Model Answer
0 min readIntroduction
Loose smut, caused by the fungal pathogen *Ustilago tritici*, is a significant seed-borne disease affecting wheat ( *Triticum aestivum* ) globally, including India. It results in substantial yield losses, impacting food security and farmer livelihoods. The disease is characterized by the replacement of wheat grains with black, powdery spores, leading to a ‘loose’ or disintegrated head. Understanding its symptoms, causal organism, and effective control strategies is crucial for sustainable wheat production. The disease has been a concern for decades, with ongoing research focused on developing resistant varieties and improved management practices.
Disease Symptoms
The symptoms of loose smut vary depending on the stage of the plant’s development:
- Seed-borne stage: Infected seeds appear normal externally but contain fungal mycelium.
- Seedling stage: No visible symptoms are apparent.
- Vegetative stage: Initially, infected plants appear healthy.
- Heading stage: This is when symptoms become prominent. Instead of producing healthy grains, the infected spikelets are replaced by a mass of black, powdery spores. These spores are easily disseminated by wind. The head appears ‘loose’ and disintegrates readily.
- Grain stage: The infected grains are replaced by teliospores, which are black, elongated, and easily detached.
Causal Organism
The causal organism of loose smut of wheat is the fungus *Ustilago tritici* (syn. *Urocystis tritici*). It belongs to the Basidiomycota phylum, Ustilaginaceae family.
- Classification:
- Kingdom: Fungi
- Phylum: Basidiomycota
- Class: Ustilaginomycetes
- Order: Ustilaginales
- Family: Ustilaginaceae
- Genus: *Ustilago*
- Species: *Ustilago tritici*
- Life Cycle: *U. tritici* is a heterothallic fungus, requiring two mating types for sexual reproduction. The life cycle involves:
- Infection of wheat seeds through wounds.
- Growth of mycelium within the seed.
- Germination of spores and infection of seedlings.
- Formation of teliospores in the spikelets.
- Dissemination of teliospores by wind.
Control Measures
Effective control of loose smut requires an integrated approach combining preventive and curative measures:
Preventive Measures
- Seed Treatment: This is the most effective control measure. Treating seeds with systemic fungicides like carboxin, vitavax, or triadimefon significantly reduces infection.
- Use of Resistant Varieties: Developing and using wheat varieties resistant to loose smut is a sustainable long-term solution. Breeding programs continuously aim to incorporate resistance genes.
- Seed Certification: Using certified, disease-free seeds ensures the absence of fungal inoculum.
- Rouging: Removing and destroying infected plants before spore dispersal can help limit the spread of the disease.
- Crop Rotation: While not a primary control, rotating wheat with non-host crops can reduce the build-up of inoculum in the soil.
Curative Measures
Curative measures are limited once the plant is infected. However, the following can help reduce disease spread:
- Foliar Spray: Foliar application of systemic fungicides like propiconazole or tebuconazole at the early stages of disease development (booting stage) can provide some control, but is less effective than seed treatment.
- Deep Ploughing: Deep ploughing after harvest can bury the teliospores, reducing their survival and dispersal.
Recent Advancements: Research is ongoing to identify and utilize novel resistance genes and develop biocontrol agents to manage loose smut effectively. Molecular markers are being used to screen for resistance genes in wheat varieties.
Conclusion
Loose smut of wheat remains a significant threat to wheat production, necessitating a proactive and integrated disease management strategy. Seed treatment with systemic fungicides and the use of resistant varieties are the cornerstones of effective control. Continued research and development of new control measures, coupled with farmer awareness and adoption of best practices, are crucial for minimizing yield losses and ensuring food security. The increasing focus on sustainable agriculture also emphasizes the importance of breeding for durable resistance and reducing reliance on chemical control.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.