Crop production is one huge investment in agriculture, also considered as the most lucrative and sought after agribusiness. Crop farmers are really make handsome profit from harvested crops especially during the dry season when most crops are limited with unlimited demand. Though, looks expensive but as it is popularly said “the best is not always cheap” and “anything worth doing is worth doing fine and well”, so also crop production. Farmers with irrigation facilities are the real farmers making real money.

I will be exposing to you the disease causing agents of crops, the organisms responsible for diseases in crop production and how to prevent and control them. They can also be called Biotic disease factors because they are actually living things, though myopic but very destructive.
There are basically four Biotic disease-causing agents in crops, they are
1.     Fungi.
2.     Bacteria.
3.     Virus.
4.     Nematodes.
Fungi are non-chlorophyll plant organisms, that are not differentiated into roots, stems and leaves and they do not have well developed vascular system as in higher plants. Fungi are saprophytic in nature because the feed on dead plant materials. They reproduce in prolific manner and they can be recognized by formation of spores which are usually large in numbers. They are usually airborne, seed borne or soil borne, they are very common and can be easily dispersed to infect plants. Most fungi diseases develop as a result of activities of vertebrate and invertebrate pests on plant, that is, the fungi can be taken from one plant to another by the activities of pest like movement from one plant to another. They are the most important plant pathogens as they are responsible for more than 70% of plant diseases.
Examples of fungi diseases are:
• Downey mildew of maize
• Fruit rot of tomato
• Rice blast.
• Smut in maize.
• Maize rust.
• Damping off of tomatoes.
• Panama diseases of banana.
• Tuber rot in yam.
• Gummosis in citrus
• Freckle disease of oil palm.
• Black pod of cocoa
• Anthracnose disease of cassava.
Control and prevention
Fungi disease can be controlled using good fungicide and pesticide, proper cleaning of farm equipment to avoid spreading and contamination of health crops. However, they can also be prevented through proper land preparation, use of resistant variety.
As known, they are everywhere and otherwise called germs. Bacteria are microscopic organisms with reproduce through fission. Bacteria being ubiquitous in nature can infect most plants through wounds made on the plant. These wounds can be created insects or the farmer, unknowingly, during cultural operations. Bacteria are so hardy that they can survive for years in the soil even when the host plant is no longer there. They are most destructive in the tropical regions where sunlight is abundant, this favours their growth and operations.
Bacteria diseases are grouped according to the symptoms. Those that stimulate growth at the base of the plant are called crown galls; those that penetrate into the water conducting system of the plant making the plant dry up are called wiltwhile those that kill plant tissues are called blights. The dead spots usually emit exudates in the morning and spread faster to other plants than fungi diseases. They may persist in the soil but susceptible to desiccation. Bacteria are saprophytic in nature, thereby helping in decomposing organic matter from plants and animals.
Examples of bacteria diseases are
• Leaf blight in cassava.
• Leaf curl disease of tobacco.
• Bacteria blight of cotton.
• Halo blight of beans.
• Brown rot of potatoes.
• Leaf streak disease of rice.
• Bacteria rot of yam.
Control and prevention
Bacteria disease can be controlled  by using resistant variety and conventional tillage practice to expose them to desiccation . Infected plant should be harvest to avoid spreading. Also, use of clean tools is advised.
Viruses are protein containing bodies which are part of living tissues. They are smaller than bacteria and multiply only within the living cell of a suitable host. Virus reproduces itself by invading a living plant. As it multiplies, it usually spread to every parts of the plant.
Virus diseases in plant are very destructive that they do not have any precise control measure, they are usually characterized by the rapid progress of symptoms such as wilting, dwarfness and unproductivity. Some take distinctive pattern beginning at the leaf, spreading within a few days to the stem, from there to the top of the plant before penetrating into the plant system. Most virus diseases are transmitted by insects chiefly Aphids, leafhoppers, mealy bugs, mites, thrips, whiteflies and nematodes.
Infected plants spread the disease through the viral particle or contagion to healthy ones through farm tools e.g hoes, cutlass and planting materials. Once a virus disease is confirmed, destruction of the plants becomes the rule followed by vigorous spraying programme to protect the rest of the plants from the insect vector.
Examples of virus diseases are:
• Streak disease of maize.
• Mosaic disease of cassava.
• Rosette in groundnut.
• Tomato mosaic virus.
• Leaf curl virus of celosia.
• Cowpea yellow mosaic virus.
• Bunchy top disease of banana
• Cocoa swollen shoot disease
These are non-segmented round worms. They are naturally found in the soil and water, they feed on other microorganisms in the soil but many are destructive plant parasites. The male nematode has cylindrical body while the female are often swollen. They posses stylet with which they puncture plant tissues. They reproduce through the fertilization of eggs or pathenocarpic reproduction, that is, without being fertilized.
Examples of nematode disease are:
• Root knot nematode of solanaceae.
• Cyst nematode of potato.
• Root knot nematode of yam

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