Diseases Of Farm Production



The high and often fairly stable temperatures of tropical regions, linked with the high relative humidity which prevails even in regions of moderate rainfall, encourages the year-round activity of many plant diseases, often referred to as plant pathogens.

Moisture on leaves and wet soil conditions provide suitable conditions for the formation of fungal spores, the means by which many plant diseases are propagated. Fungi belong to a primitive group of plants, the Thallophyta, but have no chlorophyll and depend for their food on the materials produced by more highly developed plants and animals. They are often saprophytic that is, they live on dead organic material.

In addition to fungi, many diseases of crop plants are caused by bacteria and viruses. These last two groups of organisms are also microscopic, and are extremely difficult to deal with since, once a plant has become infected, there is no treatment which can restore it to normal growth.

Some plants have a natural resistance to diseases, dụe to either the structure of their leaf surfaces or their internal plant chemistry which reacts to prevent the development of many disease organisms.
Exrgal diseaseN can, however, be treated by various means, since the spores or reproductive bodies, which can be compared with the seeds of flowering plants, an he killed with various chemicals, if they are growing on the surface of leaves.

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When the fungus has penetrated into the leaf or stem, it is very difficult to control. The mycelium, or thread-like vegetative parts of many fungi which functions as both root and stem, can also be destroyed by contact with chemicals. A typical fungus is as shown in Fig. 5.2 showing details of mycelium and spores. The hypha is the term given to a single strand or filament of the mycelium.
The fruiting bodies produced by many fungi are termed sporangia, it is within these that the mass of spores which spread the disease are produced. The symptoms of fungal diseases are very varied but spotted area: on leaves, sunken lesions on stems, striped streaks on both stems and leaves, coloured and often raised areas or pustules, and powdery or netted webs of mycelium covering the leaf are some of the commonest signs of fungal infection. Many diseases cause internal breakdown of the cell tissues, often preventing the normal flow of nutrients to different parts of the plant.

Many fungi have a life cycle which can be divided into at least two main stages: the pathogenic phase, in which it depends on the food materials produced by a living plant and the independent phase in which it may be dormant in the soil or feeding on the remains of dead organic material as a saprophyte. The length of these phases varies widely with different fungi and is
often linked with seasonal variation in the weather such as the change from dry to wet seasons. Fungi can attack almost any part of a plant, such as the seeds, fruits, leaves, stems and roots but many fungi are able
to enter into the plant tissues most easily through particular parts such as the roots, leaves or wounds in stem tissues.

Virus diseases are, like bacterial diseases, impossible to treat once plants have become infected. They are ultra-microscopic and can only live and reproduce in living tissue although some can remain inactive in dried plant material for considerable periods. They often cause yellowing or a mixture of yellow and green streaks or patches which is often termed a mosaic, as in cassava mosaic virus. Leaf distortion, curling and reduction in leaf size, often termed 'dwarfing', are other symptoms of virus infection.
In some instances, plants infected with viruses do not show any external symptoms but death or reduction of yield may be the result of infection.

Viruses are often carried by insects known as vectors which feed on infected plants by sucking the contents of the leaf tissue. When these insects then feed on healthy plants, the virus is transmitted to the leaf cells. Aphids are typical plant virus vectors and the control of these insects will lead to a reduction in the rate of spread of any virus present in a crop.

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Viruses are rarely transmitted by seeds but, in some instances, virus-infected plants can produce seeds which are also infected; this is an additional reason for destroying infected plants as soon as they are seen.
Infection can also be carried on knives used in budding; if these are first used on infected plants, virus infection transmitted to the ext stock or scion used.
Bacterial diseases are serious in that infected plants cannot be successfully treated once they have become infected. Bacteria can only enter plants through wounds or through the natural openings in the leaf known as stomata. Bacterial rots of stored tubers such as those of yams often result from damage which occurs during the harvesting operation. 

Preventive measures include the use of chemicals such as formalin to sterilise the soil and maintaining a good standard of hygiene on the farm or garden by removing
dead plants and weeds. All diseased plants should be removed and burned as soon as they are observed to prevent further spread of the disease. Crop rotation is often the most effective means of reducing infection on following crops.

Not all bacteria are harmful to plants by causing diseases, the Rhizobium bacteria which infect the roots of leguminous plants are highly beneficial to the farmer since they are the means by which both the and soil can receive additional amounts of crop nitrogen without the use of fertilisers.


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