Study symptoms of the diseases in locally available crop plants

Objective

To collect and study symptoms of diseases in locally available crop plants.  

 

Theory

Plant disease is defined as the abnormal physiological functioning of a plant due to pathogenic organisms such as fungi, bacteria, and viruses.  

All species of plants, whether wild or cultivated, are subject to disease. The occurrence may vary depending on environmental conditions, season, the presence of the pathogen, and the crop variety produced. 

According to the nature of their causal agent, plant diseases can be classified into,  

  1. Infectious plant diseases 
  2. Non-infectious plant diseases 

 

1. Infectious plant diseases 

Infectious plant diseases result from pathogenic organisms such as fungi, bacteria, viruses, nematodes, or parasitic flowering plants. These contagious agents can reproduce inside or on their host, spreading from one susceptible host to another. 

 

Common diseases in crop plants 

Parasitic microbes  

Parasitic microbes are organisms that live on or in the host and feed on it. It can cause diseases by infecting and living off the plant. They absorb nutrients from the host and may kill it. If the plant diseases caused by these microbes are unchecked, they may spread to other plants, leading to significant economic and agricultural implications. The parasitic microbes affect commercial crops like cereals, pulses, vegetables, fruits, etc. Examples include Bacterial blights, white and black rust, and tobacco mosaic infection. 

 

Bacterial blights 

  • It is a bacterial infection caused by Pseudomonas and appears as dark spots on the leaves.  
  • Symptoms start as light green spots on leaves, growing to brown tissue in the centre, surrounded by a lemon-yellow ring.  
  • Bacterial blights are commonly found in potatoes, tomatoes, chilli plants, etc. 

                                                                   

                                                                               Fig.1 Bacterial blight caused by Pseudomonas in tomato plants 

 

White rust 

  • It is a fungal infection caused by several fungus-like oomycetes in the genus Albugo, which attack many herbaceous plants.  
  • Symptoms of the infection include light yellow spots on leaves, chalky-white spots turning into powdery pustules, and darkening on the undersurface of leaves and other plant parts above ground. 
  • White rust occurs on various crops, including cabbage, cauliflower, collard, etc.            

 

                                                                    

                                                                                  Fig.2 White rust caused by the fungus Albugo in cabbage

 

Tobacco mosaic 

  • It is a viral infection caused by the tobacco mosaic virus (TMV), belonging to the Tobamovirus genus. 
  • Symptoms of tobacco mosaic infection are characterized by small, light green mosaic patterns on leaves, which progress with mottling or malformation, ultimately resulting in yellow-to-brown spotting. 
  • Tobacco mosaic primarily affects tobacco plants. 

 

 

                                                                    

                                                                 Fig.3 Tobacco mosaic caused by Tobacco mosaic virus (TMV) in tobacco plants 

 

Black rust 

  • It is a fungal infection caused by the fungus Puccinia graminis, which causes significant diseases in cereal crops. It primarily infects the wheat plants. 
  • Symptoms of this infection are that it causes small brown pustules on leaf blades, and if severe, brown patches develop throughout the leaf. 

 

                                                                    

                                                                         Fig.4 Black rust caused by fungus Puccinia graminis in wheat plants 

 

Seed-borne diseases 

Pathogenic organisms can infect plant seeds and cause seed-borne illnesses. Pathogens infect the seeds of a plant either externally or internally. If the infected seeds are used for planting, they infect the new plant and spread the disease throughout the plant population.    

Examples, 

  • Bacterial Fruit Blotch (BFB) in watermelon, caused by Acidovorax citrulli, results from infected seeds and develops fruit blotches and lesions. 
  • Wheat Loose Smut in wheat seeds, caused by the fungus Ustilago tritici, resides in seeds, leading to smut-infected plants. 
  • Bacterial leaf streaks in rice, caused by Xanthomonas oryzae, result in lesions and streaks on rice leaves. 

 

2. Non-infectious plant diseases 

Non-infectious plant diseases are caused by unfavourable growing conditions, including extreme temperature, moisture, etc. Unlike infectious agents, non-infectious causal agents cannot reproduce within the host and are thus non-transmissible.  

Examples,  

  • Physiological Disorders in plants such as blossom end rot in tomatoes and peppers due to calcium deficiency.  
  • Nutrient deficiencies result from a lack of essential nutrients such as nitrogen, phosphorus, potassium, magnesium, iron, etc., leading to stunted growth and yellowing of leaves 

 

Learning Outcomes

Student understands: 

  • Plant diseases and their causes. 
  • Difference between infectious and non-infectious diseases in plants. 
  • Common types of diseases in plants and their symptoms. 
  • The diseases in stored cereal at home. (To know more - Study how Insect pests spoil Food Grains)