1, insufficient lighting, no ventilation and light transmission.

2, fertilization is unscientific, and there is too much nitrogen fertilizer.

3. It may be a degraded variety.

If you need to fight, when the seedlings are over-fertilized, spray an inhibitory regulator such as chlorpromazine once, usually 2-3 times, to control vigorous growth.

Pay attention to the reasonable collocation of nitrogen, phosphorus and potassium when applying compound fertilizer. The sweet potato field may be in front of the house and behind it. Due to years of domestic garbage, the soil acidity and nitrogen content in these places are too heavy.

Extended data:

Cultivation techniques of sweet potato;

1. Soil preparation and fertilization: Soil preparation should be carried out on sunny days, and the soil should be broken and finely ground to make it loose.

2. Seedbed seedling raising: In mid-February, the seedling raising time is appropriately advanced, and the seed potatoes are moved into the protected seedbed for seedling raising.

3. Adequate base fertilizer: loam and sandy plots are selected, and 2-3 tons of farmyard manure, 0/5 kg of urea/kloc and 20 kg of special phosphate fertilizer are applied in early spring.

4. Plastic film mulching: Plastic film mulching can play a role in warming and preserving moisture, improving the physical and chemical properties of soil and increasing the temperature difference between day and night.

5. Timely early planting: timely early planting and reasonable close planting: mid-April is the suitable time for seedling raising, and it is basically stable when the temperature rises in early spring all year round. The suitable time for seedling planting is after the end of late frost, when the ground temperature reaches above 10 cm, seedlings can be planted. Last winter or at the beginning of the year, the seedlings were well sunned and ridged again when planted.

Reference source: Baidu Encyclopedia-Sweet Potato