Environmental disorder also interferes with normal growth and structure of plants.

Muhammad Ibrahim Khaskhali
B. Sc. Agri. Hons. Plant Pathology
Sindh Agriculture University, Tandojam

Apr 19 - 25, 2004

Most of us are un-aware to friends as well as enemies of our lovely crop plants. Most probably, there are three main factors: pathogen (casual organism), plant (susceptible host plant which can not overcome the disease), and favorable environmental condition. If any one of these factors is not present, the disease will not occur. The relationship of these three factors is known as "Disease Triangle".

Now it can be understood through 'Disease Triangle' that why majority of the plants are not affected by many thousands of diseases, which exist in nature. Actually, plant disease is an interaction among susceptible host, pathogen and favorable environmental condition, because, plants are not only affected by the pathogen but environmental disorders also interferes with normal growth and structure of plants. Therefore plant diseases are categorized into two groups: Infectious or Biotic or Parasitic plant diseases and Non-infectious or a-biotic or non-parasitic plant diseases.


The diseases caused by living casual organisms (pathogen) are termed as biotic diseases. These are capable to transmit from diseased to healthy plants and cause disease if the conditions are favorable, therefore, also termed as infectious or parasitic diseases. The pathogens causing such type of diseases are fungi (e.g. rust, smut etc), bacteria (rots, leaf spots etc), viruses (cotton leaf curl virus or CLCV, banana bunchy top virus or BBTV etc) and nematodes (root knot diseases etc). The infectious disease causing organisms may be Soil borne (remain in the soil or disseminated through soil by cultural practices), Windborne or airborne (spreading through wind or air), Seed borne (spreading through seed) and or Insect borne (spreading through insect).


The diseases, not caused by living causal organisms (pathogen) and not capable to transmit from diseased to healthy plant are known as a-biotic or non-infectious diseases, because no any single plant parasite is involved in such type problem. These diseases are caused by environmental factors such as: Low or high temperature or moisture, unfavorable effects of gasses, deficiency or excess of minerals and lightening injuries.

The study of plant diseases is called Plant Pathology (Phytopathology). The Greek words: phyton means plant, pathos means suffering or trouble and logos means science or discourse or study. Hence, in its broader meaning, Phytopathology or Plant Pathology is science of nature, causes, preventive and curative control measures of plant diseases. Plant diseases, caused by biotic agents are known as infectious (because these may spread rapidly), whereas, a-biotic or phsiogenic diseases are non-infectious (because these could not transmit from diseased to healthy one). However, disease is very complicated problem, therefore couldn't be defined easily in a single sentence. Plant disease is considered as any disturbance or deviation in the normal structure or function of the plant, either due to the attack of pathogen (disease causing organism e.g. fungi, bacteria, virus, nematode, mycoplasmas or viroids etc) or by environmental factors (such as low or high temperature, moisture, light or deficiencies or excess of nutrients).



Pathogens are living microorganism that initiates (cause) a disease. Such as, fungi, bacteria, virus, nematode, mycoplasmas, viroid etc. each one has different life cycle and specific infectious stage. For example, in most of the nematode second stage is the infectious stage.


Host is the any plant on which pathogen survives. Most of the causal organisms (pathogen) not only have their specific life cycle but also having particular host specie, genus or family on which they survive. For example, Root rot of cotton caused by Rhizoctonia species will not attack maize. Black spot of rose will not attack marigold. While other, such as powdery mildew caused by Erysiphae polygoni produces similar symptoms on many other (different) plants.


Various environmental conditions are responsible for the existence of plant diseases. No doubt, if the pathogen (causal organism) is present but favorable environmental conditions are not present no disease will developed/occurs. Specific environmental conditions are needed for the existence, multiplication and development of micro-organism (pathogen), after which, disease will appears. Most of the plant diseases may not exist due to unfavorable environmental conditions, because such conditions varies from one area to another. For example, in the winter Rust disease producing spores (propagating unit) are disseminated by wind from northern to southern (hilly to plain) areas, where they develop. These spores carried away by wind, reach on the host, come in active in humid conditions and cause infection on the host plant (wheat, barley etc).

Moisture, temperature, wind, sunlight, nutrient and soil quality not only affect the pathogen development but also interfere in the growth of plant. Among all theses factors, if one of them is out of balance, plant may not grow properly. Such type of plant becomes susceptible to diseases.


Moistened condition in the life of plant comes form humidity, dew, rainfall or irrigation. In moist condition, most of the fungal spores (propagating unit) multiply and spread to cause the disease on other plant. Well-known diseases such as Rust (disease of wheat), black spot, firblight and apple scabe are spread in the moist condition to infect new host.


The disease-causing organism has a specific temperature of their growth and development. The optimum temperature for the fungal growth is 20-300C. Some of the pathogen multiply in warm weather, while other in cool weather. For example, powdery mildew diseases are late summer and warmer temperature. Soil temperature is also helpful in providing the favorable condition for the multiplication of root diseases causing organism.


Most of the airs borne diseases (spreading through air) are spreading through wind from one plant to another and from one area to another even many miles. But the combination of both sun and wind are helpful in producing the disease incidence by drying the soil surface and plant surface. Sunlight is very essential for the photosynthesis process of plant. If the proper proportion of sunlight don not receives by plant it becomes stressed. Such conditions make them to susceptible to disease.

Now, it could be easy to know about friends and enemies of crop plants, let us try to confirm them and to find out (adopt) the ways by which we can protect the crops from all the enemies, which are actually hidden.