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Rice bacterial blight is a bacterial disease that affects huge cultivation of rice every year. It is one of the most deadly and destructive infections for cultivated rice (Oryza sativa and O. glaberrima). Sometimes millions of hectares are infected and crops more than 75% can get destroyed. The causal agent of bacterial blight of rice is a species of bacterium Xanthomonas. The disease was first observed in Kyushu, Japan from the year 1884 to 1885. In 1911, the causal agent bacterium Xanthomonas oryzae pathovar oryzae was identified but it was known as Bacillus oryzae at that time. It is seen that they thrive in warm and humid environments and are commonly seen in rice growing regions of Asia, Australia, Caribbean, Latin America and western coast of Africa.
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Key Terms: Rice, Plants, Cultivation, Disease, Bacteria, Leaves, Crops, Genes, Mutation, Climate, Environmental Changes, Seedling, Infection
What is Rice Bacterial Blight?
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Bacterial blight is one of the most deadly diseases that occurs in rice cultivation.
The bacterial blight of rice was first evident when water soaked streaks which spread from leaf tips and margins started releasing milky ooze that dries into yellow droplets.
When the infection occurs in the booting stage, the yield is not destroyed but results in a poor quality of grains and higher proportion of broken kernels, thereby reducing quality of the cultivated rice.
Xanthomonas oryzae pv. Oryzae is the causal agent of the bacterial blight of rice. The disease occurs in areas with high numbers of weeds and can spread easily from one infected plant to another. A suitable temperature of 25-34 degree celsius along with a relative humidity of 70% is favourable for the infection. It is seen that it can occur in both tropical and temperate environments where there is rainfed and irrigated lowlands.
Bacterial Blight
Symptoms of Rice Bacterial Blight
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In the beginning, infected leaves turn greyish green and roll up. As the disease progresses, the leaf turns yellow and then straw yellow and then slowly it dries up and dies. The virus shows different symptoms at different stages of plant life. Some of the symptoms are mentioned below:
- When the seedling is affected, the infected leaves first turn yellow and then go on to being straw yellow.
- After sometime, they wilt, dry up and die.
- In the mature plants, water-soaked streak formation is seen and the green colour slowly turns into greyish green.
- Then they form a yellowish lesion with uneven edges.
- In the final stage, a milky bacterial ooze is seen dripping from the leaves. The leaves eventually dry out and a white crust is seen.
Effect of Bacterial Blight
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The pathogen, Xanthomonas oryzae, can spread wind, rain and water used in irrigation. The disease spreads more due to bad weather conditions like high humidity with more than 70% bad rainfall, heavy wind and warm temperature ranging from 25-34o. The use of nitrogen fertilisers further enhances the effectiveness and spread of the disease.
Both tropical and temperate environments are favoured, especially in lowland water-fed areas. This is also the reason why rice plants are highly affected by this disease as they require lowland with rain stagnant areas to grow.
Read More: Strategies for Enhancement in Food Production
Methods of Controlling Rice Bacterial Blight
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Below mentioned are some of the methods:
- Biological Method: Biological Methods of controlling the disease have not been that fruitful because it mostly consists of using bacterial antagonists of the pathogen which has proven to be limited in effectiveness.
- Chemical Method: Chemical control is also ineffective in minimising the blight of rice due to various safety concerns, biological effects and overall effectiveness. Sometimes rice seeds are treated with certain antibiotics like copper sulphate and copper oxychloride. However, some countries have restricted or banned the use of antibiotics hence these methods are not an universal method.
- Another method which has been proven to be helpful is cultivating the rice with the gene that provides resistance to Xoo infection. There are over 30 resistance genes, running from Xa1 to Xa33, that have been identified in rice plants. Some, like Xa21 have been incorporated in commercial rice strain genomes. These resistant varieties of rice are very successful and have reduced crop loss drastically.
Preventive Measures
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Mentioned below are some of the preventive measures to stop bacterial blight from destroying fields:
- To avoid nitrogen usage in the field in the form of urea.
- One of the most effective measures is to use seedlings of blight-resistant rice.
- By using healthy seeds from certified and reliable sources.
- Ensuring good drainage of water so that the water doesn't become stagnant.
- To remove weeds and straw, which act like a disease host of the bacteria.
- Using a combination of copper and mancozeb-containing fungicides.
Things To Remember
- Bacterial blight of rice is one of the deadliest diseases for rice which occur in both tropical and temperate climates where there are irrigated and rainfed lowlands.
- When there is continuous rainfall along with heavy rain, the infection is seen spreading rapidly from one plant to another through ooze droplets on lesions of infected rice plants.
- The bacteria causes wilting of seedlings (also called kresek) with yellowing and drying of leaves.
- The effective control of this disease is limited. The biological control for this disease is to use the bacterial antagonists of the pathogen whereas the chemical control involves the use of antibiotics such as copper sulphate and copper oxychloride.
- The most helpful method in controlling the disease is by cultivating the rice with a gene that provides resistance to the infection. Xa21 genes have been incorporated in the rice genome to reduce crop loss drastically from bacterial blight.
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Sample Questions
Ques. In the protoplast fusion experiment, name the physical barriers of a cell. How to overcome the barriers? (2 marks)
Ans. In an experiment, the cell wall is the most significant physical barrier. In order to overcome it, it is treated with pectinase and cellulase enzymes which can digest the cell wall efficiently and expose the protoplasm with only the cell membrane.
Ques. List a few biofortified crops and how they are helpful. (3 marks)
Ans. The list of biofortified crops consists of maize,wheat, rice, carrot, spinach.
- Fortified Rice - It has a high content of iron
- Maize hybrids - It has twice the amount of amino acids
- Wheat - The new and fortified wheat has high protein content.
Ques. Explain the germplasm collection. What are its advantages? (3 marks)
Ans. Germplasm refers to the collection of all the diverse alleles of all the genes of a crop.
The advantages of it is as follows -
- It provides a background of all the entire gene and allele information of a particular crop to the breeder.
- The most desired gene can be manipulated by the breeder to get the best quality of crop.
Ques. What are the biological and chemical methods used to control blight in rice? (2 marks)
Ans. The biological method used to control blight in rice is by using bacterial antagonists of the pathogen but it has limited effectiveness. The chemical method of control is by treating rice seedlings with certain chemicals like copper sulphate and copper oxychloride.
Ques. Which is the proven method for effective control of blight in rice? (2 marks)
Ans. The proven effective method of control for blight in rice is by gene mutation. The genes such as Xa21 have been helpful in improving the varieties and qualities of commercial rice. The rice with new and improved genes has shown more resistance to blight pathogen and as a result there is a drastic change in the loss of rice crops.
Ques. Explain what is the meaning of biofortification. (2 marks)
Ans. Biofortification refers to the process of breeding crops in order to increase the nutritional quality of it. It can be achieved by selective breeding or genetic engineering.This reduces the need to consume multiple food items because those nutrients can be obtained from a singular item. For example, Golden rice has been fortified with Vitamin A for improved consumption of nutrients.
Ques. Explain the process of somatic hybridization. (2 marks)
Ans. In somatic hybridisation, isolated protoplasts from different varieties of plants having desirable characters are fused together. This fusion can be done by mechanical, spontaneous or induced fusion methods. They produce a hybrid protoplast which can further grow into a new variety of plant. These are called somatic hybrids and the process is called somatic hybridization.
Ques. How is plant cell totipotency being used for plant propagation and improvement? (4 marks)
Ans. During the 1950s, it was seen that a plant could be generated from explants which means that any part of the plant can be taken and grown in a test tube under sterile conditions in a special nutrient medium. This capacity of generation of whole new plants from any cell or explant is called totipotency.
- This method has been useful in creating new somaclones of plants like bananas, tomatoes and apples in a shorter period of time. This is called micropropagation and each plant is identical to the original plant.
- The possibility of producing disease free plants has also been possible through this method. The meristem of the plant which is disease free is extracted and cultivated in-vitro like in the case of sugarcane and bananas.
Ques. How are mutations beneficial for plant breeding? Discuss with an example. (3 marks)
Ans. Mutation has been used to create genetic variations through the changes in the base sequence within genes resulting in new character creation or traits which are not found in parental plants. The usage of gamma radiations and chemicals makes it possible to artificially induce mutations in plants. The plant with desirable traits is selected and used as a source of breeding, For example, the resistance to yellow mosaic virus in mung bean and powdery mildew has been induced by mutations.
Ques. What are the various steps taken to release a new variety of plant breed in the market? (4 marks)
Ans. The various steps are as followed -
- Variability Collection: The root of a breeding programme is gene variability. Wild genes are preserved and collected as they serve as a primary source of productive exploitation of natural genes in a population.
- Selection and Evaluation of Parents: The germplasm available is used to assess the trait needed from the parent plant to incorporate in the new variety of breed. The selected plant is thus multiplied and utilised in the process of hybridization. Then a pure line is formed.
- Cross Hybridization: The desired characters from selected parents are combined manually. This is a long and tedious process.
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