Citrus canker seedling diseases seedling blight pythium

MBO-09 Vardhman Mahaveer Open University, Kota

Course Development Committee

Chair Person
Prof. Ashok Sharma

Director (Academic) (Special Invite)

Vardhman Mahaveer Open University, Kota

•Dr. P.K. Sharma
Department of Botany MSJ College,
Bharatpur

•Dr. P.P. Paliwal
Department of Botany
Govt. PG College, Banswara

Writers:

Department of Botany
University of Rajasthan, Jaipur

Academic and Administrative Management

MBO-09 Vardhman Mahaveer Open University, Kota Index
Plant Pathology

MBO-09

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Unit - 1

•Concept and Scope of plant pathology

•Different types of pathogens and diseases caused by them

Thegrowth of the plants depend on the availability of nutrients and water in the soil where they grow and the normal ranges of environmental factors such as light, temperature, and pH. Plants, however, do get sick. Anything that affects the health of plants may affect their growth and may seriously reduce their usefulness to themselves and to mankind. Plant pathogens, unfavorable weather, and insect pests are the most common causes of reduction or destruction of plant growth and production. Plants suffer from diseases whose causative agents are similar to those affecting animals and man. Although there is no evidence that plants feel pain and discomfort, the development of disease follows the same steps and is usually as complex in plants as it is in animals and man.

Plant pathology is the study of (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 disease-causing agents and the diseased plant; and (4) the methods of preventing disease, alleviating the damage it causes, or controlling a disease either before or after it develops in a plant.

more of these essential functions is interfered by any pathogen or adverse environmental factor, the normal activity of the cells is hampered in some or the other way, then the plant becomes diseased. The primary causes of disease are either pathogens or factors in the physical environment, but the specific mechanisms by which diseases are produced vary considerably with the causal agent and sometimes with the plant. At first the reaction of the plant to the disease-causing agent at the site of affliction, is of a chemical nature, and is invisible. Soon, however, the reaction becomes more widespread and histological changes take place that manifest themselves macroscopically and constitute the symptoms of the disease. Affected cells and tissues of diseased plants are usually weakened or destroyed by the disease-causing agents. The ability of such cells and tissues to perform their normal physiological functions is reduced or completely eliminated; as a result, plant growth is reduced or the plant dies. Thus, Disease in plants can be defined “as the series of invisible and visible responses of plant cells and tissues to a pathogenic microorganism or environmental factor that result in adverse changes in the form, function and integrity of the plant and may lead to partial impairment or death of the plant or its parts.”

Plants are considered to have a disease or a disorder when they are not growing up to expected standards. There are two visual keys for identifying “sick” plants—symptoms and signs.

1.2.1 The Disease Triangle

A disease episode requires the interaction of three components: the host, the pathogen, and the environment. This interaction is known as the disease triangle (Fig. 1).

vironment must be conducive (favorable) for disease development. That is the temperature, moisture, nutrients and wind must all favor the pathogen’s growth and development. Some diseases (notably those caused by viruses) require a vector (transmitting agent) for infection. In these cases, the vector is an additional component of this interaction. The degree to which these three components interact relates to the disease severity. For example, if the host is highly susceptible, the pathogen is highly virulent, and the environment is highly conducive, then the disease will be very severe. Severity also includes an element of time - the longer the environment remains favorable for disease development, the greater the severity of the disease.

Successful disease management requires the disruption of some part of the disease triangle. Accurate diagnosis of the causal agent is required for effective use of management strategies in general and specifically for effective chemical control. Using an inappropriate chemical will not only be ineffective against the disease agent, but also can lead to additional disease problems by killing beneficial microorganisms in the environment. There are several different strategies for disease management. But, overall, the most successful strategy depends on the integrated use of available control methods.

sacrifices of reddish colored dogs and cattle in an attempt to appease the rust god Robigo.

•470 B.C.- Pliny was the first to use a fungicide, amurce of olives, to control

The vineyards were also troubled with pilferers. They began applying a mixture of copper sulfate and lime to the plants along the edges of the fields. It was also observed that these plants held onto their leaves throughout the season.

•1900- White Pine Blister Rust caused by Cronartium ribicola. The

discovered near the Georgia border and was eradicated in 1931. The pathogen was found 400 miles away in Dade County in 1912. The pathogen

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•1995- Sudden Oak Death, caused by Phytophthora ramorum, was discovered in California. A large number of tanoaks were found to be declining with no known cause. It took five years to isolate and identify the causal agent and is also called ramorum blight and ramorum dieback.

Although the disease was first observed in the United States in tanoaks, it is also found to infect manyother plant species.

2004, apparently entering on winds of Hurricane Ivan. It was found in 9 States shortly thereafter. It was found in Florida early in 2005 on Soybean and new detections for that season remained in the South.

1.4 Scope of Plant Pathology

Table 1.1 : Examples of severe losses caused by Plant Diseases

anthracnoses, rusts, smuts)

•The plant organ they affect (root diseases, stem diseases, foliage diseases,

Localized disease: affecting only a part of the plant.

Systemic disease:affecting the entire plant.

plants causing diseases. e.g. Blight, rust, powdery mildew.

Seed borne: The micro organisms are carried along with seeds and cause

of potato is endemic to Darjeeling.

Epidemic or epiphytotic: A disease occurring periodically but in a severe

moderate to severe form e.g., leaf blights, wilt.

Pandemic: Diseases occurring throughout the continent or sub-continent

1.Non parasitic disease: The causal factors of these are mainly physiological

or environmental like freezing injury caused by low temperature, high

On this basis, plant diseases are classified as follows: I. Infectious Plant Diseases
1. Diseases caused by fungi
2. Diseases caused by bacteria
3. Diseases caused by parasitic higher plants
4. Diseases caused by viruses
5. Diseases caused by nematodes
II. Noninfectious or Abiotic Diseases

1. Nutrient deficiencies
2. Mineral toxicities
3. Lack or excess of soil moisture
4. Too low or too high temperature
5. Air pollution
6. Lack of oxygen
7. Lack or excess of light
8. Soil acidity or alkalinity (pH)

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(8) cankers (9) fruit rots etc.

4.Fruit diseases: Crown gall of stone fruits, citrus canker.

5.Seedling diseases: seedling blight (Pythium, Phytophthora, Fusarium, Corticium)

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development and reproduction. Thus, in many cases, parasitism is intimately associated with pathogenicity, since 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, both the plant and the microorganism are beneficial to the other's development, and this phenomenon is known as symbiosis. Of the large number of groups of living organisms, only a few members of a few groups can parasitize plants: fungi, bacteria, and parasitic higher plants (all three belonging to the plant kingdom), nematodes (of the animal kingdom) and viruses. These parasites, to be successful, must be able to invade a host plant, feed and proliferate in it and withstand the conditions in which the host lives. Some parasites, including viruses, nematodes, and among the fungi, those causing downy mildews powdery mildews and rusts, can grow and reproduce only on living hosts, and they are called obligate parasites. Other parasites (most fungi, bacteria) can live on either living or dead hosts and are, therefore, called non-obligate parasites. Some non-obligate 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 (facultative saprophytes), whereas others live most of the time and thrive well on dead organic matter but, under certain circumstances, may attack living plants and may become parasitic (facultative parasites). There is usually no correlation between the degree of parasitism of a pathogen and the severity of disease it can cause, since many diseases caused by weakly parasitic pathogens are much more damaging to the plant than others caused even by obligate parasites.

Parasitism is inherent to life. Parasites are found in every organism of all existing species on earth. Since the beginning of life parasitism was adopted by certain organisms to multiply. Actually parasitism must have occurred at an early stage of evolution. All living organisms have a uniform biochemical composition that points to a common origin in a common ancestor that lived a billion years ago. Life on earth was only possible as a consequence of parasitism in what was still a molecular world. In fact, life appeared on earth around 4.4-3.8 billion years ago as a consequence of molecular parasitism and the present day life forms still display relics of these ancient associations in their genomes. Later diversification of life forms and species radiation was also the consequence of these multiple associations. These ancient events represent the first steps towards a host-parasite way of life although still at the level of molecules. Molecular parasitism is clearly exemplified by transposable elements of the genome. Indeed, the DNA sequences called transposable elements are actually recognized as molecular parasites.

Transposable elements occur in both prokayotes and eukaryotes. Transposition is the insertion of an identical copy of the transposable element into a new genomic site of the host. These insertions can cause deletions, inversions, and chromosome fusions that result in considerable genome plasticity, thus contributing to biodiversity. They have a common origin with viruses and retroviruses and were identified first as 'control elements' by McClintock (1984).