Symptoms are the manifestation of diseases expressed as a result of successful alterations in normal metabolic reactions occurring in the body of the host due to effect of pathogen-activities. In this way, symptoms refer only to the appearance of diseased plants or plant tissues. However, in most of the diseases, the pathogens grow and produce a variety of structures like mycelium, sporophores, spores, fructifications, etc. on the infected surface of the host. These structures are called “signs” and are quite distinct from symptoms.
In the mildews, for example, one sees mostly the “signs” consisting of a characteristic growth of mycelia, sporophores, spores, and fruiting bodies onto the “symptoms”, which consist of merely the chlorotic or necrotic areas developed on the surface of the infected parts of the host plant. For any disease in a given species of plants, there are characteristic symptoms usually occurring in a certain sequence during the course of disease-development. Such a series of symptoms is generally called the “syndrome”.
Most of the diseases of plants are manifested by means of their characteristic symptoms, which are clearly evident on the body of the host. Such symptoms are termed “morphological symptoms”. But, there are pathogens (certain viruses), which infect their hosts without ever causing development of morphological symptoms on them. Here, the symptoms are exhibited by individual cells and are called “cytological symptoms” or “internal symptoms”.
The viral pathogens that cause cytological symptoms are usually called “latent viruses” and the respective hosts are called “symptomless carriers”. Also, there are cases where plants usually develop symptoms after being infected with certain viruses but, they become temporarily symptomless under certain environmental conditions. Such symptoms are called “masked symptoms. However, only morphological symptoms are taken into account. Generally the pathogens cause three main types of morphological symptoms.
They are:
(1) Necrosis (killing of plant tissues),
(2) Hypertrophy and hyperplasia (over-development), and
(3) Hypotrophy and hypoplasia (under-development; stunting).
Symptom # 1. Necrosis:
Necrosis death of cells, tissues and organs as a result of parasitic activity is the commonest and most destructive type of effect a pathogen renders onto the infected plant or on its parts. Almost all types of pathogens (fungi, bacteria, viruses, and so on), cause necrosis on their hosts. However, in some cases, the pathogens bring destructive necrotic effect on host which quickly leads to the killing of host tissues. But in many other cases, the pathogens primarily do not kill the host tissues; the killing is evidenced during late stage.
Necrotic symptoms are the common viral symptoms occurring on plants in localized areas. The necrotic areas are generally differentiated by the presence of dark brown border around them. Necrosis in viral diseases may remain localized but, sometimes, it spreads as long streaks producing systemic necrosis resulting in the death of the whole plant. One strain of TMV develops streaks on the stems and causes complete necrosis of leaves and fruits resulting in the death of the infected tomato plant.
Besides viral necrotic symptoms, the fungal and bacterial pathogens cause a variety of necrotic symptoms on their hosts.
Some important ones are as follows:
i. Spot:
Spots are probably the most familiar necrotic symptoms occurring on green leaves, floral parts, and fruits. They represent well-defined localized (restricted) areas of usually brown or tan necrotic tissue that may be surrounded by margins of purple or some other dark colour. Tiny spots are called “flecks” or “specks”.
When dark threads of a fungal pathogen can be seen on the surface of a spot, the symptom is called “blotch” which is, therefore, both a symptom of a disease and a sign of the pathogen. Blotch is characteristic of the black spot disease of rose.
Fungal leaf-spots are generally of two types – tar-spots and shot-hole spots. Tar spots appear as raised and black-coated giving the appearance of a flat drop of tar on leaves and are characteristically caused by Phyllachora and Rhytisima fungal pathogens. The shot hole spots are those in which the dead and collapsed tissue in localized lesion shrink and fall off from the healthy ones after sometime leaving a hole. These spots are characteristically caused by Cercospora and some other pathogens.
In bacterial leaf-spot, the bacteria invade the host leaf through stomata and cause necrosis of tissues around the sub-stomatal spaces resulting in the appearance of leaf spots on leaf surface. These spots generally appear brown or water-soaked and usually remain localized. Example – leaf spot of mango (Pseudomonas mangiferae-indicae).
ii. Blight:
Blight (burnt appearance) represents general and rapid browning of infected plant parts primarily and then its rapid and extensive death. The dead tissues soon disintegrate and give burnt or scorched appearances. Examples – late (Phytophthora infestans) and early (Alternaria solani) blight of potato, and bacterial blight of rice (Xanthomonas campestris pv. oryzae).
iii. Blast:
Blast represents the sudden death of unopened buds and inflorescence; the blasted organs are usually shed off the plant.
iv. Scorch:
Scorch resembles blight, but the scorch occurs in irregular patterns usually along the margins of leaves and, sometimes, between the veins. Example of scorch is elm leaf scorch disease caused by fastidious vascular bacteria.
v. Scald:
Scald is the blanching of epidermal and adjacent tissues of fruit and, occasionally, of leaves; example – plum leaf scald disease caused by fastidious vascular bacteria.
vi. Canker:
Canker (dead area in the bark or cortex of the infected plant) Fig. 7.3 represents a localized wound or necrotic lesion of the bark that is often sunken beneath the surface of the stem or twig of woody plants. Cankers generally begin at a wound or at a dead stub. From that point, they expand in all directions but much faster along the main axis of the stem, branch, or twig. Examples are Nectria canker disease of apple, Cytospora canker of peach, and citrus canker (Xanthomonas campestris pv. citri).
vii. Die-Back:
Severe necrosis of twigs or branches beginning at tip and advancing towards their bases is called “die-back” or “stag-head”. The symptom frequently becomes apparent during the dormant season or shortly thereafter. Example is die-back disease of chillies (Colletotrichum capsici).
viii. Damping-Off:
Damping-off is the symptom referring to rapid death and collapse of seedling tissues near the soil-line resulting in the fall of the seedlings on the ground. Example is damping- off disease of seedlings (Pythium spp.).
ix. Anthracnose:
It is a sunken ulcer-like lesion of necrotic cells on the infected pan of the host plant. Example – mango anthracnose (Colletotrichum gloeosporioides).
x. Streak and Stripe:
Streak represents elongated necrotic area along stems and leaf-veins. Stripe is also elongated necrotic area but confined to leaf-tissues between the veins, particularly in monocotyledons.
xi. Ring Spots:
Ring spots are the common type of symptoms produced by viral pathogens. They are characterized by the appearance of sometimes chlorotic but mostly necrotic rings on the leaves or also on the fruits and stem. The ring spots may be present singly or in groups developing concentrically on infected parts of the host plant. Examples are tobacco ring spot disease (tobacco ring spot virus) and tomato ring spot disease (tomato ring spot virus).
xii. Rot:
Rots represent dead and decomposed tissue on various parts of the host plant as a result of maceration and disintegration caused under the influence of various enzymes secreted by the pathogen. Though the rotting may result due to non-parasitic causal agencies, the great majority of rots are caused by fungal or bacterial pathogens.
Rots may be grouped according to the structures or organs the pathogens attack thereupon. They are – root rot (Fig. 7.4C; root rot of turmeric by Pythium aphanidermatum); foot rot (foot rot of citrus trees by Phytophthora parasitica or P. citrophthora); stem rot (Fig. 7.4C; stem rot of paddy by Leptosphaeria salvini); bud rot of palms by Phytophthora palmivora; fruit rot (Fig.7.4D; example, fruit rot of cucurbits by Pythium aphanidermatum); rhizome rot (rhizome rot of ginger by Pythium spp.); ear rot (yellow ear rot or tundu disease of wheat by Corynebacterium titici’); leaf rot (black rot of crucifers by Xanthomonas campestris pv. campestris), and; tuber rot (Fig 7.4E; brown rot of potato by Pseudomonas solanacearum).
Besides, the rots may be dry or soft. Rots of woody and pithy stems are often dry (example – red rot of sugarcane and trees rots;) whereas the rots on pulpy fruits and vegetables are usually wet hence soft. Example is bacterial soft rots of vegetables by Erwinia spp.
xiii. Wilt:
Wilt is usually a generalized secondary symptom in which leaves and/or shoots lose their turgidity, become flaccid and droop (wilt) because of plugging of xylem vessels of the root or stem either by the pathogen itself, or by substances secreted by it. Examples are wilt disease of arhar (Fusarium oxysporum f.sp. udum), bacterial wilt of cucurbits (Erwinia tracheiphila) and bacterial wilt of solanaceous crops (Pseudomonas solanacearum).
xiv. Scab:
Scabs are rough, crust-like localized lesions usually slightly raised or sunken and cracked giving a scabby appearance. Though they can occur on every part of the host but are frequently, evident on fruits and vegetables. Example is scab disease of apple (Venturia inaequalis).
xv. Rust:
Rusts are the small pustules of spores, usually breaking through the host epidermis. However, they appear as relatively small local areas of various colours on leaves and/or stems usually imparting rusty appearance. The pustules contain enormous number of spores which come out on the surface usually by breaking the epidermis. Plant rusts, caused by basidiomycetous fungal pathogens of the order uredinales, are among the most destructive plant disease, and result in formation of swellings and even galls. Examples are black rust of wheat (Puccinia graminis tritici) and rust of linseed (Melampsora lini).
xvi. Smut:
Smuts refer to shooty or charcoal-like powdery mass usually appearing on floral organs, particularly the ovary, though also on leaf, stem, and root. They are caused by basidiomycetous fungal pathogens belonging to the order ustilaginales. Smut infected tissues are either destroyed or replaced by black smut spores or they are first stimulated to divide and enlarge to produce a swelling or gall of varying size and then destroyed and replaced by smut spores. Example, loose smut of wheat (Ustilago tritici), corn smut (U. maydis), covered smut or bunt of wheat (Tilletia carries and T.foetida) and kernel smut of rice (Neovossia barclayana).
xvii. Mildew:
Mildews represent the chlorotic or necrotic areas usually covered with pathogens as a result of their vigorous growth thereupon. Two types of mildews, “downy” and “powdery”, are popular.
A downy mildew is one in which the chlorotic or necrotic areas on the host surface are covered with extensively growing sporan giophores and sporangia of the pathogen grown in downy (like down, i.e., soft hairs, soft feathery hairs of birds) fashion; the mycelia remain confined internally under the host surface.
The downy mildews are primarily foliage blights that attack and spread in young, tender green tissues including leaves, twigs, and fruits. The characteristic downy mildews are caused by a group of oomycetous fungal pathogens belonging to family peronosporaceae. Examples are downy mildew of peas (Peronospora pisi), downy mildew of grapes (Plasmopara viticola), downy mildew of crucifers (Peronospora parasitica), and downy mildew of maize (Sclerospora sacchari), etc.
A powdery mildew is one in which the chlorotic or necrotic areas are covered with superficially growing mycelia and fructifications of the pathogen giving a dusty or powdery appearance usually of white to gray colour. Powdery mildews are probably the most commonly occurring widespread symptoms generally on all kinds of plants – cereals and grasses, vegetables, ornamentals, trees, shrubs, weeds, etc. Powdery mildew causing fungal pathogens belongs to family erysiphaceae of class ascomycetes and is obligate parasites. Examples are powdery mildew of grape (Uncinula necator), powdery mildew of peas (Erysiphe polygoni), and powdery mildew of apple (Podosphaera leucotricha).
Symptom # 2. Hypertrophy and Hyperplasia (Over-Development):
In some diseases caused by fungi and bacteria, affected cells get stimulated by pathogen-activities and abnormally enlarge in size (hypertrophy) or abnormally increase in number as a result of rapid cell divisions (hyperplasia). Such hypertrophied or hyperplasic cells result in the development of usually abnormally large, or abnormally proliferating organs, or the production of amorphous outgrowths on normal looking organs. Cells so stimulated not only divert much of the available foodstuffs to themselves and away from the normal tissues but damage adjacent normal tissues by their excessive growth and thus interfere with the physiological activities of the plant.
However, the most common hypertrophy and hyperplasia symptoms are as follows:
i. Galls/Tumours:
Galls/tumours represent elongated portions of plants usually filled with fungal mycelium. They are often produced on herbaceous parts of plants and also sometimes on woody stems and on roots. Formation of galls/tumours is accomplished by hypertrophy and hyperplasia. Examples are stem gall of coriander (Protomyces macrosporus), club root of crucifers (Plasmodiophora brassicae), and crown gall disease (Agrobacterium tumefasiens).
ii. Warts:
Warts are the protuberances developed on tubers and stems generally giving warty or discoloured cauliflower-like appearances. Good example is wart disease of potato (Synchytrium endobioticum).
iii. Leaf Curls:
Parts of or entire infected leaves are thickened, swollen, distorted, and curled downward and inward as a result of hypertrophy and hyperplasia mainly confined on one side of the infected part. Peach leaf curl disease (Taphrina deformans) is a typical example.
iv. Witches’ Broom:
Witches’ broom is a special type of over-development in which a large number of slender branches arise from a localized (restricted) area and run upward more or less parallel to each other giving the appearance of a broom. Witches’ broom development is in contrast with horizontal growth habit of normal shoot. Witches’ broom are caused by certain fungal pathogens (Taphrina, some rusts), bacteria (Corynebacterium fasciens), and viruses but, they are characteristic to plant diseases caused by phytoplasmas (= MLOs)
v. Enations:
Enations are the outgrowths generally occurring in veins or midrib on the lower surface of leaves. They may be small, large, papillate, or spine-like in shape and also vary in number. In some cases like pea enation mosaic virus, the outgrowths develop between or adjacent to veins and look like leaves, funnels, wings, cups, etc.
Some infected plants develop elongated internodes and become abnormally tall (rice infected with Gibberella fujikuroi); the floral parts of bajra infected with Sclerospora graminicola get proliferated; infected inflorescence of Brassica campestris with Albugo candida becomes large, green and fleshy with stamens converted into leafy structures.
Symptom # 3. Hypotrophy and Hypoplasia (Under Development):
In some diseases, infected cells and tissues slowdown in development resulting in the infected plant parts thereof are being arrested in their growth as a result of degeneration of the cells (hypotrophy) or subnormal cell divisions (hypoplasia) under the influence of the pathogen.
Though apparent in some diseases caused by fungal and bacterial pathogens, the symptoms due to under-development of cells and tissues occur most commonly in diseases caused by viruses and phytoplasmas (= MLOs).
“Atrophy”, however, represents the extreme case of under-development in which the infected plant parts fail to develop at all.
i. Chlorosis:
Chlorosis (weakened green colour) is a general term used to denote destruction or inhibition of formation of chlorophyll subsequent to infection and usually occurs when young growing leaves are infected. Chlorotic symptom experiencing cells contain less chlorophyll thus appearing pale green or yellowish in colour. Extreme case of chlorosis is “blanching” or “bleaching” in which chlorophyll content in infected areas completely disappears. Though evident in some diseases caused by different group of pathogen, chlorotic symptoms occur frequently in viral diseases.
Variety of chlorotic symptoms, however, is evident in diseases caused by viruses; they are:
Mosaic – irregular intermingling of normal green with light green, yellow, or white chlorotic areas imparting a mosaic-like pattern (tomato mosaic, mosaic of bhindi, etc.); mottle – this chlorotic symptom is closely related to mosaic and is applied when the discoloured areas are round (peanut mottle disease); vein-chlorosis and vein-clearing – the former represents chlorosis restricted to veins while the latter represents the translucence of veins; vein-banding – represents the symptoms in which the chlorotic areas occur along the veins in a regular manner interrupted with green-coloured (non-chlorotic) bands.
ii. Leaf Narrowing:
Here the infected leaves generally become narrow due to reduced growth of laminar tissue but the veins and midrib remain normal in growth. In extreme cases only the main veins remain normal while laminar tissue is almost completely absent. This forms “fern leaves” or “shoestring” like leaves as happens in tomato infected with TMV.
iii. Leaf Curling:
These symptoms represent irregular and extensive wrinkling and furrowing of leaves due to reduced growth of veins in comparison to the growth of laminar tissue resulting in sunken veins and raised up laminar tissue leading to the curling of the leaves. Examples are leaf curl diseases of papaya and tomato.
iv. Stunting or Dwarfing:
These symptoms represent the cases wherein the infected plants show nearly proportional growth reduction in all its organs. Such plants almost remain normal in their morphology but become dwarf or stunted. Examples are barley yellow dwarf disease and chrysanthemum stunt disease.
v. Rosetting:
In rosetting, the internodes of the infected plants become shortened due to reduced growth. This brings the leaves together at the top of the branches giving rosette-like appearance. Examples are dwarf bunt of wheat (Tilletia contraversa), and tobacco rosette complex disease.
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