Crop Diseases

Plant/crop pathology means the study of plant/crop diseases. Crop pathology is the study of the identification, classification, disease cycle, etc., of the pathogen causing the disease. Many diseases of field and horticultural crops and their management are additionally known for diseases like powdery mildew, late blight in potatoes, etc.

Crop Diseases

Crop disease is defined as a complication with the crop that reduces its yield or distorts its appearance. A crop is claimed to be diseased when it shows a harmful deviation from its normal functioning in terms of physiological processes. 

Causative agents: Causative agents are fungi, bacteria, viruses, mycoplasma, viroid, nematode, or parasitic flowering plants that cause crop diseases.

Classification of crop diseases: Crop diseases may be classified under the following categories:

1. Based on the causative agent: Supported by the causative agent, the crop diseases are classified as follows:

• The diseases that are caused by infectious microorganisms like fungi, bacteria, viruses, nematodes, parasitic flowering plants, etc., are called infectious diseases. The causative agents may reproduce within or on their host and spread rapidly from one host to another.
• Non-infectious crop diseases are caused by unfavourable growing conditions like extremely high or low temperatures and an abnormal concentration of an essential mineral.

2. Based on the occurrence and geographic distribution: Based on the occurrence and geographic distribution, the crop diseases are classified as follows:

• Endemic disease: A disease that is consistently present in a particular region is only referred to as an endemic disease; for instance, wart disease in potatoes.
• Epidemic disease: A disease that spreads rapidly to a larger proportion of hosts in a given region in a short time period is known as an epidemic disease. For example, rust.
• Sporadic diseases: These are crop diseases that occur irregularly and in very small numbers. For example, leaf blight and wilt.

3. Based on infection level: Based on infection level, the crop diseases are classified as follows:

• Localised diseases: These are diseases that affect only certain limited parts of the crop.
• Systemic diseases: Pathogens spread along the whole crop body, being linked to almost every phase of the crop’s life cycle.

4. Based on pathogen generation: Based on pathogen generation, the crop diseases are classified as follows:

• Monocyclic disease: These diseases are also referred to as simple interest diseases as they appear only in a single generation in one crop season. Loose smut is an example of a wheat crop disease.
• Polycyclic disease: These diseases are also called Compound interest diseases as they appear in more than one generation in a crop season. Late blight is an example of a potato crop disease, which is also a prominent disease in the field and horticulture.
• Polyetic disease: These are polycyclic diseases that reoccur in successive crop seasons and complete their disease cycle in one or more than one years. Apple cedar rust is a crop disease.

5. Based on symptoms: Considering the symptoms, the crop diseases can be classified as follows:

• Smuts crop disease: It is generally caused by fungus-like loose smut of wheat crop diseases, which is caused by Ustilago nuda tritici. 
• Rots crop disease: It is a wheat crop disease that infects the underground part of a plant and is caused by bacteria, fungi, or nematodes.
• Leaf spot crop disease: It is an example of a wheat crop disease commonly caused by a bacteria called Xanthomonas campestris.
• Powdery mildew crop disease: It is a well-known wheat crop disease that causes damage to stems, flowers, and fruits; for example, grapes, cucumbers, etc.

Diseases of Field and Horticultural Crops and Their Management

Diseases that come under the category of field and horticulture crops are as given below, along with the methods which are mostly preferred and widely accepted for the management of crops from diseases:

• Greening by phloem-limited bacteria, Liberobacter asiaticum in the citrus fruits: This disease of field and horticultural crops can be managed by the use of pesticides.
• Powdery mildew caused by Oidium mangiferae (common name – Acrosporum mangiferae): It is one of the most common fields and horticulture crop diseases and can be managed by dusting the plant with sulphur. One of the major factors responsible for this is the high humidity.
• Moko disease caused by Pseudomonas solanacearum or Burkholderia solanacearum in bananas can be managed by removing the infected plants and exposing the soil to sunlight.

Crop diseases caused by Nematodes: Nematodes are unsegmented roundworms that usually thrive in soil and attack the small roots of plants, but some species of nematodes stay on and feed on buds, stems, leaves, or flowers of plants. Nematodes that are parasitic to plants obtain their food by sucking juices from the plants. 

Prevention: Prevention of crop diseases requires identifying the host, learning to distinguish between biotic and abiotic causative factors and examination of crops for symptoms and signs on a regular basis.

Conclusion

Any disease to a plant can hamper its yield, growth and sustainability. Hence, diagnosis of the disease, identification of the pathogen, and proper eradication and management of the disease are essential for crop health. The symptoms appearing on the plant’s body play a crucial role in identifying the disease.

The examination of leaves for their colour, size and shape acts as the best starting point in the diagnosis of a disease.