Symptoms of Diseases: Meaning and Classification

In this article we will discuss about the meaning and classification of symptoms.

Meaning of Symptoms:

The manifestation of a disease is known as symptom. After successful penetration, the pathogen colonises on the host, as a result many structural and physiological changes occur within the host, and finally the expression of the disease i.e., the symptom develops.

The symp­tom may be external or internal. The pathogen which is visible externally on the host surface is the sign. Sometimes disease caused by an organism may produce more than one type of symptom and they are collectively referred to as syndrome (wilt of pigeon pea, c.o. Fusarium udum).

Classification of Symptom:

In most parasitic diseases, the growth of the pathogen takes place inside the host tissue, thus the parasite is invisible. The expression of the disease in different shapes, colours etc., are visible from outside.

Symptoms are classified into three categories:

1. Necrotic,

2. Atrophic and

3. Hypertrophic.

1. Necrotic Symptoms:

These symptoms are evidenced by the death (necrosis) of cells, tissues or organs of the infected host due to parasitic or non-parasitic causal agencies.

The common and important necrotic symptoms produced by the different types of parasitic causal agents are:

a. Spots,

b. Anthracnose,

c. Blight,

d. Scald,

e. Die- back,

f. Streak or Stripe,

g. Canker,

h. Damping off,

i. Rot,

j. Blotch, and

k. Blast.

(a) Spots:

When necrotic region develops on the host surface, formed by killing of tissue in a limited area, it is called spot. The spots may be circular, angular or irregular in outline (Fig. 5.4A-C). Diffe­rent types of leaf spot diseases are found in plants, e.g., brown spot of rice (Helminthosporium oryzae), ring spot of sugarcane (Leptosphaeria sacchari), etc.

(b) Anthracnose:

These are more or less angular and elongated spots which appear on veins of the lower surface of leaf, and may spread on the upper sur­face in severe cases (Fig. 5.4E). Stem, petioles and fruits (Fig. 5.4D) may also become affected during severe infec­tion, e.g., anthracnose of bean (Colletorichum lindemuthianum), anthracnose of mango (C. gloeosporioides) etc.

(c) Blight:

Blight can be defined as the rapid killing of different plant parts, such as leaves, blossoms etc., and the killed tissues may become slimy and often emit pungent odour, e.g., late blight disease of potato (Phytophthora infestans) (Fig. 5.4F), early blight of potato (Alternaria solani) (Fig. 5.4G) etc.

(d) Scald:

A white or pale appearance of leaves and/or fruits resembling that developed by scalding water is referred to as scald, e.g., leaf scald of sugarcane (Xanthomonas albilineans).

(e) Die-back:

The symptom appears due to gradual dying of twigs from apex towards the base due to attack of pathogen, e.g., ripe fruit rot and die- back of chilli (Colletotrichum capsici).

(f) Streak or Stripe:

It appears as narrow elongated, hydrotic, necrotic or other type of lesions on leaf veins or stems, e.g., bacterial leaf streak of rice (Xantho­monas campestris pv. oryzicola).

(g) Canker:

It appears as sunken necrotic areas present in the bark and cortex of woody stems, on leaves and fruits (Fig. 5.4H). Sometimes the diseased portion becomes separated from the healthy region by cracks. The callus formation may take place at the healthy margin, e.g., citrus canker (Xanthomonas axonopodis).

(h) Damping off:

The pathogen attacks the stem of seedlings at the ground level and causes toppling down. Wet-rot type lesions appear at the infected region, e.g., damping off of tobacco seedling (Pythium aphanidermatum), damping off of jute seedling (Rhizoctonia bataticola). In addition to the above, other fungi like Fusarium and Phytophthora also cause damping off diseases.

(i) Rot:

It is a type of necrosis, where the dead tissues are in a more or less advanced stage of disintegration in the infected region. It may be caused by parasitic (bacteria-soft rot of potato, c.o. Erwinia carotovora; bacterial stalk rot of maize, c.o. E. dissolvens or fungi-red rot of sugarcane, c.o. Colletorichum falca- tum; stem or foot rot of papaya, c.o. Pythium aphanidermatum) or non­parasitic causal agents (adequate soil moisture causes blossom-end rot of tomato).

The nature of rot depends on the nature of causal agents, structure of the infecting organ, and external condi­tions. Based on the nature, it is of two types white rot and brown rot; based on the texture it is of two types: soft rot and dry rot and based on the organ affected, it is of many types: foot rot, root not, bud rot, leaf rot, heart rot, sap rot and fruit rot.

(j) Blotch:

It appears as superficial dis­colouration on leaves or on fruits in which slight superficial injury is usually visible by the presence of the fungus, e.g., sooty blotch disease of apple.

(k) Blast:

This symptom appears due to sud­den death of inflorescence, young buds, young fruits and region of leaf showing burning at some of the region, e.g., blast disease of rice (Pyricularia grisea).

2. Atrophic or Hypoplastic Symptom:

This type of symptoms manifest due to slowing down in development of the affected tissue or organ. The underdevelopment takes place due to hypoplasia i.e., subnormal cell divi­sion or rarely due to degeneration of cells.

Some important types of atrophic symptoms are:

a. Chlorosis,

b. Dwarfing,

c. Vein- clearing,

d. Wrinkling etc.

(a) Chlorosis:

It appears as yellow or lighter green in colour either due to degradation of chlorophyll or its synthesis at a slower rate. Chlorosis may be systemic (Aster-yellows) or it may be of different patterned (Mosaic). In Mosaic, light green areas or yellow regions alternate with deep green areas in various patterns, e.g., yellow vein mosaic of lady’s finger (Abelmoschus esculentus).

(b) Dwarfing:

Some organ or the entire plant becomes reduced in size due to infection, e.g., little leaf of brinjal (Solanum melongena), c.o. Mycoplasma like organism.

(c) Vein-Clearing:

The green colour fails to develop along the veins, thus appearing translucent. This is very common in virus infection.

(d) Wrinkling:

This is very common symp­tom associated with mosaic, where overgrowth of the green region and reduced growth of the chlorotic region cause wrinkling.

3. Hypertrophic or Hyperplastic Symptoms:

The symptoms are evident by the over­growth of tissue or organ due to enlargement of cells (hypertrophy) or due to increase in cell number by more divisions of cells (hyperplasia). Some important hypertrophic symptoms are: a. Galls, b. Curl, c. Hairy roots, d. Witches broom etc.

(a) Galls:

Galls are the abnormal, localised swelling or outgrowth, developed on the infected region of the host plants. Galls of larger size are called excres­cence or knot, while those of the smaller sizes are the wart or tubercle, e.g., wart disease of potato (c.o. Synchytrium endobioticum) (Fig. 5.5A).

(b) Curl:

This symptom appears due to localised overgrowth on one side or certain region of the leaves or shoots which become curled, arched or distor­ted, e.g., peach leaf curl disease (c.o. Taphrina deformans) (Fig. 5.5B).

(c) Hairy Roots:

Huge numbers of compact­ly arranged fine fibrous roots are deve­loped due to infection.

(d) Witches Broom:

More branches with vertical growth are developed due to infection. The branches grow vertically instead of normal horizontal pattern. Sometimes the branches and leaves become swollen.

Sign:

In some parasitic diseases, the patho­gen is partially or wholly visible with naked eye and the diseases are identified by seeing the structure of the pathogen.

Some important signs are:

a. Mildews,

b. Rusts, and

c. Smuts, etc.

(a) Mildews:

Pathogen grows superficially and appears as white, violet or brown spots on young stems, leaves and also on fruits. When the fungal spores are available in the form of spreading powdery mass on the host surface, they are called powdery mildews (powdery mildew of cucurbits, c.o. Erysiphe cichoracearum).

On the other hand, when the symptoms are visible in the form of tangled cottony or downy growth, they are called downy mildews (downy mildew of cucurbits, c.o. Pseudoperonospora cubensis).

(b) Smuts:

The symptoms are usually deve­loped as black powdery mass of spores like smut on the inflorescence, either by replacing ovary, the loose smut (loose smut of wheat, c.o. Ustilago segetum var. tritici) or the spores remain inside ovarian wall, the covered smut (covered smut of barley, c.o. Ustilago hordei).

(c) Rust:

The symptoms appear as small pustules on the epidermis of leaf and stem of the host. Pustules may be pow­dery or compact and are of different colouration, like black (black stem rust of wheat, c.o. Puccinia graminis tritici), brown (brown rust of wheat, c.o. Puccinia recondita), yellow (yellow rust of wheat, c.o. Puccinia striiformis), white (white rust of crucifer, c.o. Albugo Candida) (Fig. 5.5C) etc., depending on the species.