Thousands of plant diseases have been recorded throughout the world, many of these causing heavy crop losses. Early detection and accurate diagnosis is essential for the effective management of plant disease. Thus the first step in studying any disease is its timely detection of the diseased plant. Quick initial detection is largely based on the signs and symptoms of disease. Signs are the visible physical presence of either the pathogen itself or the structures formed by the

pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...

. Common examples of easily detected signs are those such as the fungal mycelia and

spore In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, f ...

masses of

downy mildew Downy mildew refers to any of several types of oomycete microbes that are obligate parasites of plants. Downy mildews exclusively belong to the Peronosporaceae family. In commercial agriculture, they are a particular problem for growers of crucif ...

s observed on infected leaves and the bacterial ooze of ''

Xanthomonas ''Xanthomonas'' (from greek: ''xanthos'' – “yellow”; ''monas'' – “entity”) is a genus of bacteria, many of which cause plant diseases. There are at least 27 plant associated ''Xanthomonas spp.'', that all together infect at least 400 ...

'' leaf streak disease on

rice Rice is the seed of the grass species ''Oryza sativa'' (Asian rice) or less commonly ''Oryza glaberrima ''Oryza glaberrima'', commonly known as African rice, is one of the two domesticated rice species. It was first domesticated and grown i ...

. Symptoms are the visible changes that occur in the host plant in response to infection by pathogens. For any disease in a given plant, there is the characteristic expression of symptoms, usually occurring in a sequential series during the course of the disease. This series of symptoms depicting the disease picture is referred to as the disease syndrome. Morphological symptoms may be exhibited by the entire plant or by any organ of the plant. These have been categorized into different groups for easy of study. Primarily, morphological symptoms of plant diseases can be categorized into 6 different types. * Necroses * Growth abnormalities * Metaplastic symptoms * Proleptic symptoms * Color changes * Wilts

Necroses

Necroses are caused due to

necrosis Necrosis () is a form of cell injury which results in the premature death of cells in living tissue by autolysis. Necrosis is caused by factors external to the cell or tissue, such as infection, or trauma which result in the unregulated dige ...

or death of plant cells. The affected plant tissue usually turns brown to black in color. Necrotic symptoms could appear in any part of the plant such as in storage organs, in green tissues, or in woody tissues.

Necrosis in storage organs

Death of cells in storage organs terminates in

decomposition Decomposition or rot is the process by which dead organic substances are broken down into simpler organic or inorganic matter such as carbon dioxide, water, simple sugars and mineral salts. The process is a part of the nutrient cycle and is e ...

or decay referred to as a rot. Two types of rots are identified as

Dry rot Dry rot is wood decay caused by one of several species of fungi that digest parts of the wood which give the wood strength and stiffness. It was previously used to describe any decay of cured wood in ships and buildings by a fungus which resul ...

and

Wet rot Wet rot is a generic term used to define a variety of fungal species, such as ''Coniophora puteana'' ( otherwise known as cellar fungus) and '' Choanephora cucurbitarum''. Some species obtain their food by breaking down the cell walls of wood, r ...

on storage tissues. Soft rots are those where the pathogen breaks down the host cell walls, resulting in the exudation of juices from the infected tissue. The organ becomes mushy or pulpy and a foul smell often develops due to colonization by secondary invaders. Many fungi and bacteria cause soft rots on several fruits and vegetables. Species of the fungus, ''

Rhizopus ''Rhizopus'' is a genus of common saprophytic fungi on plants and specialized parasites on animals. They are found in a wide variety of organic substances, including "mature fruits and vegetables", jellies, syrups, leather, bread, peanuts, and t ...

'' and bacterium ''

Erwinia ''Erwinia'' is a genus of Enterobacterales bacteria containing mostly plant pathogenic species which was named for the famous plant pathologist, Erwin Frink Smith. It contains Gram-negative bacteria related to ''Escherichia coli'', ''Shigella'' ...

'' are two such commonly found pathogens causing soft rots. In a dry rot, the storage organ becomes hard and dry, and in some diseases, there is rapid loss of water and the infected organs become shriveled, wrinkled, and leathery. Dry rots showing such symptoms are referred to as mummifications.

Necrosis in green tissues

Necroses on green tissue are termed differently based on the nature of symptoms and the type of green tissue. The term,

damping off Damping off (or damping-off) is a horticultural disease or condition, caused by several different pathogens that kill or weaken seeds or seedlings before or after they germinate. It is most prevalent in wet and cool conditions. Symptoms There ar ...

refers to the sudden wilting and topping over of seedlings as a result of extensive necrosis of tender tissue of the roots and stem near the soil line, due to the attack of soil-borne pathogens such as fungus, ''

Pythium ''Pythium'' is a genus of parasitic oomycetes. They were formerly classified as fungi. Most species are plant parasites, but ''Pythium insidiosum'' is an important pathogen of animals, causing pythiosis. The feet of the fungus gnat are frequent ...

''. This fungus is known to cause damping off in an assortment of seedlings such as that of

brinjal Eggplant ( US, Canada), aubergine ( UK, Ireland) or brinjal (Indian subcontinent, Singapore, Malaysia, South Africa) is a plant species in the nightshade family Solanaceae. ''Solanum melongena'' is grown worldwide for its edible fruit. Mo ...

chilli Chili or chilli may refer to: Food * Chili pepper, the spicy fruit of plants in the genus ''Capsicum''; sometimes spelled "chilli" in the UK and "chile" in the southwestern US * Chili powder, the dried, pulverized fruit of one or more varieties ...

mung beans The mung bean (''Vigna radiata''), alternatively known as the green gram, maash ( fa, ماش٫ )٫ mūng (), monggo, or munggo (Philippines), is a plant species in the legume family.Brief Introduction of Mung Bean. Vigna Radiata Extract G ...

tobacco Tobacco is the common name of several plants in the genus '' Nicotiana'' of the family Solanaceae, and the general term for any product prepared from the cured leaves of these plants. More than 70 species of tobacco are known, but the ...

tomato The tomato is the edible berry of the plant ''Solanum lycopersicum'', commonly known as the tomato plant. The species originated in western South America, Mexico, and Central America. The Mexican Nahuatl word gave rise to the Spanish word ...

, and ''

Cucurbita ''Cucurbita'' (Latin for gourd) is a genus of herbaceous fruits in the gourd family, Cucurbitaceae (also known as ''cucurbits'' or ''cucurbi''), native to the Andes and Mesoamerica. Five edible species are grown and consumed for their flesh and ...

''. A

spot Spot or SPOT may refer to: Places * Spot, North Carolina, a community in the United States * The Spot, New South Wales, a locality in Sydney, Australia * South Pole Traverse, sometimes called the South Pole Overland Traverse People * Spot (produ ...

refers to a well-defined area of gray or brown necrotic tissue. Spots are very common on leaves and fruits and are probably the most familiar necrotic symptom. Sometimes the necrotic tissue within a leaf spot may crack and fall off from the surrounding green tissue leaving an empty space. Such a symptom is known as a shot hole. Minute or very small spots are sometimes referred to as flecks or specks. When dark mycelia of a fungal pathogen appear on the surface of necrotic spot, blotting the leaves, shoots, an stems as large and irregular spots, the symptom is referred to as a

blotch A stain is a discoloration that can be clearly distinguished from the surface, material, or medium it is found upon. They are caused by the chemical or physical interaction of two dissimilar materials. Accidental staining may make material, mat ...

. Both streaks and

stripes Stripe, striped, or stripes may refer to: Decorations *Stripe (pattern), a line or band that differs in colour or tone from an adjacent surface *Racing stripe, a vehicle decoration *Service stripe, a decoration of the U.S. military Entertainment ...

occur in grasses and are elongated areas having dead cells. Streaks occur along the stem and veins, while stripes are in the laminar tissues between veins. Net necrosis is a symptom resulting from an irregular pattern of

anastomoses An anastomosis (, plural anastomoses) is a connection or opening between two things (especially cavities or passages) that are normally diverging or branching, such as between blood vessels, leaf veins, or streams. Such a connection may be normal ...

between streaks or stripes.

Blight Blight refers to a specific symptom affecting plants in response to infection by a pathogenic organism. Description Blight is a rapid and complete chlorosis, browning, then death of plant tissues such as leaves, branches, twigs, or floral organ ...

s are characterized by the rapid death of entire leaves including the veins or parts of the leaves. Blights also could occur on flowers and stems. Scorches resemble blights, but there necrosis occurs in irregular patterns between veins and along leaf margins. Firing is sudden drying, collapse and death of entire leaves. Firing occurs in response to the activity of root rot and vascular wilt pathogens.

Scald The structured computer-aided logic design (SCALD) software was a computer aided design system developed for building the S-1 computer. It used the Stanford University Drawing System (SUDS), and it was developed by Thomas M. McWilliams and Lawrence ...

is the blanching of epidermal and adjacent tissues of fruits and occasionally of leaves. The sudden death of unopened buds or inflorescence is referred to as

blast Blast or The Blast may refer to: * Explosion, a rapid increase in volume and release of energy in an extreme manner *Detonation, an exothermic front accelerating through a medium that eventually drives a shock front Film * ''Blast'' (1997 film) ...

. Extensive necrosis of fruits that resemble in premature dropping is called shelling.

Necrosis in woody tissues

Necrosis of woody tissue often brings about various types of die-back symptoms.

Dieback Dieback may refer to a number of plant problems and diseases including: * Forest dieback caused by acid rain, heavy metal pollution, or imported pathogens * The death of regions of a plant or similar organism caused by physical damage, such as from ...

is the extensive necrosis of a shoot from its tip downwards. Restricted necrosis of the bark and cortical tissue of stems and roots is termed as a

canker A plant canker is a small area of dead tissue, which grows slowly, often over years. Some cankers are of only minor consequence, but others are ultimately lethal and therefore can have major economic implications for agriculture and horticultur ...

. In cankers, necrotic tissue in the sunken lesions is sharply limited, usually by a callus from adjacent healthy tissue. When woody tissues are diseased, they may exude different kinds of substances. When the exudate is gummy, the symptom is called

gummosis Gummosis is the formation of patches of a gummy substance on the surface of certain plants, particularly fruit trees. This occurs when sap oozes from wounds or cankers as a reaction to outside stimuli such as adverse weather conditions, infections, ...

, while it is resinosis when it is resinous. If the exudate is neither gummy nor resinous, it is described as bleeding.

Abnormalities in growth

Many disease symptoms are associated with growth changes in diseased plants. These could be caused by either reduced growth due to hypoplasia and atrophy or excessive growth due to hyperplasia and hypertrophy.

Hypoplasia and atrophy

Hypoplasia Hypoplasia (from Ancient Greek ὑπo- ''hypo-'' 'under' + πλάσις ''plasis'' 'formation'; adjective form ''hypoplastic'') is underdevelopment or incomplete development of a tissue or organ.Atrophy Atrophy is the partial or complete wasting away of a part of the body. Causes of atrophy include mutations (which can destroy the gene to build up the organ), poor nourishment, poor circulation, loss of hormonal support, loss of nerve supply t ...

is the reduction in the size of plant cells produced. This also results in stunted plants or plant parts.

Dwarfing Dwarfing is a process in which a breed of animals or cultivar of plants is changed to become significantly smaller than standard members of their species. The effect can be induced through human intervention or non-human processes, and can include ...

is the failure of a plant or a plant part to attain its full size. Rosetting is a condition where the internode of a plant do not elongate, and hence, the leaves appear close together in a cluster.

Hyperplasia and hypertrophy

Hyperplasia Hyperplasia (from ancient Greek ὑπέρ ''huper'' 'over' + πλάσις ''plasis'' 'formation'), or hypergenesis, is an enlargement of an organ or tissue caused by an increase in the amount of organic tissue that results from cell proliferati ...

is the enlargement of a plant tissue due to excessive increase in the number of plant cells produced. Hyperplasia results in overdevelopment in size of plants or plant organs.

Hypertrophy Hypertrophy is the increase in the volume of an organ or tissue due to the enlargement of its component cells. It is distinguished from hyperplasia, in which the cells remain approximately the same size but increase in number.Updated by Linda J ...

is excessive growth due to the enlargement of individual cells. This condition also results in the overdevelopment in size of plants or plant organs. Hyperplasia and hypertrophy could result in the enlargement of leaves and fruits, and the enlargement of stems and roots.

Enlargement of leaves and fruits

Several symptoms expressing enlargement of leaves and fruits are commonly observed among diseased plants. Curling, which is the bending of the shoot or the rolling of the leaf, is a result of over-growth on one side of an organ. Often viral diseases cause such leaf distortions due to irregular growth of the

lamina Lamina may refer to: Science and technology * Planar lamina, a two-dimensional planar closed surface with mass and density, in mathematics * Laminar flow, (or streamline flow) occurs when a fluid flows in parallel layers, with no disruption betwee ...

. Leaf curl of papaya is caused by papaya leaf curl virus (a begomovirus). Extreme reduction of the leaf lamina brings about the symptom known as the shoe-string effect. The puckering or crinkling of leaves due to different growth rates in adjacent tissue is known as savoying. Overgrowth of

epidermal The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water relea ...

and underlying tissues of leaves, stems, fruits and tubers may result scab formation. Scab consists of raised, rough, and discrete lesions. These are often sunken and cracked, giving a typical scabby appearance. Localized swellings or enlargement of epidermal cells due to excessive accumulation of water or fungal structures is termed intermuscence and the diagnostic symptom is the appearance of a

blister A blister is a small pocket of body fluid (lymph, serum, plasma, blood, or pus) within the upper layers of the skin, usually caused by forceful rubbing (friction), burning, freezing, chemical exposure or infection. Most blisters are filled wi ...

Enlargement of stems and roots

Symptoms causing enlargement of stems and roots are termed differently based on their nature. Excessive accumulation of food material in stems, above a constricted area produces a swelling termed sarcody. Localized swellings that involve entire organs are termed tumefaction. Commonly exhibited tumefactions are

gall Galls (from the Latin , 'oak-apple') or ''cecidia'' (from the Greek , anything gushing out) are a kind of swelling growth on the external tissues of plants, fungi, or animals. Plant galls are abnormal outgrowths of plant tissues, similar to be ...

clubs Club may refer to: Arts, entertainment, and media * ''Club'' (magazine) * Club, a ''Yie Ar Kung-Fu'' character * Clubs (suit), a suit of playing cards * Club music * "Club", by Kelsea Ballerini from the album ''kelsea'' Brands and enterprises ...

, and

knots A knot is a fastening in rope or interwoven lines. Knot may also refer to: Places * Knot, Nancowry, a village in India Archaeology * Knot of Isis (tyet), symbol of welfare/life. * Minoan snake goddess figurines#Sacral knot Arts, entertainme ...

. Excessive development of adventitious organs results in

fasciculation A fasciculation, or muscle twitch, is a spontaneous, involuntary muscle contraction and relaxation, involving fine muscle fibers. They are common, with as many as 70% of people experiencing them. They can be benign, or associated with more serio ...

, that is the clustering of organs around a focal point. Such examples include witch's broom and hairy root.

Witch's broom Witch's broom or witches' broom is a deformity in a woody plant, typically a tree, where the natural structure of the plant is changed. A dense mass of shoots grows from a single point, with the resulting structure resembling a broom or a ...

is a broom-like mass proliferation due to the dense clustering of branches of woody plants while

hairy root Hairy root culture, also called transformed root culture, is a type of plant tissue culture that is used to study plant metabolic processes or to produce valuable secondary metabolites or recombinant proteins, often with plant genetic engineering. ...

results due to excessive development of roots.

Fasciation Fasciation (pronounced , from the Latin root meaning "band" or "stripe"), also known as cresting, is a relatively rare condition of abnormal growth in vascular plants in which the apical meristem (growing tip), which normally is concentrated aro ...

is the broadening or flattening of cylindrical organs such as stems. The continued development of any organ after it has reached a stage beyond which it normally does not grow is known as proliferation. The outgrowth of tissue in response to wounding is known as a

callus A callus is an area of thickened and sometimes hardened skin that forms as a response to repeated friction, pressure, or other irritation. Since repeated contact is required, calluses are most often found on the feet and hands, but they may o ...

. Callus formation is found to form around most cankers.

Metaplastic symptoms

Metaplastic symptoms are those which form when tissues change from one form to another. Such symptoms include

phyllody Phyllody is the abnormal development of floral parts into leafy structures. It is generally caused by phytoplasma or virus infections, though it may also be because of environmental factors that result in an imbalance in plant hormones. Phyllo ...

, the development of floral organs into leaf-like structures, juvenillody, the development of juvenile seedlings on mature plants and russeting, a superficial browning of surfaces of fruits and tubers due to suberization.

Proleptic symptoms

Proleptic symptoms result from the development of tissues earlier than usual. Examples include prolepsis, the premature development of a shoot from a bud, proleptic abscission, the premature formation of

abscission Abscission () is the shedding of various parts of an organism, such as a plant dropping a leaf, fruit, flower, or seed. In zoology, abscission is the intentional shedding of a body part, such as the shedding of a claw, husk, or the autotomy of a ...

layers and restoration, the unexpected development of organs that are normally rudimentary.

Color changes

Changes in the color of plant tissue are a common symptom of plant disease. Often these color changes are brought about by the

yellowing Jaundice, also known as icterus, is a yellowish or greenish pigmentation of the skin and sclera due to hyperbilirubinemia, high bilirubin levels. Jaundice in adults is typically a sign indicating the presence of underlying diseases involving ...

of normal green tissue due to the destruction of

chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words , ("pale green") and , ("leaf"). Chlorophyll allow plants to a ...

or a failure to form chlorophyll. Such repression of leaf color may be complete or partial. When color repression is complete, it is known as albication. However, the more common, partial repression is referred to as

chlorosis In botany, chlorosis is a condition in which leaves produce insufficient chlorophyll. As chlorophyll is responsible for the green color of leaves, chlorotic leaves are pale, yellow, or yellow-white. The affected plant has little or no ability to ...

. Patches of green tissue alternating with chlorotic areas are described as a

mosaic A mosaic is a pattern or image made of small regular or irregular pieces of colored stone, glass or ceramic, held in place by plaster/mortar, and covering a surface. Mosaics are often used as floor and wall decoration, and were particularly pop ...

. Mosaic is a symptom caused by many viruses. Based on the intensity and the pattern of discoloration, mosaics are termed differently. Irregular patches of distinct light and dark areas are known as

mottling Mottle is a pattern of irregular marks, spots, streaks, blotches or patches of different shades or colours. It is commonly used to describe the surface of plants or the skin of animals. In plants, mottling usually consists of yellowish spots o ...

. Streaking and ring spots are still other distinct types of discolorations. Ring spots are circular masses of chlorosis with a green center. Vein clearing and vein banding are yet other common color changes on leaves. Chlorophyll may also develop in tissues normally devoid of it. Thus usually white or colored tissue becomes green in color. This is called as

virescence Virescence is the abnormal development of green pigmentation in plant parts that are not normally green, like shoots or flowers (in which case it is known as floral virescence). Virescence is closely associated with phyllody (the abnormal developmen ...

. Anthocyanescence is due to the overdevelopment of anthocyanin and result in the development of a purplish coloration. Color changes can also take place in flowers. Such an example is the color break virus-affected tulips.

Wilts

Wilting Wilting is the loss of rigidity of non-woody parts of plants. This occurs when the turgor pressure in non-lignified plant cells falls towards zero, as a result of diminished water in the cells. Wilting also serves to reduce water loss, as it m ...

is due to loss of

turgor Turgor pressure is the force within the cell that pushes the plasma membrane against the cell wall. It is also called ''hydrostatic pressure'', and is defined as the pressure in a fluid measured at a certain point within itself when at equilibriu ...

in plant tissue resulting in the drooping of plant parts. They are common symptom in diseases where the pathogen or the toxic metabolites it produces affects the

vascular tissue Vascular tissue is a complex conducting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally. There ...

of the host plant. Interference in water transport brought about by the infection of these vascular pathogens leads to wilting. Unlike wilting due to low

soil moisture Soil moisture is the water content of the soil. It can be expressed in terms of volume or weight. Soil moisture measurement can be based on ''in situ'' probes (e.g., capacitance probes, neutron probes) or remote sensing methods. Water that enters ...

, wilting due to the activity of these pathogens cannot be overcome by watering the plants. Infected plants eventually die.http://global.britannica.com/EBchecked/topic/644850/wilt

References

External links

International Society for Plant Pathology

Australasian Plant Pathology Society

American Phytopathological Society

British Society for Plant Pathology
* Food Security Journal
Contributions toward a bibliography of peach yellows, 1887–1888
Digital copy of scientist Erwin Frink Smith's manuscript on peach yellows disease.
Erwin Frink Smith Papers
Index to papers of Smith (1854–1927) who was considered the "father of bacterial plant pathology" and worked for the United States Department of Agriculture for over 40 years.
Plant Health Progress, Online journal of applied plant pathology

Pacific Northwest Fungi, online mycology journal with papers on fungal plant pathogens

Rothamsted Plant Pathology and Microbiology Department

New Mexico State University Department of Entomology Plant Pathology and Weed Science

Pathogen Host Interactions Database (PHI-base)

Grape Virology
{{Plant disorders Agronomy Pathology