**IV. Major Pests and Diseases**
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1. **Virus Disease**
Viral diseases are a major concern for farmers, as they can cause significant crop losses. The symptoms vary depending on the type of virus, the plant species, and environmental conditions. In many cases, multiple infections occur simultaneously in the field. Common symptoms include mosaic patterns on leaves, where yellow and green patches appear unevenly. As the disease progresses, the leaves become distorted, wrinkled, or stunted. New leaves may be pale, narrow, or thin, and the overall growth of the plant is reduced. In severe cases, plants may wither and die.
**Control Measures:**
(1) Use healthy seedlings.
(2) Remove and destroy infected plants early.
(3) Control insect vectors such as whiteflies and aphids.
(4) Avoid mechanical transmission by using clean tools and practicing good hygiene.
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2. **Bacterial Wilt**
Bacterial wilt is a vascular disease caused by bacteria that thrive in warm, humid conditions. Infected soil is the main source of the pathogen, which enters through wounds in the roots. Early symptoms include wilting of the lower leaves, followed by progressive wilting and death of the entire plant. The stems often develop adventitious roots, and when cut, the vascular tissue appears brown and exudes a milky, viscous liquid. A simple test involves placing the cut stem in clear water; if bacterial wilt is present, the water will turn cloudy within minutes. This distinguishes it from other bacterial diseases, which do not produce the same milky discharge.
**Control Measures:**
(1) Plant resistant varieties.
(2) Use healthy transplants.
(3) Practice crop rotation.
(4) Apply SH soil additives.
(5) Maintain field hygiene.
(6) Avoid planting in contaminated areas.
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3. **Bacterial Spot**
Bacterial spot affects leaves, fruits, petioles, stems, and inflorescences. Initially, small, water-soaked spots appear on the leaves, which then enlarge into irregular, dark brown lesions with a sunken center and raised edges. The disease spreads rapidly under wet conditions and can lead to defoliation and fruit damage.
**Control Measures:**
(1) Use certified, disease-free seeds and seedlings.
(2) Apply 86% copper hydroxide wettable powder at 800 times dilution at the first sign of infection. Stop application 6 days before harvest.
(3) Strengthen prevention during rainy seasons.
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4. **Late Blight**
Late blight is common in cool, humid environments. It infects leaves, stems, flowers, and fruits. Initial symptoms include dark green, water-soaked lesions that expand quickly, leading to moldy growth and spore production. On fruits, the infection starts as gray-green, water-soaked spots that eventually turn brown and develop a wavy pattern. White fungal growth may appear, but no soft rot occurs.
**Control Measures:**
(1) Apply 75% chlorothalonil wettable powder (600 times), 58% mancozeb (400 times), or 80% mancozeb.
(2) Follow safe harvesting intervals.
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5. **Seedling Phytophthora Blight**
This disease occurs in high humidity and temperature. It primarily affects young seedlings, causing stem rot near the soil line. Lesions start as pale brown spots, then darken and cause the seedling to collapse. The pathogen spreads via irrigation water and rain.
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6. **Southern Blight (White Blight)**
Southern blight thrives in hot, humid conditions. After the pathogen's sclerotia germinate, white mycelium spreads over the plant surface. Symptoms include leaf yellowing, stem base rot, and the formation of brown sclerotia. Fruits in contact with the soil may also rot.
**Control Measures:**
(1) Solarize the soil before planting.
(2) Apply 50% fosetyl-Al WP diluted 5,000 times after planting. Repeat after 15 days.
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7. **Early Blight**
Also known as *Alternaria*, early blight begins with small, water-soaked spots on older leaves, which grow into leathery, dark brown lesions with a yellow halo. Severe infections lead to leaf drop and fruit rot.
**Control Measures:**
(1) Maintain field hygiene.
(2) Apply 86% copper hydroxide WP (800 times), 81.3% Jiake copper (1000 times), or 37.5% copper hydroxide (800 times).
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8. **Black Mold (Leaf Mold)**
Black mold affects leaves, petioles, and stems. Initial symptoms include small gray spots on lower leaves, which later turn gray-brown and develop conidiophores. Infected leaves may dry but do not fall off easily.
**Control Measures:**
(1) Improve air circulation.
(2) Apply 80% Zn Mn NP (400 times) mixed with 50% Lai Lai.
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9. **Root-Knot Nematodes**
These pests damage roots, causing galls, stunted growth, yellowing, and poor fruit development. Infected plants show weak growth and reduced yields.
**Control Measures:**
(1) Practice crop rotation with rice.
(2) Apply nematode control additives based on styrene.
(3) Apply nematicide emulsion (80%) 15 cm deep in the soil before transplanting. Wait 5 days before planting.
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*To be continued...*
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