Watermelon fruit rot, also known as "frozen prickly heat," is a common issue that affects the quality and marketability of watermelons. The symptoms include water-soaked spots around the seeds, which turn purple or dark brown. In severe cases, the seeds become rotten, making the fruit inedible and reducing its value. According to field surveys, several factors contribute to this condition.
First, the choice of rootstock plays a significant role. Comparisons between different plots showed that rootstocks with white pumpkin seeds had a lower incidence (about 5%), while those with loquat seeds had higher rates, up to 90% in some cases. Areas like Sunrise Office and Boluopu Town, where gourd-based rootstocks are more common, experience more severe fruit rot. In contrast, towns like Tangchi and Shagangzi, which use more white pumpkin seed rootstocks, report fewer issues.
Second, insufficient nutrition in the seedling stage can lead to weak plants. When both rootstocks and scions are sown in the same small nutrient container, overcrowding and high night temperatures can cause elongated growth and weakened resistance.
Third, overuse of certain fertilizers, particularly chicken manure, can increase soil alkalinity and reduce permeability, negatively affecting plant health.
Fourth, cold weather after transplanting can be devastating. For example, in 2006, a farmer in Xihu Village, Boluopu Town, planted 5 acres of greenhouse watermelons on April 8th. Six days later, a sudden cold snap dropped the temperature to 2.5°C. The seedlings suffered from poor root development and yellow leaves, and it took nearly two weeks for them to recover. This delay severely impacted the crop, leading to 85% of the fruits being affected and a financial loss of 90,000 yuan despite an investment of 20,000 yuan.
Fifth, inadequate temperature differences and excessive humidity create ideal conditions for disease. High night temperatures hinder plant assimilation, and high humidity increases the risk of fungal infections such as anthracnose.
Sixth, delayed harvesting and exposure to strong sunlight can cause sunburn and further damage. When watermelons mature, they should be harvested promptly to avoid overexposure to intense light, which can lead to sunburn and prickly fruit syndrome.
Seventh, virus infections during the seedling and early fruiting stages can also lead to fruit rot. Infected plants often show stunted growth, yellowing leaves, and uneven leaf texture.
To prevent these issues, several measures can be taken:
1. Use disease-free seeds for both rootstocks and scions. Avoid introducing seeds from infected areas and treat them with dry heat or chemicals before planting.
2. Grow seedlings in disease-free soil. Choose fertile soil far from previous melon fields and mix it with a nursery agent to sterilize and strengthen the seedlings. A typical mix might involve 500 bags of nursery mother with one bag, enough for about 1,500 pots.
3. Practice crop rotation. Rotate watermelon crops with non-cucurbit plants like eggplants, legumes, or green onions every 2-3 years to reduce disease buildup.
4. Use white pumpkin seeds as rootstocks, as they are less prone to fruit rot.
5. Disinfect tools regularly. Grafting blades should be soaked in 75% alcohol after each use. Avoid damaging plants during farming to prevent virus transmission.
6. Apply fertilizer wisely. Use bio-organic compound fertilizers as a base, along with well-decomposed chicken manure and balanced mineral nutrients.
7. Prepare for late spring frosts. Plant watermelons in early April, when they are most vulnerable. Have protective materials ready and consider using fumigants to prevent both disease and cold damage, keeping the greenhouse temperature above 15°C.
8. Manage humidity effectively. Watermelon plants do not tolerate high moisture levels. Use drip irrigation under plastic sheets to reduce humidity and prevent disease spread.
9. Control temperature. Maintain a daytime temperature between 24-30°C and ensure the minimum temperature does not drop below 15°C. Watermelons thrive in warm, well-drained soil with good air circulation and a pH of 5-7.
10. Protect ripening fruits from direct sunlight. As watermelons mature, cover them with leaves to prevent sunburn and maintain quality.
11. Prevent viral diseases. Viruses are a major cause of fruit rot. Symptoms include small, curled, yellow-green leaves. Along with using disease-free seeds and crop rotation, effective antiviral pesticides can be applied. These agents help destroy virus structures and restore plant health. Field tests have shown promising results in managing viral outbreaks.
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