Model Answer
0 min readIntroduction
Plant diseases pose a significant threat to global food security, causing substantial economic losses in agriculture. Citrus canker and late blight of potato are two devastating diseases that affect commercially important crops worldwide. Citrus canker, caused by a bacterium, severely impacts citrus production, while late blight, a fungal disease, historically led to the Irish potato famine and continues to be a major concern for potato growers. Understanding the etiology, symptoms, and effective control measures for these diseases is crucial for sustainable agricultural practices.
Citrus Canker
Citrus canker is a contagious bacterial disease affecting citrus plants, caused by Xanthomonas citri subsp. citri. It is a major limiting factor in citrus production in many tropical and subtropical regions.
Causal Organism
Xanthomonas citri subsp. citri is a Gram-negative bacterium. It enters the plant through wounds and natural openings like stomata and hydathodes.
Symptoms
- Leaf Spots: Raised, circular, water-soaked lesions on leaves, initially yellowish and later becoming brown and corky. A characteristic feature is a halo of chlorosis around the lesions.
- Fruit Lesions: Similar lesions appear on fruits, causing deformation, premature fruit drop, and reduced market value.
- Twig and Stem Cankers: Lesions can develop on twigs and stems, leading to dieback.
- Defoliation: Severe infections can cause extensive defoliation.
Control Measures
- Copper-based Sprays: Regular application of copper oxychloride or copper hydroxide sprays can prevent infection.
- Pruning: Removal and destruction of infected twigs, branches, and fruits.
- Sanitation: Disinfection of pruning tools and equipment.
- Resistant Varieties: Use of canker-resistant citrus varieties where available.
- Biological Control: Application of antagonistic bacteria like Bacillus subtilis.
- Quarantine: Strict quarantine measures to prevent the spread of the disease to new areas.
Late Blight of Potato
Late blight of potato, caused by the oomycete Phytophthora infestans, is a destructive disease that can rapidly devastate potato and tomato crops. It gained notoriety for causing the Irish potato famine in the mid-19th century.
Causal Organism
Phytophthora infestans is an oomycete (water mold), not a true fungus. It produces zoospores that are dispersed by wind and rain.
Symptoms
- Leaf Blight: Irregular, water-soaked lesions on leaves, often starting at the tips or margins. A white, fuzzy growth (sporulation) is visible on the underside of leaves in humid conditions.
- Stem Blight: Dark brown, water-soaked lesions on stems, leading to girdling and collapse.
- Tuber Rot: Brown, reddish-purple, and dry rot of tubers, extending deep into the flesh. Secondary bacterial infections often occur.
- Foul Odor: Infected tubers often emit a foul odor.
Control Measures
- Protective Fungicides: Application of protectant fungicides like mancozeb or chlorothalonil before disease onset.
- Systemic Fungicides: Use of systemic fungicides like metalaxyl or cymoxanil for curative control.
- Resistant Varieties: Planting of late blight-resistant potato varieties.
- Seed Treatment: Use of disease-free seed tubers.
- Crop Rotation: Rotating potatoes with non-host crops.
- Proper Ventilation: Ensuring good air circulation in potato fields.
- Early Blight Monitoring: Regular scouting for early signs of the disease.
Comparative Table
| Feature | Citrus Canker | Late Blight of Potato |
|---|---|---|
| Causal Organism | Xanthomonas citri subsp. citri (Bacterium) | Phytophthora infestans (Oomycete) |
| Host Plants | Citrus species | Potato and Tomato |
| Primary Symptoms | Leaf spots, fruit lesions, cankers | Leaf blight, stem blight, tuber rot |
| Mode of Spread | Wind, rain, insects, mechanical means | Wind, rain, zoospores |
| Control Measures | Copper sprays, pruning, sanitation | Fungicides, resistant varieties, seed treatment |
Conclusion
Both citrus canker and late blight of potato represent significant challenges to agricultural productivity. Effective management requires a combination of preventative measures, including the use of resistant varieties, sanitation practices, and timely application of appropriate control agents. Integrated disease management strategies, incorporating biological control and monitoring, are crucial for minimizing losses and ensuring sustainable crop production. Continuous research and development of new control methods are essential to combat the evolving nature of these plant pathogens.
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.