Seed Technology - IV

  Unit – 4: Seed borne diseases

4.1 Seed-Borne Diseases 

Seed-borne diseases are plant diseases caused by pathogens (fungi, bacteria, viruses, and nematodes) that are present within or on the seeds. These pathogens can survive for extended periods, often through dormancy, and infect seedlings or mature plants when the seeds germinate.

 

Common Seed-Borne Diseases:

1. Fungal Diseases

 

• Smuts:
• Loose Smut of Wheat: Caused by Ustilago tritici, this smut results in loose, black spore masses within the wheat heads. The entire head is destroyed, and the spores are easily dispersed by wind.
• Covered Smut of Barley: Caused by Hordeum vulgare, this smut forms covered, black spore masses within the barley kernels. The infected kernels are often smaller and darker than healthy ones.
• Rusts:
• Wheat Stem Rust: Caused by Puccinia graminis, this rust produces reddish-brown pustules on stems, leaves, and heads of wheat. The fungus completes its life cycle on barberry plants, where it produces teliospores that can infect wheat.
• Coffee Rust: Caused by Hemileia vastatrix, this rust infects coffee plants, causing reddish-brown pustules on leaves. The fungus can severely reduce coffee yields and quality.
• Powdery Mildews:
• Powdery Mildew of Wheat: Caused by Blumeria graminis, this fungus produces white, powdery patches on leaves, stems, and heads of wheat. The fungus feeds on the plant's cells, causing leaf yellowing and stunting.
• Grapevine Powdery Mildew: Caused by Uncinula necator, this fungus infects grapevines, producing white, powdery patches on leaves, shoots, and berries. The fungus can reduce grape quality and yield.
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2. Bacterial Diseases: 

• Blights:
• Fire Blight of Apple and Pear: Caused by Erwinia amylovora, this bacterium infects apple and pear trees, causing leaves, flowers, and twigs to turn black and die. The bacterium can spread rapidly, causing severe damage to orchards.
• Bacterial Blight of Rice: Caused by Xanthomonas oryzae pv. oryzae, this bacterium infects rice plants, causing water-soaked lesions on leaves and stems. The bacterium can reduce rice yield and quality.

3. Viral Diseases

• Mosaic Diseases:
• Tobacco Mosaic Virus: This virus infects a wide range of plants, including tobacco, tomatoes, and cucumbers. It causes mottled patterns on leaves, stunting, and yield loss.
• Cucumber Mosaic Virus: This virus infects a variety of plants, including cucumbers, squash, and beans. It causes mosaic patterns on leaves, yellowing, and stunting.

4. Nematode Diseases

• Root-Knot Nematodes: These nematodes infect plant roots, causing galls or knots to form. The galls can interfere with water and nutrient uptake, leading to stunted growth and reduced yield.
• Cyst Nematodes: These nematodes also infect plant roots, but they form cysts that contain eggs. The cysts can survive in the soil for many years, allowing the nematodes to persist in infected fields.

These are just a few examples of common seed-borne diseases caused by various pathogens. Understanding the specific pathogens involved can help in developing effective control measures.

 

Transmission Methods:

1. Direct Seeding:
• When infected seeds are directly sown into the soil, the pathogens can infect the seedling as it emerges.
2. Nursery Production:
• Infected seeds used in nurseries can transmit diseases to seedlings, which are then transplanted to the field.
3. Mechanical Transmission:
• Farm equipment, tools, or even human hands can carry pathogens from infected seeds to healthy plants.
4. Vector Transmission:
• Insects, mites, and other vectors can transmit seed-borne pathogens from infected plants to healthy ones.
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Prevention and Control:

• Seed Certification: Obtain seeds from reputable sources that certify their freedom from seed-borne diseases.
• Seed Treatment: Treat seeds with fungicides, bactericides, or nematicides to kill pathogens.
• Crop Rotation: Rotate crops to break the disease cycle and reduce the buildup of pathogens in the soil.
• Sanitation: Clean farm equipment and tools to prevent the spread of diseases.
• Resistant Varieties: Plant varieties that are resistant to common seed-borne diseases.

By understanding the transmission and prevention of seed-borne diseases, farmers and gardeners can take appropriate measures to protect their crops and ensure healthy yields.

 

4.2 Seed Health Testing Methods for Detecting Microorganisms

Seed health testing is crucial to ensure the quality and safety of seeds, as well as to prevent the spread of plant diseases. Several methods are employed to detect microorganisms in seeds, including:

 

1. Cultural Methods

• Isolation: This involves plating seed extracts or macerates onto selective media to isolate and identify specific microorganisms.
• Incubation: The inoculated plates are incubated under appropriate conditions to allow the growth of microorganisms.
• Colony Counting: The number of colonies formed is counted to estimate the microbial population.

 

2. Microscopic Examination

• Direct Observation: Seeds or seed extracts can be examined under a microscope to directly observe microorganisms.
• Staining: Staining techniques can be used to enhance the visibility of microorganism.

3. Immunological Methods

• ELISA (Enzyme-Linked Immunosorbent Assay): This method uses antibodies to detect specific antigens of microorganisms.
• Immunofluorescence: Fluorescently labeled antibodies bind to specific antigens, allowing for visualization under a microscope.

4. Molecular Methods

• PCR (Polymerase Chain Reaction): This technique amplifies specific DNA or RNA sequences of microorganisms, allowing for detection even in low numbers.
• Real-time PCR: This is a quantitative version of PCR that provides information on the amount of target DNA or RNA present.
• Sequencing: Sequencing can be used to identify the exact species of microorganisms present.

 

5. Biological Assays

• Host Plant Tests: Seeds can be sown in a controlled environment and monitored for disease symptoms.
• Bioindicator Plants: Specific plant species can be used as indicators of the presence of certain microorganisms.

 

6. Rapid Diagnostic Tests

• Lateral Flow Assays: These are rapid tests that provide results within minutes.
• Immunochromatographic Assays: These tests use antibodies to detect antigens in a sample.

 

The choice of method depends on the specific microorganisms being targeted, the desired sensitivity and specificity, and the available resources. A combination of methods is often used to ensure accurate and reliable seed health testing.

 

 

4.3 Management of Seed-Borne Diseases

 

Seed-borne diseases can significantly impact crop yields and quality. Effective management strategies are essential to prevent their spread and minimize losses. Here are some key approaches:

 

Preventive Measures

• Seed Certification: Obtain seeds from reputable sources that certify their freedom from seed-borne diseases.
• Seed Treatment: Apply fungicides, bactericides, or nematicides to seeds before sowing to kill pathogens.
• Crop Rotation: Rotate crops to break the disease cycle and reduce the buildup of pathogens in the soil.
• Sanitation: Clean farm equipment, tools, and storage facilities to prevent the spread of diseases.
• Resistant Varieties: Plant varieties that are resistant to common seed-borne diseases.

 

Cultural Practices

• Seed Selection: Choose high-quality seeds that are free from disease symptoms.
• Planting Depth: Plant seeds at the appropriate depth to ensure proper germination and reduce the risk of infection.
• Field Sanitation: Remove and destroy infected plants to prevent the spread of diseases.
• Weed Control: Weeds can harbor pathogens and compete with crops for resources.

 

Chemical Control

• Seed Treatments: Apply fungicides, bactericides, or nematicides to seeds before sowing.
• Soil Treatments: Apply fungicides or nematicides to the soil to control pathogens that may infect seedlings.
• Foliar Sprays: Apply fungicides or bactericides to plants during the growing season to control diseases that develop after germination.

 

Biological Control

• Biocontrol Agents: Use beneficial microorganisms, such as bacteria or fungi, to compete with harmful pathogens.
• Resistant Varieties: Plant varieties that naturally resist certain diseases.

Integrated Pest Management (IPM)

• Combine Multiple Strategies: Use a combination of preventive, cultural, chemical, and biological methods to manage seed-borne diseases effectively.
• Monitor for Pests and Diseases: Regularly inspect crops for signs of infection and take appropriate action.
• Minimize Pesticide Use: Use pesticides sparingly and only when necessary to avoid environmental damage and the development of resistant pests.

By implementing a comprehensive seed-borne disease management program, farmers and gardeners can protect their crops and ensure healthy yields.

 

4.4 Seed Treatment Methods

 

Seed treatment is a crucial practice in agriculture to protect seeds from diseases, pests, and environmental challenges. It involves applying various substances to seeds before sowing to enhance their germination, vigor, and resistance to adverse conditions.

 

Common Seed Treatment Methods

 

1. Chemical Treatment:
• Fungicides: Used to control fungal diseases.
• Examples: Captan, benomyl, thiram
• Bactericides: Used to control bacterial diseases.
• Examples: Streptomycin, copper oxychloride
• Nematicides: Used to control nematode pests.
• Examples: Carbofuran, aldicarb
• Insecticides: Used to control insect pests.
• Examples: Imidacloprid, thiamethoxam
2. Biological Treatment:
• Biocontrol Agents: Using beneficial microorganisms to suppress harmful pathogens.
• Examples: Trichoderma species, Bacillus subtilis
• Plant Growth-Promoting Rhizobacteria (PGPR): Bacteria that promote plant growth and health.
• Examples: Azospirillum species, Pseudomonas species
3. Pelleting:
• Coating seeds with a protective film to improve their handling, germination, and resistance to pests.
• Examples: Clay, polymers
4. Inoculation:
• Introducing beneficial microorganisms, such as nitrogen-fixing bacteria or mycorrhizal fungi, to the seeds.
• Examples: Rhizobium species, Glomus species
5. Hot Water Treatment:
• Immersing seeds in hot water to kill certain pathogens.
• Example: Used for treating wheat seeds to control bunt diseases.
6. Cold Storage:
• Storing seeds at low temperatures to prevent germination and preserve their viability.
• Example: Used for long-term storage of vegetable and flower seeds.
7. Solarization:
• Exposing seeds to sunlight to kill pathogens.
• Example: Used for treating seeds of certain crops, such as tomatoes and peppers.

 

The choice of seed treatment method depends on various factors, including the type of crop, the specific pathogens or pests involved, and the environmental conditions. A combination of methods may be necessary for optimal protection and germination.

 

4.5.  Spraying and dusting

 

Seed treatment is a crucial aspect of managing seed-borne diseases. It involves applying chemicals or biological agents to seeds before sowing to protect them from pathogens. Here are two common methods: spraying and dusting.

 

Spraying

• Process: Seeds are soaked in a liquid solution containing fungicides, bactericides, or nematicides. The solution can be applied by spraying or immersing the seeds.

 

• Advantages:
• Can be applied to large quantities of seeds.
• Provides uniform coverage.
• Can be combined with other treatments, such as pelleting.

 

• Disadvantages:
• Requires specialized equipment.
• May be less effective for small seeds.

 

Dusting

• Process: Seeds are mixed with a dry powder containing fungicides, bactericides, or nematicides. The powder can be applied by hand or using a mechanical duster.

 

• Advantages:
• Simple and easy to apply.
• Suitable for small and large quantities of seeds.
• Can be combined with other treatments, such as pelleting.

 

• Disadvantages:
• May not provide uniform coverage.
• Can be messy and dusty.

 

Common Seed Treatment Chemicals:

 

• Fungicides: Used to control fungal pathogens. Examples include captan, benomyl, and thiram.
• Bactericides: Used to control bacterial pathogens. Examples include streptomycin and copper oxychloride.
• Nematicides: Used to control nematode pathogens. Examples include carbofuran and aldicarb.

 

Factors to Consider When Choosing a Seed Treatment Method

 

• Type of pathogen: The choice of treatment method and chemical depends on the specific pathogen causing the disease.
• Seed characteristics: The size, shape, and surface texture of the seeds can influence the effectiveness of different treatment methods.
• Environmental conditions: The climate and soil conditions can affect the survival and activity of pathogens.
• Regulatory requirements: Some countries have specific regulations regarding the use of seed treatment chemicals.