In this article we will discuss about the general symptoms of plant diseases.
The changes in the host plant which serve to recognise the disease are called the signs and symptoms of the disease. The sign of a disease is the external appearance of some portion of the pathogen of the host. It may be some portion of the mycelium of the parasite or some spore stage.
The best examples are the rusty uredosori and black teleutosori of the Stem Rust of wheat, the smut spore stage of Ustilago and white blisters of white rusts (Albugo). These stages of the pathogens are at first covered but become exposed as the spores mature.
The symptoms are the visible effects which the parasite induces on the host plant. Any visible deviation on the host plant (both physiological and morphological) from the normal in structure and function is called a symptom. Generally the symptoms are growth responses.
These are induced by the causal agent operating on the host. They furnish clues to find out the nature of the disease. Symptoms may affect the whole plant or be restricted to a particular organ or parts of an organ. They are, as a matter of fact, the danger signals of a disease.
The modem plant pathologists do not make any distinction between the two terms (signs and symptoms). They club them together as symptoms of a disease and define the term as external signs on the host plant which are characteristic of a given disease.
These are the result of interaction between the host and the pathogen.
They divide the symptoms of a disease into two categories:
A. Symptoms due to the external appearance of the pathogen or some portion of the pathogen on the host (signs):
The somatic phase in most of the pathogens is usually invisible. It lies within the tissues of the host plant. During the disease cycle some portion of the parasite such as the reproductive or resting structures become visible by rupturing the overlying tissues of the host.
Some pathogens, however, are ectoparasites. In their case both the somatic and reproductive structures are visible externally and furnish a clue to the diagnosis of the disease.
Common examples of such symptoms (signs) are:
1. Mildews:
These are a group of important fungal diseases of seed plants in which the parasite is seen as a superficial growth on the host surface (leaves, green stems and fruits) in the form of patches of varying sizes and colours.
The mildews are of two kinds, downy mildews and powdery mildews. The downy mildews are all internal obligate parasites. They are characterised by superficial downy growth consisting of conidiophores and conidia on the host lesions in damp and warm weather.
The powdery mildews are external parasites in which the mycelium forms whitish patches on the surface of the leaves of the host plant. The patches appear dusty or powdery with the formation of numerous white conidia which form a coating on the host surface.
2. Smuts:
These are the fungal diseases of cereals and other members of grass family that cause the ears (particularly the ovaries) to turn black or sooty. The smut symptoms may as well appear on other parts (leaves, stem or roots) of the host plant.
3. Rusts:
These are fungal diseases of grasses and other plants which appear on the host surface as small, coloured pustules-red, brown, yellow, orange, or black in colour.
4. Sclerotia:
The conspicuous phase of some fungal diseases such as Ergot of rye is the formation of sclerotia in the position of kernels in the spike. The sclerotium is a compact, hard mass of dormant fungal hyphae. It may be black, greyish violet, dark brown or purplish in colour.
5. White blisters:
These are white, shining, blister-like pustules found on the leaves of cruciferous plants caused by Albugo Candida, the white rust. These pustules expose the white, powdery masses of spores.
B. Symptoms which are the visible effects induced by the parasite on the host plant:
These are grouped under three categories necrosis, hypoplasia and hypertrophy. The other important symptoms are wilts, and damping off.
1. Necrosis:
Death or killing of the host tissues induced by the attack of a pathogen is called necrosis. It may be caused by rots, blights, wilts, die back and cankers and may be general or local, gradual or rapid. When necrosis is general it is called decay and rotting.
The causal organism may be a fungus or a bacterium. Rotting is universal in plants. It takes a heavy toll of useful plants. The rots are of many kinds. The common ones are dry rots, wet rots, soft rots, hard rots, brown rots, and black rots. These rots cause general necrosis and progress slowly or rapidly.
(a) Lesions:
In many cases necrosis is limited in extent. It is confined to small areas and is called local necrosis. It results in localised spots of diseased tissues. These are called the lesions. The tissue in the spot or the lesion area is dead. Examples of local necrosis are leaf and fruit spots.
They are local lesions caused by fungi, bacteria and viruses. The spots are minute, circular or sub-circular and sometimes angular in outline. They are of different colours such as brown, dark, orange, red or white.
The colour of the spot depends upon the nature of fungal spores present in the area. The leaf spot causes reduction in leaf surface and sometimes defoliation. The dead tissue of the leaf spot may fall out leaving circular or irregular perforations. These are called shot holes. The leaf shot holes are sharply bordered.
(b) Cankers:
Local necrosis also results in open wounds often sunken in stems and surrounded by living tissue. These are called the cankers. Canker is a dead lesion. Sometimes it is surrounded by a raised margin.
Cankers may be due to slow rots of the outer parts of herbaceous and woody stems. They are thus usually limited in size. They may also be restricted in extent due to the formation of cork around and edges of the wound in the woody plants.
When the canker partly encircles the branch its growth is subnormal. The branch is underdeveloped. If the canker completely encircles the branch the part of the branch beyond the canker dies.
(c) Blight:
In many cases the leaves, stems or twigs, in response to the attack of the pathogen undergo rapid discolouration followed by death. The dead parts become dark or brown in colour. This condition of the affected organ is called the blight.
(d) Rot:
It is a disease in which the affected tissues die and undergo decay. The rot diseases may be classified on the basis of the organ attacked such as leaf, stem or root rots, bud rot and fruit rot. On the basis of type of dissolution brought about by the causal agent, the rots may be classified as soft rot, dry rot, black rot and wet rot.
2. Hypoplasia:
It is a subnormal cell production in response to the attack of the pathogen. It results in the subnormal growth of the parts attacked resulting in stunted growth and dwarfing of the host plant.
3. Hypertrophy (overgrowths):
It is an abnormal increase in size of one or more organs of a plant in response to the attack of a pathogen. It results in abnormal growth causing distortions, swellings, leaf curls and galls. The excessive growth may be due to two processes, hyperplasia and hypertrophy.
The former consists in rapid cell division and thus increase in the number of cells of which the organ is composed. Hypertrophy leads to abnormal increase in the size of cells only.
The attack of Com smut stimulates abnormal and rapid cell division (hyperplasia) and enlargement of individual cells (hypertrophy) of the infected organs resulting in the formation of overgrowths called the corn galls or tumours (Fig. 22.6 E).
Witches broom is another result of hypertrophy. It is a compact cluster of fine slender branches generally developing from an enlarged
axis and the whole looking like a broom (Witches broom of Cherry). The hypertrophied parts become inedible.
4. Wilts:
The drooping of the entire plant due to loss of turgidity is a common symptom of disease. The causes may be varied. The wilting due to a disease is permanent and eventually leads to death of the plant.
5. Damping off:
Species of Pythium and Rhizoctonia are most important among the fungi which cause damping off disease of seedlings. Either the stem is attacked near the soil level or the crown of roots. The attacked region becomes weak and thus is not able to bear the load of the upper portion of the seedling.
Consequently the seedling collapses. It topples down and dies. The seedling of many plants such as chilli, tobacco, tomato and mustard are prone to the damping-off disease.
