Plant diseases damage the crop, reduce the yield, lower the quantity and quality, increase the cost of production and require cash outlays for material and equipment for control measure. The chemicals used for disease control, are equally poisonous for all livings including human being and are also main cause of environmental pollution. In certain cases, fields have been found to show as much as 100% damage.

The readers including farmers, students, and research and extension workers would be able to understand:

What is plant disease and disease complex?

How the plant diseases are important?

Which are the causes of plant diseases?

To what we may call the symptom?

Basic principles for identification of diseases.

Dissemination or Transmission of diseases.

Disease management or control.

BACKGROUND

Biology (Greek words: Bios=life and Logos=study or knowledge) deals with the study of living organisms. About more than one and a half million species of organisms have been identified by the scientists. Bacteria are the smallest organisms of 0.0001mm size. Whale is the largest animal (length up to 40 meters and weight up to 150 tons). Red wood tree is tallest tree (over 300 feet in height)

Pathology (Greek words: Pathos=suffering) comprises both the sciences of understanding the nature and cause of disease as well as is an art of treating the suffering one. The Plant Pathology or Phytopathology (Greek: Phyton=plant) is a science, which deals with the studies of nature, causes, prevention and other aspects of plant disease.

WHAT IS Disease? 

·         Any disturbance of a plant that interferes with its normal structure, function or economic value is called plant disease.

·         It is a continuous harmful process in plants that is usually caused by parasitic microorganism or either is a change in plant brought about by one or the other environmental factor.

·         The situation (diseased plant) is manifested by visible morphological changes.

Disease complex: A plant disease caused by the interaction of two or more species or types of pathogens.

Disease incidence: Percentage of plants or plant parts with a disease.

Disease severity: Percentage of diseased plant.

MOST COMMON DISEASES OF CROPS

Ø             Root rots e.g. root rot of cotton

Ø             Powdery mildews e.g. P/M of mango/ jujube

Ø             Downy mildews e.g. D/M of cucurbits

Ø             Rusts e.g. rust diseases of wheat

Ø             Smuts e.g. whip smut of sugarcane

Ø             Bunts e.g. bunt diseases of wheat

Ø             Wilts e.g. wilt of tomatoes, chilies, cucurbits etc

Ø             Damping off e.g. D/Off of papaya, vegetable crops etc

Ø             Blights e.g. blight of rice, sunflower etc

Ø             Canker diseases e.g. citrus canker

IMPORTANCE OF PLANT DISEASES

 EXAMPLES OF SERIOUS AND SEVERE DISEASE PROBLEMS

PREVIOUS RECORDS
The black stem rust of wheat, Phyllody of sesame, whip smut of sugarcane, wilt and storage rot of potato, wilt of gram, early blight of potato, long smut of sorghum, ring rot, tip burn and early blight of potato, grain smut of sorghum, red rot of sugarcane, wilt or root rot and boll rot of cotton, blight of rice, grain smut of sorghum are the examples of most serious and severe disease problems from 1906 to 1930 at various different localities in Sindh.

RECENT PAST ISSUES
The wilt and viral diseases of chilies and tomatoes, powdery mildew of cucurbits, mango and jujube, downy mildew of different crops, banana bunchy top virus (BBTV) and cotton leaf curl virus (CLCV) also take a heavy toll and the total losses in production, since last two decades.

CURRENT PROBLEM
The mango tree mortality (decline or sudden death) is the most recent example, because of which trees are dried up and farmers have no way to control. Therefore, they are going to cut off their fruit bearing mango trees from orchards.

CAUSES OF PLANT DISEASES 

It is also pertinent to mention here that any injury or abnormality brought about by insects, ticks, mites, birds, rodents and other animals belongs to the field of entomology. The remaining causes of all disturbance and deviations are taken place in the field of plant pathology.

SYMPTOMS OF PLANT DISEASES

·         The plant disease may associate with various external and/or internal changes, reactions or alteration in the plant, which we call symptoms.

·         Where one particular casual agent is involved, there are often a number of symptoms, which together form a characteristics symptoms picture or syndrome to which a specific disease name is given.

·         The symptoms that appear for a long period of time are termed as chronic symptoms.

THE MOST COMMON SYMPTOMS

·         The symptoms may also be categorized on the basis of common names of plant diseases which are given on the basis of major symptoms developed due to disease e.g. rusts, smuts, mildews etc.

·         The symptoms are further termed on the basis of disease causing agent e.g. symptoms caused by fungi, bacteria, nematode, mycoplasma; or symptoms caused by deficiency or excess of nutrients, low or high temperature/ moisture, lightening injury or  symptoms caused by injurious atmospheric gasses etc.

Attention

Students are advised to refer different books on plant diseases and collect different specimens of diseased plants for examining above explained symptoms and giving example to each one.

IDENTIFICATION OF PLANT DISEASES

One can identify the specific plant disease, by means of their symptoms in the field of a particular crop.

The identification of a pathogen of a new disease requires the study of its life cycle as well as the study of its cultural, physiological and biochemical properties.

This type of study includes isolation of organism from host; it's culturing on media or on plants and its examination under various conditions of nutrition and environment.

For doing all this, one should be familiar with the fundamental procedure including laboratory techniques for handling with diseased plant, their preservation, culture development, physiology and biochemistry of disease causing organism and infected plant.

On a whole, a critical experienced eye is necessary in the identification of plant diseases. The plant diseases are conformably identified through Microscopic and Macroscopic Studies in laboratory and in the field.

Macroscopic study (Identification through external symptoms either by naked eye or with the aid of a hand lens)

Microscopic study (Careful microscopic examinations of transverse and longitudinal section cuttings of diseased plant tissue or teasing of the material with the help of a needle)

Dissemination/ Transmission OF DISEASES 

Dispersal or spread of inoculum (disease causing organism) from its source (diseased plant and/or other) to healthy plant is called dissemination, whereas, transfer of a pathogen (disease causing organism) from one plant to another is termed as transmission.

 Wind, water or rain, and equipment can spread the disease causing organisms (pathogen) viz. fungus, bacterium, nematode or virus, once introduced in a field. The grower or field worker itself, the insect pests, birds, animals also help the disease-causing organism to spread more. Even the higher pathogens may become carrier for another. For example, viruses are transmitted through fungi and nematodes. The dry and wet as well as hot and cool climatic conditions sometimes help to reduce but may also increase development and spread of the disease. The disease inciting agents perpetuates (survive from season to season) through the seed given from diseased crop, weed plants, infested fields and all plant debris.

PLANT Disease Management
Effective disease management is essential for high quality production and the identification of diseases is first step in effective management. Control of the diseases may includes crop rotation; planting resistant varieties when available or sowing or planting of certified, disease-free seed or seedlings; timely sowing or planting in well-prepared, fertile soil; controlling weeds and insect pests; following a fungicide spray program when recommended and practising clean plough down after harvest. It must be kept in mind that in case of any doubt or incorrect identification of diseases may lead to the use of wrong management practices, wastage of time and wasted expenses that will result the failure of crop. For example, insecticides do not control diseases caused by bacteria, or viruses are also not controlled with fungicides. Similarly, a particular fungicide may control only one fungal disease, but not another. Therefore, the growers should must learn to recognise the more common diseases by their symptoms and they have sufficient knowledge of disease development to select appropriate management practices for the particular disease situation.

Integrated pest management (IPM) involves the use of several different strategies and the judicious use of pesticides (Fig.) for management of diseases and other pests of the crops. The most beneficial, better and more economical control of all diseases and insect pests may be achieved through IPM as compared to any one, a single management practice, such as pesticide application. The weed and insect management are also important components of disease control in an IPM system, because infection and spread of some pathogens is associated with the presence of certain weeds (alternate hosts) and insects (vectors).

However, the Integrated Disease Management (IDM) strategies that are effective components of an IPM system are given below, with a hope that the growers will adopt these for the benefit of themselves as well for nature, because the chemicals applied for the control of diseases and insect pests are equally poisonous for all livings and is major source of environmental pollution. Their effectiveness in controlling specific diseases could be learnt through regular study and observation.

Crop rotation: The fungi, bacteria, and nematodes cause soil and seed borne as well as foliar diseases. These pathogens may survive from season to season in the soil or on seed and other crop debris in soil and build up to damaging levels with repeated cropping; therefore 3-4 year rotation with non-host crops is recommended.

Site selection: Most crops are best grown on sandy loam, sand, or silt loam soils with a pH of 6 to 7. Growth on acid and/or poorly drained soils often results in increased incidence of Fusarium wilt, fruit rots and some other diseases. Therefore, maintaining records of the disease history of fields is beneficial for avoiding disease problems or implementing preventive measures. In such type situation, late plantings should not be situated near early plantings where a disease already exists.  

Sanitation: Several destructive diseases of various crops can be spread from infested fields to clean fields in soil and crop debris carried on equipment and workers. Therefore the equipments and boots should be washed to remove all clinging soil and debris; when leaving infested fields to avoid contamination of clean fields. Clean fields can also be worked before entering infested fields.

Variety selection: Resistance is the most effective and economical means of disease control. For some diseases, resistance is the only effective control. Therefore, available disease resistant varieties should be planted where possible.

Soil fumigation: Soil fumigation is expensive and potentially dangerous for inexperienced applicators, but increases yields, earliness, and controls soil borne diseases. However, row fumigation may be economically feasible mostly for vegetables.

Planting time: The planting time sometimes favors or may disfavors the multiplication of disease inciting organisms; therefore, change in planting time is recommended in various cases. For example, cultivation of early sowing and early maturing varieties are recommended for the control of rust in wheat.

Disease-free seed and seedlings: Some diseases may be seed borne or introduced into fields on infected seedlings (transplants). Efforts should be taken to obtain disease free seed and transplants. Seed dressing fungicides are found effective in minimizing the potential for seed borne diseases caused by fungi and to help assure establishment of an adequate stand. Only healthy or treated seed and seedlings should be used to initiate plantings.

Weed control: Mostly the weeds serve as alternate hosts or sources of infection for virus diseases. Therefore, effective weed control practices should be utilized in and around the crops.

Insect control: The insects are major source to transmit virus and bacterial diseases. Therefore, insects must be controlled when warranted in and around the crops.

Irrigation: Excessive irrigation or frequent irrigation with small amounts of water as well as shortage of irrigation (drought conditions, due to long irrigation intervals) favors spread and development of many diseases. Therefore, the crops should be irrigated properly.

Fertilizers: Heavy doses of nitrogenous fertilizers may maximize but judicious use of potassic fertilizer help in minimizing the susceptibility of plants. Therefore, proper use of fertilizers is recommended.

Chemical control: Fungicides protect healthy plants from infection, but do not cure diseased plants. These are needed for effective management of some foliar diseases caused by fungi. Copper sprays are also useful in reducing foliar diseases caused by bacteria. The diseases are more difficult to control once established; hence timing of the first spray is critical. The first spray should be made before symptoms appear where diseases are anticipated or shortly after symptom appearance. Moreover, an adequate spray volume is needed to achieve thorough coverage of infected plant or tree. Thereafter, an approved schedule should be maintained. Sprays should also be applied before an anticipated rain event rather than after because this affords protection during periods favorable for infection. Most soil borne diseases cannot be controlled by foliar fungicide application, but drenching may be found useful in such condition.

Scouting: Scouting allows for early detection of all pests and diseases so that timely management practices can be implemented. Plantings should be scouted regularly (at least once per week), for assessment of the effectiveness of management programs already implemented.

Residue management: Many of the pathogens survives in and on the plant debris or pruned parts of plants or trees, therefore theses should be incorporated into the soil by ploughing or disking after harvest to hasten decomposition.

CONCLUSION