Wednesday, July 17, 2024
General Agriculture

Causes of Plant Diseases and Causal Agents of Crop Diseases

Plant diseases can be caused by various agents either acting singly or in combination with another, and the study of these agents is term etiology of the disease. Disease-inciting agents that are themselves living organisms are called pathogens.

The term parasite and host describe a nutritional relationship between two organisms, but the growth of a parasite in its host usually results in changes which are detrimental to the plant and considered on its ability to induce disease, a parasite can also be a pathogen.

Parasites causing plant disease can be classified according to their dependence upon the host plant as: Obligate, and Facultative.

1. Obligate Parasite

These organisms can only grow directly upon the host plant and cannot generally grow saprophytically on non-living organic matter. Their survival in the absence of a suitable host depends upon dormant resting stages in the life cycle, such as spores.

An obligate parasite depends critically upon the existence of the host. They cause only fairly mild symptoms such as growth malformation, stunting and discoloration; they would not kill the host.

2. Facultative Parasites

These are usually well adapted to a saprophytic existence and can survive long periods in an active stage in the absence of a suitable host. The destruction of the host is of less consequence to the facultative parasites which therefore cause more immediate and drastic damage, such as necrosis and wilting.

Mode of Action

Parasites produce pathogenic symptoms in their host plants in a variety of ways. Obligate parasites live directly on host cells and even this physical penetration of the plant tissues and utilisation of plant food may produce little immediate effect.

Facultative parasites may produce a variety of toxins and enzymes which kill the plant tissues as they are penetrated.

Many disease symptoms are the result of the defence reactions of the host, and include the production of gums and resins, the suberisation of cell walls, the blocking of xylem vessels by tyloses, and the shedding of infected plant parts.

Causal Agents of Crop Diseases

The casual agents of crop diseases include the following;

1. Fungi

The majority of plant diseases are caused by various parasitic fungi. Most parasitic fungi are facultative although some are specialised obligate parasites, such as powdery mildews (Erysiphaceae) and rusts (Uredinales).

Fungal pathogens, although diverse in form are characterised by the production of spores which enable them to spread between plants.

Many parasitic fungi disperse their spores through water, in rain splashes or are carried in air currents. Some fungi attack crops at or below the soil level while others are dispersed by insects or through seeds.

The dispersal of spores is aided by the fact that most fungal spores are very small and are also produced in large numbers. When the spores of these fungi fall on a suitable host plant they grow into its tissue, absorb food and develop reproductive sporangia.

Some common diseases caused by this genus phytophorainclude:

Phytophthora palmivora which causes black pod disease of cocoa.

Phytophthora infestans which causes potato and tomato blights.

Phytophthora parasitica which causes stem rot of tomato.

Some common diseases caused by the genus Pythiuminclude a number of soil inhibiting fungi which usually enter the host plant through wounds and subsequently cause rotting.

Seedlings infected by Pythium spp. turn black and rapidly die; this is often referred to as damping off diseases.

Many crops are attacked at the seedling stage by Pythiumdebaryanumwhich rapidly causes death of the seedlings.

Watery wounds rot of potato tubers is often caused by Pythiumultimum.

The genus Peronospora includes species which are widely referred to as downy mildew diseases. Examples are:

Peronospora destructor; which infests crops such as onion.

Sclerospora graminicola which attack guinea corn.

The genus Puccinia, include many different types of rust and smut diseases. They form rust colored spore patches which develop on the epidermis of the infected host plants.

These fungi infect graminaceous crops such as maize, guinea corn and rice, making the grains worthless for both food and planting materials.

2. Bacteria

These microscopic organisms are generally capable of survival where other living organism cannot exist, such as water, the tissue of plants, dust particles and damp soils.

Bacteria usually enter into the tissue of crops through wounds, stomata, flowers or fruits. The symptoms of bacterial infection are varied, but the most common ones are decay, accompanied by an unpleasant odour.

Examples of bacterial diseases are: blight diseases of guinea corn and bacterial wilt of tomatoes, tobacco, garden eggs and peppers. Affected plants rapidly wilt, collapse and die. Citrus and mango fruits are liable to infection due to bacteria entering the wounds made by sucking insects or birds.

Read Also : Integrated Pest Management and Symptoms of Plant Diseases

3. Viruses

Viruses are a group of extremely minute particles which are visible only through a powerful electron. They are very highly specialised obligate parasites and can only exist within living plant cells.

Causes of Plant Diseases and Causal Agents of Crop Diseases

Frequently they cause obscure symptoms easily confused with mineral deficiencies of other environmental effects.

Plants infected with diseases due to viruses show varying symptoms such as change in leaf color, malformation such as swollen shoots, mosaic leaf patterns and distortion, reduced leaf formation, leaf spot, rings and streak on leaves and stunting.

Most viruses spread between plants by means of living vector, usually insects or nematodes, which themselves become infected with the virus, after feeding on a diseases plant.

Many viruses can be carried by insects, particularly sucking insects such as aphids, mealy bugs and leaf-hopper. The knives used in budding and grafting may also transmit viruses if used on infected plants. Viruses are rarely disseminated through seeds.

Examples of common viruses which affect crops are:

Tobacco mosaic virus (TMV).

Cassava mosaic virus (CMV).

Capsicum leaf curl virus (CLCV).

Cocoa swollen shoot virus.

Tristeza virus which affects citrus.

Control measures against virus diseases are usually aimed at the vector, but resistant crop varieties and use of clean planting material are also important.

Read Also : Composite Wastes Complete Management Guide


Benadine Nonye is an agricultural consultant and a writer with several years of professional experience in the agriculture industry. - National Diploma in Agricultural Technology - Bachelor's Degree in Agricultural Science - Master's Degree in Science Education - PhD Student in Agricultural Economics and Environmental Policy... Visit My Websites On: 1. - Your Comprehensive Practical Agricultural Knowledge and Farmer’s Guide Website! 2. - For Effective Environmental Management through Proper Waste Management and Recycling Practices! Join Me On: Twitter: @benadinenonye - Instagram: benadinenonye - LinkedIn: benadinenonye - YouTube: Agric4Profits TV and WealthInWastes TV - Pinterest: BenadineNonye4u - Facebook: BenadineNonye

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