Basic Science of Plant Diseases

 Forking in carrots

Introduction

A disease in plants can be defined as any disturbance brought about by an agency/factor which interferes with manufacture, translocation or utilization of food, mineral nutrients and water in such a way that the affected plant changes in appearance and or yields less than a normal, healthy plant of the same variety.

Causes of Plant Diseases

Plant diseases are of two types viz., infectious and non-infectious. The infectious type are caused by fungi, bacteria and viruses. Nutritional deficiencies, mineral toxicities, lack or excess of soil moisture and light, low or too high temperatures, soil acidity or alkalinity constitute the non-infectious type.

Usually, a disease causes a progressive and continuous disturbance of cellular activities that eventually become manifest as symptoms. A symptom is a visible or otherwise detectable abnormality arising from disease. Disease symptoms should be closely observed, as they often give indications about the cause of the disease and thus have a diagnostic value. Environmental factors must always be taken into account. Symptoms may either be mild in terms of extension or intensity, or they may be acute or severe, the latter usually leading to the death of plants or plant parts. The spreading of a disease depends on two factors, i.e. time and environment. The time determines the course of the disease as the pathogen population increases with the passage of time. Environmental factors, such as climatic and soil conditions, and cultivation methods, have a great influence on the expression of the disease’s symptoms.

1. Non-infectious Diseases

Maize wilting due to drought

Many serious disorders are caused by external physical factors which adversely affect the proper growth of plants. Very often, the disorder is a direct result of a deficient or excessive supply of an essential growth factor, such as light, oxygen, soil moisture and heat. These are indispensable for the various life processes in plants including water and nutrient uptake, respiration and photosynthesis. A strong interaction exists between these growth factors. Generally, each plant has the capacity to balance its requirement for each of them. However, abrupt changes in environmental conditions or a severe deficiency or injurious overabundance of one or more of the growth factors may disrupt a plant’s life process and bring about disease symptoms. Since these disorders are not caused by parasites, they are not infectious.

• Light: The requirement for light is very variable, some plant species prefer shade, others full sunlight. In many species the total hours of daylight – long day versus short day varieties determine the transgression from the vegetative to the generative phase. However, too much light, especially in combination with high temperature and lack of water, produces wilting or scalding in broad-leaved plants. Too little light produces chlorosis and etiolated or “lanky” growth.
• Oxygen: An adequate supply of oxygen to above - ground parts of the plants is essential for effective respiration. A shortage of oxygen in the soil owing to poor soil structure, compaction or water-logging hampers the development of a healthy root system.
• Water: Regular availability of water is essential for photosynthesis and metabolic processes in the plants. Plants absorb water mainly through the root hairs, this assists in the uptake of nutrients from the soil and their distribution throughout the plant. Evaporation of water from the leaves aids in cooling of leaf surfaces. Water contained within the walls of plant cells lends firmness (turgor) to plants, juicy fruits and succulent leaves may contain more than 90% water. A low relative humidity (dry air) may accelerate evaporation of water from the leaves and cause wilting when water pressure (turgor) cannot be maintained in the plant by replenishment from the roots. Early stages of wilting may be irreversible and the tissues may be damaged permanently.
• Temperature: The supply of heat, or the lack of it, determines to a large extent the functioning of life processes in plants. Plants have an optimum temperature range for healthy development and a particular total requirement for heat in order to reach their full productivity.
• Nutrients: Overabundance and deficiency of nutrients may cause a whole range of symptoms from discoloration to rot, die-back or stunted growth. Commonly, the productivity of a plant is lowered and its resistance to parasites weakened. The appearance of characteristic deficiency symptoms on leaves does not necessarily mean that the nutrient is in short supply in the soil. In fact, it might be sufficiently available, but its uptake may be impaired by an unfavourable acidity rate (pH) or by the shortage of another element. Soil salinity or alkalinity are serious problems threatening irrigated agriculture, where no possibilities exist to flush and drain the salt from the top layer of the soil. Continued fertilizing with ammonium sulphate may lead to high acidity of the soil and in time to toxic accumulation of elements such as iron, manganese and aluminium.

Symptoms of Plant Diseases

The way plant diseases express themselves by way of specific symptoms often holds important clues for diagnosis and treatment. The diagnosis must, however, take into account all environmental conditions, since these affect symptom expression. For instance, soil-borne diseases and nematodes may inhibit root development and thus cause poor growth. Likewise, a hard and compact soil structure near the surface may prevent the development of plant roots with a similar result. The following are the symptoms that generally occur with plant diseases:

• Damping off - Sudden collapse and death of seedlings in the seed bed or field, e.g. damping off in tomato, cabbage and cotton.
• Wilt - Sudden or gradual wilting and death of grown up plants as a result of disturbance in the vascular systems e.g. cotton wilt, green beans wilt, fusarium/bacterial wilt in peas.
• Spot - Localised necrotic lesions on leaves/fruits consisting of dead or collapsed tissues, e.g. Brown spot of rice.
• Blight - General and extremely rapid browning of leaves, branches, twigs and floral parts resulting in scorching and death of affected part.
• Mildew - White, grey, brownish patches of varying size on leaves, stem and fruit usually covered with mycelium and fructification of the fungus e.g. Downy mildew of rose cut flowers and powdery mildew of cucurbits.
• Canker - sunken out growths on leaves, stem, fruit which may be either smooth or rough e.g. citrus canker.
• Scab - Roughed or crust like lesion giving a freckled appearance of the diseased organ e.g. potato scab.
• Galls - Malformations of globose, elongated or irregular shape as a result of excessive cell division and cell enlargement e.g. crown galls on rose cut flowers.
• Mosaic - Alteration of light green, yellow, or white patches with the normal green of the leaves or fruits e.g. tomato mosaic.
• Yellows - Uniform discolouration or chlorosis of the foliage without any sporting pattern.
• Leaf curl - Distortion, thickening and curling of leaves e.g. tomato leaf curl.
• Ring spot - Appearance of chlorotic or necrotic rings on the leaves and fruit e.g. tomato sported wilt virus.
• Vein banding - Retention of bands of green tissue along the veins while the tissue between the veins is chlorotic e.g. vein banding mosaic.
• Vein clearing - Destruction of chlorophyll adjacent or in the veinal tissue e.g. yellow vein mosaic.
• Witches broom - Profuse upward branching of twigs e.g. potato witches broom.

2. Infectious Diseases

a. Plant Pathogen Fungi

Fungi are nucleated, spore bearing, achlorophyllous organisms which generally reproduce sexually and whose usually filamentous, branched somatic structures are typically surrounded by cell walls containing cellulose or chitin or both. Fungi reproduce by means of spores which are specialised propagated or reproductive bodies which are formed asexually or sexually. Asexual reproduction is more important for the propagation of the species, as it results in production of numerous individuals and is usually repeated several times during the season. The spread of the disease mainly takes place with the help of asexually formed spores like zoospores, conidia and oidia. Sexual reproduction normally results in the production of resting spores like oospores, ascospores etc., which help the pathogen in tiding over unfavourable conditions. Sexual stage of many fungi is produced once in a crop season. In general, it is the imperfect state that is the active pathogen, the perfect state (sexual) occurring in the terminal stages of the fungus spore and certain amount of growth which in turn depend moisture, temperature etc., which vary for different pathogens. Some fungi penetrate directly through cuticle and epidermis, others through stomata and wounds. The success of a fungus as a pathogen also depends on efficient transmission. Seed or other vegetative propagules, soil debris of a previous crop, weed hosts, alternate hosts are the major means of survival of the fungal pathogens. Dissemination (local and long distance) occurs through irrigation water, wind and insects. Effective means of control depends on the knowledge of the various means of survival and dissemination. Fungal pathogens are registered among many orders of fungi, their identification is done by using standard descriptions and keys.

b. Plant Pathogenic Bacteria

Symptoms and identification of bacterial wilt in tomato plants

Bacteria are an extremely small, microscopic, unicellular micro-organisms that reproduce by fission. They exhibit three shapes viz, spherical or cocci, rods or bacilli and spiral or spirilli. The important genera of bacteria that cause diseases in crop plants are: Xanthomonas, Pseudomonas, Erwinia, Corynebacterium, Agrobacterium and Streptomyces). They enter the plants through wounds or natural openings (stomata) and interfere with physiological functions by destruction of structural integrity by acting upon cell wall components, interference with transport system by mechanical plugging or by producing toxins, interference with host metabolism or by alteration of genetic control mechanisms. These physiological disturbances at the cell level are expressed externally on plant by producing symptoms such as spots, blights, wilts, rots, cankers, galls etc. They are transmitted by a variety of agencies in or on seed and other planting material, soil, wind, rain, insects and nematodes and by cultural practices. The importance of these methods of transmission varies with the host and the pathogen and sometimes more than one method may be involved in any one disease. With regard to control, many seed borne bacterial pathogens can be eliminated either by heat/hot water treatment or by pre-treating the seed with suitable seed dressing chemicals.

c. Plant Pathogenic Viruses

Viruses reported on tomato crop

Viruses are submicroscopic organisms, multiply only inside living cells and have ability to cause disease. All viruses are parasitic in cells and cause a multitude of diseases to all forms of living organisms. Viruses are much smaller than fungi and bacteria and as such they cannot be visualised under microscope. Viruses can be observed under electronic microscope.

Virus Transmission

Viruses cannot actively penetrate their host cells. Some agency should directly introduce them into the host cell to cause infection. A virus is commonly transferred from an infected host to a non-infected one by a vector, which is often an insect, mite or nematode, but may also be fungus or parasitic flowering plant (e.g. Cuscuta spp, dodder). Sucking insects, such as aphids, leaf and plant hoppers, thrips and whitefly, dominate among the insect vectors, whereas nematodes and fungi play a role in the transmission of soil-borne virus disease. One particular vector may be able to transmit only one virus or many viruses. One particular virus may have one or more vectors. Some viruses may also be transmitted mechanically without the intervention of a vector insect. The sap of an infected plant can reach a healthy one either by means of tools used in cutting and grafting, by rubbing one plant against another.

Symptoms of Virus-induced Diseases

The symptoms of diseases caused by viruses are numerous and highly variable as their expression is influenced by the condition of the plant and environmental factors such as light, temperature and humidity. Visible symptoms may include local necrotic lesions, mosaic or mottled pattern of lighter and darker green leaf-tissues (chlorosis), yellowing or other discolouration including ring-spots or other distinct patterns, virescence (greening), vein clearing, reduction of plant growth leading to dwarfing or stunting, distortion due to unequal growth of cells, excessive tillering, flower and fruit variegations, wilting eventually followed by defoliation and yield depression. A plant may be infected by several viruses at the same time, the symptoms of which may be manifested concurrently and thereby reinforce or mask one another. The study of symptoms may be used for diagnosing the virus disease, but one must take into account that similar symptoms may be caused by different viruses, or even by nutrient deficiencies and genetic disorders. The severity of the symptoms depends on the sensitivity of the plant and virulence of the virus.

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