**IV. Major Pests and Diseases**
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1. **Virus Disease**
Viral diseases are a major concern for farmers, causing significant damage to crops. The symptoms vary depending on the type of virus, the plant species, and environmental conditions. Compound infections are common in fields, especially under stress. The most typical signs include mosaic patterns on leaves, with uneven yellow and green areas. Infected leaves may become distorted, stunted, or have an irregular texture. New leaves often appear pale, narrow, or thin, and the overall plant growth is weakened, leading to severe stunting or even death.
**Control Measures**:
- Use healthy seedlings.
- Remove infected plants early.
- Control vector insects like whiteflies and aphids.
- Avoid mechanical transmission through tools or hands.
2. **Bacterial Wilt**
Bacterial wilt is a vascular disease caused by bacteria that thrive in warm and humid conditions. The main source of infection is contaminated soil. Bacteria enter through wounds in the roots and spread through the vascular system. Early symptoms include wilting of the upper leaves, followed by gradual drying of lower leaves. Stems may develop adventitious roots, and when cut, the vascular bundles appear brown and exude a milky, viscous fluid. A simple test involves placing a cut stem in clear water; if bacterial wilt is present, the water will turn milky within minutes.
**Control Measures**:
- Plant resistant varieties.
- Use disease-free transplants.
- Rotate crops with rice.
- Apply SH soil additives.
- Maintain field hygiene.
- Practice crop rotation (Lai Tin).
3. **Bacterial Spot**
This disease affects leaves, fruits, stems, and petioles. Initially, small water-soaked spots appear on the leaves, which later expand into irregular, dark brown lesions with sunken centers and raised edges.
**Control Measures**:
- Use clean seeds and seedlings.
- Apply 86% copper hydroxide wettable powder diluted 800 times at the first sign of disease. Stop application 6 days before harvest.
- Strengthen prevention during rainy seasons.
4. **Late Blight**
Late blight thrives in cool, moist environments and can affect all parts of the plant, including leaves, stems, flowers, and fruits. Initial symptoms include dark green, water-soaked spots that rapidly expand, forming fuzzy, white mycelium. On fruits, it causes gray-green, water-soaked lesions that eventually turn brown and hard.
**Control Measures**:
- Apply 75% chlorothalonil wettable powder diluted 600 times.
- Use 58% mancozeb WP at 400 times dilution.
- Follow safe harvesting intervals.
5. **Seedling Phytophthora Blight**
This disease occurs in hot and humid conditions, affecting young seedlings. It causes stem rot near the soil line, leading to wilting and death.
**Control Measures**:
- Ensure proper drainage.
- Avoid overwatering.
- Apply fungicides as needed.
6. **Southern Blight (White Blight)**
Southern blight is common in high humidity and temperature. White fungal threads grow on the plant surface, causing leaf wilting and stem base rot. Sclerotia form around the infected area, and fruits may also be affected.
**Control Measures**:
- Solarize the soil before planting.
- Apply 50% fosetyl-Al WP diluted 5,000 times after planting. Repeat after 15 days.
7. **Early Blight**
Also known as *Alternaria*, this disease starts with small, dark brown spots on older leaves. Lesions expand into leathery, wrinkled patches with yellow halos. Severe cases lead to defoliation and fruit rot.
**Control Measures**:
- Maintain good field hygiene.
- Apply 86% copper hydroxide WP at 800 times dilution.
- Use 81.3% copper oxychloride at 1000 times dilution.
8. **Black Mold (Leaf Mold)**
This disease affects leaves, petioles, and stems. Small gray spots appear on the lower leaves and gradually turn into brownish-gray lesions.
**Control Measures**:
- Improve air circulation.
- Apply 80% Zn Mn NP wettable powder at 400 times dilution. Mix with 50% chlorothalonil.
9. **Root-Knot Nematode**
These microscopic worms attack the roots, causing galls and root decay. Infected plants show stunted growth, yellowing, and poor fruit production.
**Control Measures**:
- Rotate with rice.
- Apply nematode control additives based on styrene.
- Treat soil with 80% nematicide emulsion before transplanting, with a 5-day waiting period.
**To be continued...**
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