Loose smut of wheat
is a significant disease affecting wheat crops worldwide, caused by the fungus Ustilago tritici.
Causal Organism
- Name: Ustilago tritici
- Classification:
- Kingdom: Fungi
- Phylum: Basidiomycota
- Class: Ustilaginomycetes
- Order: Ustilaginales
- Family: Ustilaginaceae
- Genus: Ustilago
- Species: tritici
Symptoms
- Seedling Stage: Infected seedlings appear normal and symptomless.
- Heading Stage: Symptoms become visible when the wheat heads emerge. The fungus transforms the kernels into masses of dark, powdery spores (teliospores).
- Spore Production: The smutted heads appear loose and have a black, powdery mass instead of healthy grains.
- Plant Development: Infected plants often appear stunted and produce fewer tillers compared to healthy plants.
Etiology
- Pathogen Structure: The fungus produces teliospores that are responsible for the infection.
- Infection Mechanism: The infection primarily occurs in the flowering stage when the fungus invades the ovary of the wheat plant.
- Spore Survival: Teliospores can survive in the soil or on seed surfaces for several years, retaining their viability.
Disease Cycle
1. Teliospore Germination: Teliospores in the soil or on seed surfaces germinate and produce basidiospores under favorable conditions.
2. Infection of Ovary: Basidiospores infect the ovary of the wheat plant during flowering. This infection remains latent, meaning the fungus does not immediately produce symptoms.
3. Systemic Growth: The fungus grows systemically within the developing plant, remaining symptomless until the plant reaches the heading stage.
4. Smut Formation: As the plant heads out, the fungus converts the developing kernels into masses of teliospores, which are then dispersed by wind or other means to initiate new infections.
Management
Effective management of loose smut involves an integrated approach combining resistant varieties, chemical treatments, and good agricultural practices to minimize the spread and impact of the disease on wheat production.
1. Seed Treatment:
- Fungicide Application: Treating seeds with systemic fungicides (e.g., carboxin, tebuconazole) before planting can effectively reduce infection rates.
2. Resistant Varieties:
- Breeding Programs: Use of wheat varieties that are resistant or less susceptible to loose smut can significantly minimize the impact of the disease.
3. Crop Rotation:
- Rotation Practices: Implementing crop rotation with non-host crops can reduce the buildup of teliospores in the soil.
4. Sanitation:
- Field Hygiene: Removing and destroying infected plant debris can help reduce the source of inoculum.
5. Chemical Control:
- Treat the seed with Vitavax @ 2g/kg seed before sowing
Karnal Bunt of Wheat
Causal Organism
- Name: The fungus Tilletia indica Mitra is responsible for Karnal bunt of wheat. This pathogen is also known as Neovossia indica in some classifications.
Symptoms
- Affected Parts: Mainly affects the grains.
- Visible Symptoms:
- Bunted grains: Infected grains show black, powdery fungal spores replacing part of the endosperm. The infection may be localized or affect the entire grain.
- Odor: Infected grains often emit a fishy smell due to the presence of trimethylamine.
- External appearance: Externally, bunted grains may appear slightly discolored or show a dark smudge.
- Ear development: Infected wheat heads may appear normal until grains are examined.
Etiology
- Primary Host: Wheat (Triticum spp.).
- Environmental Conditions:
- Temperature: Optimal range is between 15°C to 22°C.
- Humidity: High humidity and wet conditions favor the development of the disease.
- Soil: Presence of inoculum in the soil from previous crops can be a source of infection.
Disease Cycle
- Primary Infection:
- Soil and Seedborne: The primary source of infection is often from spores present in the soil or contaminated seeds.
- Sporidia: Teliospores in the soil germinate to produce sporidia, which infect the wheat plants.
- Secondary Spread:
- Wind and Rain: Dissemination of sporidia by wind and rain leads to secondary infections.
- Infection Timing: Infection usually occurs during the flowering stage of the wheat plant.
- Overwintering:
- Teliospores: The fungus survives as teliospores in the soil or on seed debris, which can remain viable for several years.
- Seed Contamination: Infected seeds can introduce the pathogen to new areas if not properly managed.
Management
- Cultural Practices:
- Crop Rotation: Rotate with non-host crops to reduce soil borne inoculum.
- Field Hygiene: Remove and destroy crop debris post-harvest to limit sources of infection.
- Clean Seed: Use certified, pathogen-free seed to prevent introducing the fungus to new areas.
- Chemical Control:
- Seed Treatment: Fungicidal seed treatments can reduce the risk of seedborne infection.
- Fungicide Application: Application of fungicides at critical growth stages may help reduce infection levels, although effectiveness varies.
- Resistant Varieties: Developing and planting wheat varieties that are resistant to Tilletia indica.
- Quarantine Measures:
- Regulations: Adhere to quarantine regulations to prevent the spread of Karnal bunt to new areas, especially in countries where the disease is not present.
- Monitoring and Inspection: Regular field inspections and monitoring to detect early infections.