontoxic substances, and, to some extent, use of biological agents antagonistic to the microorganisms that cause plant disease.

 The challenges for plant pathology are to reduce food losses while improving food quality and, at the same time, safeguarding our environment. As the world  population continues to increase while arable land and most other natural resources continue to decrease, and as our environment becomes further congested and stressed, the need for controlling plant diseases effectively and safely will become one of the most basic necessities for feeding the hungry billions of our increasingly overpopulated world.

Plants Invisible Enemies

Plants, through their ability to fix carbon dioxide by photosynthesis, are the primary producers of the food that feeds the world’s human population as well as the many animals and other organisms that are heterotrophic for carbon compounds. It is not surprising, therefore, that among the latter there is a considerable number which, in order to have first call on these rich pickings, have adopted the parasitic mode of life. 

 They range from higher plants themselves, the parasitic angiosperms, to viroids, naked fragments of nucleic acid, in some instances less than 300 nucleotides in length. Between these extremes of size, there are plant pathogenic organisms among the fungi, nematodes, algae, Oomycetes, Plasmodiophoromycetes, trypanosomatids, bacteria, phytoplasmas and viruses.

In almost all of these categories there are organisms that cause catastrophic plant diseases, affecting the lives of millions of people by competing for the plant products on which they depend for food, fibre, fuel and cash. In this chapter all 11 classes of plant pathogenic agent will be introduced and those that are particularly destructive will be highlighted together with the impact that they have had on the people who have been most seriously affected. However, the first imperative of a plant pathologist is to establish unequivocally the cause of disease.

The correct diagnosis of a plant disease and its cause is not always an easy task. In the first instance symptoms may be ill defined which make their association with any organism problematic (Derrick and Timmer, 2000) and, secondly, plants grow in environments which are notably non-sterile. In particular, besides supporting a microflora on their aerial parts, the phylloplane, they are rooted in soil which may contain in excess of 1 million organisms per gram. 

The plant pathologist is therefore faced with trying to determine which, if any, of the organisms associated with the diseased plant is responsible for the symptoms. 

This is normally achieved by the application of the postulates of Robert Koch, a German bacteriologist of the 19th century, which for plant pathogens may be stated as follows:

(1) The suspected causal organism must be constantly associated with symp-toms of the disease.

(2) The suspected causal organism must be isolated and grown in pure culture.

(3) When healthy test plants are inoculated with pure cultures of the suspected causal organism they must reproduce at least some of the symptoms of the disease.

(4) The suspected causal organism must be reisolated from the plant and shown to be identical with the organism originally isolated.

Clearly, these criteria can only be met with organisms that can be cultured, ruling out all obligate pathogens which include a number of important fungi, many phytoplasmas and all viruses and viroids. Establishing these organisms as causal agents of disease usually involves purification of the suspected agent rather than culture and the demonstration that these purified preparations reproduce at least some of the disease symptoms.

Why Fungi Cause Catastrophic Plant Disease

There are at least five reasons why fungi may cause catastrophic plant disease.

(1) They sporulate prolifically, the spores providing copious inoculum which may infect further plants.

(2) Their latent period, i.e. the time between infection and the production of further infectious propagules, usually spores, may be only a few days.

(3) The spores, if they are wettable, may be spread as high-density inoculum in surface water or in droplets by rain-splash. Alternatively, non-wettable spores may be carried long distances by the wind.

(4) They may produce compounds that are phytotoxic and/or a battery of enzymes that destroy the plant’s structure.

(5) Biotrophic pathogens, such as the rusts and mildews, draw nutrients away from the economically valuable part of the plant by the production or induction of growth regulators such as cytokinins and consequently depress yields.