Plant pathogens

Microorganisms involved in plant diseases.

Introduction:

The branch of agricultural, botanical or biological sciences which deals with the study of causes of diseases is known as Plant pathology. It can be defined as a physiological disorder or structural abnormality which is deleterious to plants or its parts that reduce the economic value of the plant.

Definitions of the plant disease:

The pathogen which interferes with one or more essential l functions of the plant cell and alters the plant system is called plant disease.

Marshall Ward (1901) : Disease represents a condition in which functions of the plant are not properly discharged.

British Mycological Society (1950) : Disease is the harmful deviation from the normal functioning of physiological processes.

A plant disease is any abnormal condition that alters the appearance or function of a plant. Plant pathogenicity is a physiological process that affects some or all plant functions. Diseases are the result of some disturbance in the normal life process of the plant. The result of interaction between host, pathogen, and environment, can be the cause of plant pathogenicity.

Fig: Plant disease triangle

Conditions for Plant disease:

Fig: Condition for plant disease

• The host should be susceptible .
• Pathogen should have the high degree of virulence.
• The environmental condition should be favorable for the disease.

#How does the Pathogens affect the plant ?

Plant get in diseased conditions by various pathogens. Some of the mechanisms under which pathogens plants are as follow:

1. Plant pathogen utilizes the host cell contents.
2. They interfere or kill the host's metabolic system through their toxins, enzymes, etc.
3. They weaken the host through continuous loss of the nutrients.
4. They also alter the translocation of minerals, food , land water, etc.

Fig: Disease cycle of the plant pathogen.

Causes , Symptoms, and Signs of plant pathogen:

# Causes and types of plant disease:

The disease can be caused by various living or non-living causes. The living factors like fungi, bacteria, and viruses can cause the disease and non-living condition like soil compaction, wind, frost, girdling roots, soil salt damage, etc can cause the plant disease. There are two types of plant disease which are as follow:

1. Infectious or Biotic diseases
2. Noninfectious or Abiotic disease.

• Infectious or Biotic diseases:

The infectious diseases are caused by living organisms or factors like fungi, bacteria, and viruses. There are also many types of causative agents for the Biotic disease. Some the causative agents of biotic diseases are described below:

1. Diseases caused by prokaryotes: Various Eubacteria (With wall) likeXanthomonas, Pseudomonas, Rickettsia,etc. causes the biotic disease. Mollicutes like Mycoplasma, Mycoplasma-like organisms (MLO) or Phytoplasma, Siroplasma can be causative agents for the biotic disease.
2. Diseases caused by eukaryotic organisms: Various eukaryotes like protozoa, green algae, nematodes, parasitic higher plants (Phanerogams), etc. are the causative agents for the infectious or biotic disease.

• Non-infectious or abiotic plant diseases : Various abiotic or non-living factors are the causative agents for non-infectious plant diseases. The factors like too low or high temperature, lack of excess of soil moisture, lack or excess of light, lack of oxygen, air pollution, nutrient deficiencies,mineral toxicities, soil acidity or alkalinity (pH), pesticide's toxins, improper cultural practices, etc. are the causative agents of non-infectious plant diseases.

#Classification of plant disease:

The plant disease diseases can be classified into three types which classified as follow:

1. Based on type of symptoms : (like blights, rusts, smuts, rots, wilts, etc.)
2. Based on type of crop: (like cereal diseases, vegetable diseases, fruit diseases, etc.)
3. Based on type of organ affected: (like fruit diseases, root diseases, etc.)
4. Based on cause: (like infectious diseases, non-infectious diseases, etc.)

#Symptoms of Plant diseases:

The symptom can be defined as a visible abnormality seen on the plants as a result of disorder or disease condition and its totality is called as the syndrome. The different type of disease with their respective symptoms. Some of the nature of symptoms of plant disease are described below:

• Blight:

This type of disease is characterized by following symptoms:

1. There is widespread death of plant tissue,
2. There are sudden withering and death of leaves,
3. A rapid discoloration of flowers and branches and death of twigs, foliage (leaves), or flowers.

• Canker :

Often the main steam has a sunken or raised necrotic lesion and root characterized by dead area on bark or steam.

• Chlorosis:

It is characterized by yellowing as the diagnosis of chlorosis is impossible without additional details because of its generic nature.

• Damping off:

It is characterized by collapse and rot of seedlings near soil level before or soon after emergence.

• Decline and Dieback:

It is characterized by the progressive decrease in plant vigor, partial defoliation. The death of twig, branch may occur and even progressive death of shoot, branch, or root starting at the tip.

• Distortion and Downy Mildew:

1. There may be malformed plant tissue.
2. There may be the appearance of white or gray bloom on leaves and stems caused by sporangiospores and sporangia.

• Gall or gall-like and Gummosis:

1. There may be abnormal localized swelling or enlargement of parts of plants as a result of the pathogenic invasion.
2. The exudation of gum or sap is seen.

• Leaf distortion, leaf scorch, and leaf spot:

1. The twisted, cupped or rolled leaf could be seen.
2. There will be burning along the leaf margin and into the leaf from the margin.
3. There will be spot or lesion on the leaf.

• Mosaic and necrosis:

1. There will be varying patterns of normal green color (light and dark) , especially leaves in the plant tissue.
2. The plant tissue can be dead as necrotic areas are also so generic that without additional details diagnosis of it is impossible.

• Powdery Mildew and pustule:

1. The appearance of powdery bloom on plant surface with the production of fungal mycelium, conidiophores, and conidia.
2. A blister-like spore mass breaking through epidermis is seen.

• Rot, rust, and scab :

1. There will be the disintegration of tissue often caused by enzymes of toxins of pathogens and rust-colored pustules are seen.
2. Discrete superficial rough lesions are seen.

• Smut and Stunting :

1. There may be the appearance of black spore masses on leaves, stem and flowers.
2. There will be a lack of growth.

• Wilt (Vascular) and Witches broom:

1. The abnormal broom-like growth of many weak shoots and general wilting of plant or part of the plant.

Sign:

The visible presence of the pathogen such as on fruiting body, mycelia, etc.or the identification of the presence of disease-causing agents is called sign. It is a state of plant system which can be seen in association with a symptom caused by the pathogen. The actual organisms that cause the diseases, can also be called signs.

Plant diseases can have following signs during pathogenesis :

• Conks

The conks are the woody reproductive structures of fungi and fruiting body.

• Fruiting bodies:

Fruiting bodies are the reproductive structures of that could be in the form of mushrooms, rusts, conks or puffballs.

• Mildew and mushrooms:

Mildew are the whitish growth produced by fungi composed by mycelium and mushrooms are the fleshy reproductives structures of fungi.

• Rhizomorphs:

Under the bark of stressed and dying trees, these are shoestring-like fungal threads found caused by Armillaria fungi which may glow.

• Slime flux and spore masses:

Slime flux is the bacterial discharge that oozes out the plant tissues, may be dried or gooey mass. And spore masses are the 'seeds' of fungus.

Reference:

1. Alexander, M. Introduction to Soil Microbiology. Academic Press, 1961.
2. Rangaswami, G and Bagyaraja PT. Agricultural Microbiology. 2nd. Prentice Hall of India, 1993.
3. RM, Atlas and R Bartha. Microbial Ecology : Fundamental and Applications. The Benjamin Cummins Publication Co. Inc., 1998.