Diseases in melon crops typically occur under specific conditions within greenhouses and other controlled environments. These conditions include the presence of susceptible melon plants, a sufficient number of pathogens, and favorable environmental factors such as temperature and humidity.

Pathogens

The presence of pathogens is essential for the development of any disease. Common types of pathogens include fungi, bacteria, and viruses. These organisms often reside on host plants or other carriers and can spread through various transmission routes when conditions are suitable.

Transmission Routes

1. Seed Transmission: Seeds can carry pathogens, especially bacterial diseases like bacterial leaf spot or anthrax. These pathogens can survive in seeds over winter or summer.

2. Seedbed Contamination: Infected soil used for seedlings can lead to disease spread. This often happens when organic fertilizers or soil containing pathogens are not fully decomposed.

3. Diseased Seedling Introduction: If diseased seedlings are not removed before planting, they can become a source of infection in the field.

4. Residual Plant Material: After harvest, leftover plant parts such as roots, leaves, and fruits left in the field can harbor pathogens, which may later infect new crops.

5. Soil Carriers: Repeated planting in the same greenhouse leads to pathogen accumulation in the soil, increasing disease incidence.

6. Irrigation Water: Water can carry pathogens from infected areas or soil into healthy crops.

7. Insect Vectors: Insects with sucking mouthparts, such as aphids, can transmit viral diseases from infected to healthy plants.

8. Airborne Spread: High levels of spores in the air can cause infections when environmental conditions are favorable.

9. Other Sources: Equipment, tools, clothing, and even human contact can act as carriers of pathogens.

Environmental Conditions

Humidity plays a crucial role in disease development. Most fungal pathogens require high humidity for spore germination and growth. For example, fusarium wilt and gray mold thrive at relative humidity above 90%, while bacterial leaf spot requires more than 70% humidity.

Factors Contributing to High Humidity

1. Continuous Rainfall: Reduced sunlight and poor ventilation in greenhouses increase internal humidity.

2. Limited Ventilation: During cold seasons, greenhouses are often sealed, leading to higher humidity levels.

3. Excessive Watering: Overwatering increases soil moisture, creating ideal conditions for disease development.

4. Overcrowded Planting: Dense planting reduces airflow, causing humidity to build up in enclosed spaces.

5. Poor Field Preparation: Uneven land or improper irrigation leads to water pooling, which becomes a hotspot for disease.

6. Condensation on Greenhouse Film: Temperature differences between inside and outside the greenhouse cause condensation, increasing soil moisture and disease risk.

Prevention Measures

Reducing Pathogen Load

1. Use clean seedbed soil that has not been used for melons in the past three years. Ensure proper disinfection and use fully decomposed compost.

2. Implement strict seed quarantine and disinfection methods such as hot water treatment or seed coating to prevent seed-borne diseases.

3. Inspect seedlings carefully before transplanting and apply broad-spectrum fungicides in the seedbeds.

4. Practice crop rotation and avoid continuous planting of melons in the same area.

5. Disinfect the soil by solarizing it during summer, raising temperatures to 50–60°C for 10–15 days.

6. Disinfect greenhouses using sulfur smoke or chlorothalonil agents before planting.

7. Remove weeds and diseased plant parts regularly, and dispose of them by burning or burying.

Controlling Humidity

Water the fields properly before planting, and avoid excessive watering during the growing season. Use drip irrigation and mulching to reduce evaporation. Maintain good ventilation by adjusting row spacing and ensuring proper airflow.

Temperature Management

Maintain nighttime temperatures around 15–18°C and raise daytime temperatures to 25–28°C to avoid optimal conditions for diseases like downy mildew.

Breaking the Infection Cycle

Avoid cross-contamination between healthy and diseased plants. Control insect pests promptly and remove infected plants from the field immediately.

Agricultural Practices

1. Choose disease-resistant varieties such as Elizabeth or Taiwan Cuixiang.

2. Improve field management by applying organic fertilizer, maintaining proper light and temperature, and pruning regularly.

3. Use grafting techniques, such as black-seed squash rootstock, to control fusarium wilt.

4. Apply foliar sprays like a mixture of sugar and urea (0.75 kg each in 50 L water) every 5–7 days. Potassium dihydrogen phosphate (0.2%) can also help prevent disease.

Chemical Control

At the early stages of disease, apply biological or low-toxicity pesticides. For bacterial diseases, use 72% streptomycin sulfate (4000 times) or 14% copper hydroxide (200 times). For fungal diseases, consider 45% chlorothalonil smoke or 75% chlorothalonil spray (500–600 times). For virus prevention, use 500 times diluted virus A or 1.5% disease Lingshui emulsion (800–1000 times). Silver or black plastic films can also be used to block virus transmission.

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