1. Too much watering leads to water accumulation in the soil, poor air permeability and root rot, thus affecting the growth of leaves.

2. excessive fertilization caused fertilizer damage, and the leaves withered and fell off.

3. Long-term water shortage and drought lead to yellowing of leaves.

4. The soil is too alkaline to adapt to the local environment, especially in the northern region. Because the water quality is alkaline, the leaves may gradually fade and turn yellow.

5. Improper maintenance, such as closed environment, poor ventilation, or excessive fertilization and watering, may cause the leaves to turn yellow.

In order to solve these problems, you can try the following methods:

1. Control the amount of watering to make the soil moderately dry and avoid water accumulation.

2. Dilute fertilizer concentration, avoid fertilizer damage, increase organic fertilizer, and provide sufficient nutrients for vanilla.

3. Water regularly to keep the soil moist and prevent drought.

4. To improve the soil environment, we can try to replace the soil suitable for the local environment, or use acidic substances regularly to neutralize the soil alkalinity.

5. Strengthen maintenance management, keep the environment ventilated, apply fertilizer reasonably and irrigate moderately to avoid excessive or insufficient maintenance.

I hope this information can help you solve the problem. If the problem still exists, further investigation and diagnosis may be needed.