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Symptoms and Signs of Diseases in Plants

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Introduction to Plant Diseases

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Plant diseases are the disability of the normal condition of a plant that disrupts or alters its vital functions. All plant species, both wild and cultivated, are subject to disease. Although each type is susceptible to specific diseases, this is relatively small in number in each of the cases. The emergence and spread of plant diseases vary from time to time, depending on the presence of the virus, environmental conditions, and plants and species grown. Some plant species are highly susceptible to outbreaks of the disease while others are highly resistant.


Plant Diseases Caused by Bacteria

There are thousands of bacterial species in nature, which many of them perform biochemical processes essential for the continuity of life. Some of the examples are bacteria detritivores, or decomposers, feed on nonliving organic matter, recycling it through the ecosystem. There are hundreds of bacterial species in the world, which causes diseases in humans, animals, and plants. 

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General Characteristics

Bacteria are prokaryotic microorganisms that are single-celled microorganisms in which the nuclear substance is not enclosed in a membrane. Generally, there are two types of bacteria, one is eubacteria and the other is archaebacteria. They both can be classified based on their differences in the composition of the cell wall and the cytoplasmic membrane and also by certain metabolic features, plant pathogens belong to the eubacteria. 


Eubacteria can be divided into three groups, that are gram-negative bacteria, gram-positive bacteria, and mycoplasmas and spiroplasmas, called mycoplasma-like organisms (MLOs). Gram-negative and gram-positive bacteria are differentiated based on their cell wall formation, which affects the ability of bacteria to respond to Gram stain, it is one of the most important stains in bacteriologic laboratories, mycoplasma-like organisms (MLOs) belong to the taxa-mollicutes category. Plant diseases caused by substances such as mycoplasma (MLOs) have been classified as agents of "slowing down" (characterized by loss of yield, decreased fruit yield, and eventual death) as well as agents of virescence (flower ripening) and abnormal development.

 

The principal genera of plant pathogenic bacteria are Agrobacterium, Clavibacter, Erwinia, Pseudomonas, Xanthomonas, Streptomyces, and Xylella. With the exception of Streptomyces species, all are small, single, rod-shaped cells approximately half to one micrometre in width and 1.0 to 3.5 micrometres in length. Streptomycetes develop branched mycelia (narrow, threadlike growth) with curled chains of conidia (spores) on the tips of the mycelia. Streptomyces are gram-positive; most species of the other genera are gram-negative.

 

The main types of pathogenic bacterium of plants are Agrobacterium, Clavibacter, Erwinia, Pseudomonas, Xanthomonas, Streptomyces, and Xylella. With the exception of Streptomyces species, they are all small, single, rod-shaped cells about 0.5 to 1.0 micrometre (0.00002 to 0.00004 inch) wide and 1.0 to 3.5 micrometres long. Streptomycetes develop branched mycelia (small, cord-like growth) with conidia chains (spores) on the tip section of the mycelia. Streptomyces are gram-positive; many other species of the genus are gram-negative.

 

Signs and Symptoms of Bacterial Diseases in Plants

Bacterial diseases are mainly grouped into four categories depending on the amount of damage caused to the plant tissue and the symptoms that they cause, which may include vascular wilt, necrosis, soft rot, and tumours. Vascular wilt results from the bacterial invasion of the plant’s vascular system subsequent replication and closure inhibit the movement (transport) of water and nutrients through the xylem of the host plant. Drooping, wilting, or death of the aerial plant structure may occur; examples include bacterial wilt of sweet corn, alfalfa, tobacco, tomato, and cucurbits (e.g., squash, pumpkin, and cucumber) and black rot of crucifers.

 

Pathogens cause necrosis by secreting a toxin (poison), Symptoms include the formation of leaf spots, stem blights, or cankers. Soft rot diseases are caused by pathogens that secrete enzymes capable of decomposing cell wall structures. Thereby destroying the texture of plant tissue, the plant tissue becomes macerated (soft and watery). Soft rots are common in potato vegetables such as potatoes, carrots, eggplant, squash, and tomatoes. Abscesses are caused by bacteria that promote uncontrolled replication of plant cells, leading to the formation of abnormally large structures.

 

Most of the symptoms of bacterial diseases in plants are major, but a few produce a range or combination of symptoms. In general, it is not particularly difficult to say whether a plant is affected by a bacterial pathogen; however, identification of the causative agent in animal species requires the isolation and placement of the pathogen using a number of laboratory techniques.

 

Transmission and Infection

The bacterium produces a disease in a plant by invading the plant tissue and then multiplying themselves. Bacteria infuse in the plants by wounds, especially those produced by adverse weather conditions, humans, tools and equipment, insects and nematodes, or by natural openings such as stomata, lenticel, hydathode, nectar-producing glands, and leaf scars.

 

Most leaf invaders are dispersed from the plant to the planted plant by windy or dusty rain. Humans spread bacteria by cultivating, grafting, pruning, and transporting diseased plants. Animals, including insects and worms, are some of the most common transmission agents. Some bacteria, such as Stewart's causal agent, or bacterial sorghum wilt (Erwinia stewartii), not only are spread by beetles but also survive the winter of this insect.

 

When conditions do not allow for growth and replication, bacteria remain dormant inside or inside plant tissues. Some, such as gallstones, can live for months or years in the soil.

 

Most bacterial diseases are influenced by the temperature and moisture of the environment. Often, a difference of only a few degrees in temperature determines whether a bacterial disease will develop. In plants, moisture means mostly the water film on plant surfaces, which is essential for establishing an infection.

 

Control

Often, bacterial infections are difficult to control. This is due to the speed of the attack as the bacteria enter the natural opening or wounds directly. Direct introduction of the bacteria enables them to escape the toxic effects of chemical protectors. The loss of bacterial infections is reduced by the use of sterile seeds planted in dry areas. Examples of diseases that are controlled by this method include bacterial damage to beans and peas, black rot in the cob, and germ area and tomato corn.

 

The seed treatment with hot water at about 50° C (120° F) also applies to those crucified, cucurbits, carrot, eggplant, peppers and tomatoes. Bactericidal seed compounds control other bacterial infections, such as angular leaf spot cotton, gladiolus scab, and soft ornamental rot. Exchanging non-spiritual plants minimizes losses caused by alfalfa cravings, blights of bean and pea, black rot of crucifers, crown gall, bacterial spot and tomato canker. Extinction and exclusion of used plants have been instrumental in combating citrus fruits, small cotton buds, fire damage and gallbladder. Resistant crop varieties are designed to reduce losses from alfalfa, maize and tobacco; a leaf spot on the sides of cotton and tobacco; and a pustule of soy bacteria, among others.

 

Insect repellents help control bacterial infections, such as wilts of the sweet corn, cucurbits, and as well as soft iris rot. Antimicrobial sprays, paints, or tubes containing copper or antibiotics are used against the bacterial blights in the bean and edible crop with watery branches, fire blight, crown gall, delphinium blackleg, and hazelnut and walnut blights. Finally, hygienic practices - namely crop ploughing, voluntary crop and weed extraction, pruning and pruning tools - and stop planting when wet leaves, watering and spraying indoor plants, and late cutting or eating alfalfa paste and other crops, help reduce disease incidence of bacteria. 


List of Plant Diseases Caused by Bacteria

Some of the popular list of plant diseases caused by bacteria are given below:


Some Bacterial Diseases of Plants

Disease

Causative Agent

Hosts

Symptoms and Signs

Additional Features

Granville wilt

Pseudomonas solanacearum

tobacco, tomato, potato, eggplant, pepper, and other plants

Symptoms of granville wilt are stunting, yellowing, and wilting of parts above ground; roots decay and become black or brown

They occur in most of the countries where temperate and semi-tropical zones; causes crop losses of high value. 

fire blight

Erwinia amylovora

apple and pear

Symptoms of fire blight blossom appear water-soaked and shrivel which spreads to leaves and stems, causing rapid dieback

The disease observed first on bottom of the plant caused by a bacterium.


wildfire of tobacco

Pseudomonas syringae

tobacco

yellowish-green spots on leaves

wildfire of tobacco occurs worldwide; causes losses in seedlings and field plants

blight of beans

Xanthomonas campestris

beans (common blight)

yellowish-green spots on leaves

Plenty of phytopathogenic xanthomonads and pseudomonads causes necrotic spots on green parts of susceptible hosts and they might be localized or systemic


Pseudomonas syringae

beans (brown spot)

small water-soaked spots on the lower side of leaves enlarge, coalesce, and become necrotic


soft rot

Erwinia carotovora

many fleshy-tissue fruits—e.g., cabbage, carrot, celery, onion

In this disease the decay of fleshy tissues that become mushy and soft

occurs worldwide; causes major economic losses

crown gall

Agrobacterium tumefaciens

Most of the genera of woody and herbaceous plants

In this disease initially a small enlargement of stems or roots usually at or near the soil line, which increases the size, becoming wrinkled, and turning brown to black

the conversion of a normal cell to one that produces excessive cell multiplication is caused by a plasmid (a small circular piece of DNA) carried by the pathogenic bacterium

aster yellows

Mycoplasma-like organism (MLO)

many vegetables, ornamentals, and weeds

chlorosis; dwarfing malformations

greatest losses suffered by carrots; transmission by leafhoppers

citrus stubborn disease

Spiroplasma citri (MLO)

citrus and stone fruits and vegetables

chlorosis, yellowing of leaves, shortened internodes, wilting

first MLO pathogen of plant disease cultured


Requirements for Disease Development

Infectious diseases cannot occur if any of the three basic conditions do not exist given below: 

(1) the right environment, the most important environmental factors are the amount and quantity of rain or heavy dew, relative humidity, and air and soil temperatures,

(2) the presence of a toxic virus, and

(3) The susceptible host,  effective disease control measures are aimed at violating this triangular ecosystem. Disease-related losses are minimized, for example, when a host is made more resistant or protected by techniques such as plant breeding or genetic engineering. In addition, the environment can be made more resistant to pathogens and more susceptible to the growth of host plants. Eventually, the virus can be killed or prevented from reaching the keeper. These basic control methods can be distinguished by many cultural, chemical and biological practices to help control the disease.


Do you know?

Which is the first bacterial disease in plants? 

The first bacterial disease discovered was anthrax (caused by Bacillus anthracis) of cattle and sheep in 1876 and it was immediately followed by the discovery of fire blight of pear and apple (caused by Erwinia Amylovora) by T. J. Burrill from the University of Illinois (1877–1885).

FAQ (Frequently Asked Questions)

1. How to Diagnose Plant Disease?

Answer. Prompt and accurate diagnosis of the disease is needed before appropriate control measures can be proposed. It is the first step in the study of any disease. Diagnosis is highly dependent on the symptoms of the condition produced by the diseased plant. The identification of the pathogen is also important in the diagnosis.

The three steps involved in a diagnosis include careful observation and classification of facts, an examination of facts, and a sound decision about the cause.

2. How do you Identify Bacterial Diseases in Plants?

Answer. The bacterial diseases in the plants can be identified by the symptoms that are much like the symptoms of fungal plant disease. They include leaf spots, blights, wilts, scabs, cankers and soft rots of roots, storage organs and fruit, and overgrowth, and the bacterial spots is the most common symptom of bacterial disease is leaf spots.