Crop Plant Diseases


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In today’s Agricultural Science class, We will be discussing Crop Diseases. We hope you enjoy the class!


Crop Plant Diseases

A plant disease is defined as “anything that prevents a plant from performing to its maximum potential.” This definition is broad and includes abiotic and biotic plant diseases.

Abiotic or non-infectious diseases:

These diseases are caused by conditions external to the plant, not living agents. They cannot spread from plant to plant, but are very common and should be considered when assessing the health of any plant. Examples of abiotic diseases include nutritional deficiencies, soil compaction, salt injury, ice, and sun scorch

Biotic or infectious diseases:

These diseases are caused by living organisms. They are called plant pathogens when they infect plants. For the purposes of discussing plant pathology, only plant disease pathogens will be discussed. Pathogens can spread from plant to plant and may infect all types of plant tissue including leaves, shoots, stems, crowns, roots, tubers, fruit, seeds and vascular tissues


Fungal Diseases

Fungal diseases are plant infections caused by fungi. Fungi can be single or multicellular, but either way, infect plants by stealing nutrients and breaking down tissue. Fungal diseases are the most common infection in plants.

Symptoms and Signs

There are some characteristic symptoms, or observable effects of the disease, in plants. Fungi infections can be recognized by symptoms like spots on plant leaves, yellowing of leaves, and birds-eye spots on berries. With some fungal diseases, the organism itself can actually be viewed on the leaves as molds, mildew or spores. These may appear as growths or malformations on stems or the underside of leaves. These direct observations of the disease-causing organism are called signs of infection.

fungus disease crop diseases agric science classnotesng
Fungal Diseases


Bacterial Diseases

Bacteria are single-celled, prokaryotic organisms. Bacteria are everywhere and many can be beneficial, but some can cause disease both in humans and plants.

Signs and Symptoms

The signs of bacteria are often harder to detect than fungi since bacteria are microscopic. Upon cutting an infected stem, a milky white substance may appear, called bacterial ooze. This is one sign of a bacterial infection. Other signs include water-soaked lesions, which are wet spots on leaves that ooze bacteria. Eventually, as the disease progresses, the lesions enlarge and form reddish-brown spots on the leaves.

A common symptom of bacterial infection is leaf spots or fruit spots. Unlike fungal spots, these are often contained by veins on the leaf.

cowpea-bacterial wilt-crop diseases-agric science classnotesng
Cowpea Bacterial Wilt

Viral Diseases

Viruses are infectious particles that are too small to be detected by a light microscope. They invade host cells and hijack host machinery to force the host to make millions of copies of the virus.

Signs and Symptoms

Viral diseases don’t show any signs in plants since viruses themselves cannot be seen even with a light microscope. However, there are symptoms that the trained eye can observe. A mosaic leaf pattern, yellowed, or crinkled leaves are all characteristic of viral infection. This classic pattern of discolouration is where many plant viruses get their name, such as the tobacco mosaic virus. Also, decreased plant growth is also commonly seen in viral infections.

tobacco mosaic virus crop diseases agric science classnotesng
Tobacco Mosaic Virus


We have come to the end of this class. We do hope you enjoyed the class?

Should you have any further question, feel free to ask in the comment section below and trust us to respond as soon as possible.

In our next class, we will be talking about the Development of Agriculture in Nigeria. We are very much eager to meet you there.

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