The Plumeria is experiencing some stress or health issues. The darkening of the trunk and leaf drop can be symptoms of a few different problems. Here are some possible causes and solutions:

Possible Causes

1. Root Rot or Fungal Infection: Overwatering or poor drainage can lead to root rot, which often manifests as a darkening of the trunk. Fungal infections can also cause similar symptoms.
2. Pest Infestation: Pests like spider mites, mealybugs, or scale can stress the plant, leading to leaf drop and trunk discoloration.
3. Environmental Stress: Extreme temperature changes, insufficient light, or drafts can stress Plumeria plants, causing leaf drop.
4. Nutrient Deficiency: Lack of essential nutrients can lead to poor plant health, visible in the trunk’s condition and leaf loss.

Corrective Measures

1. Inspect the Roots: Gently remove the plant from its pot and inspect the roots. Healthy roots are white and firm, while rotting roots are brown, soft, and mushy. Trim away any rotting roots and repot in fresh, well-draining soil.
2. Improve Drainage: Ensure the pot has adequate drainage holes. Use a well-draining soil mix specifically designed for succulents or cacti.
3. Watering Practices: Let the soil dry out between waterings. Plumerias do not like to sit in wet soil. Water less frequently in cooler months when the plant is not actively growing.
4. Check for Pests: Examine the plant for signs of pests and treat with appropriate insecticides or natural remedies like neem oil.
5. Optimize Growing Conditions: Place the plant in a location with bright, indirect sunlight. Protect it from extreme temperature changes.
6. Fertilize Appropriately: Use a balanced, slow-release fertilizer during the growing season. Ensure it contains essential nutrients like nitrogen, phosphorus, and potassium.
7. Prune if Necessary: Remove any severely damaged or dead parts of the plant to encourage new growth.

Monitoring and Care

• After taking these steps, monitor the plant closely for changes. Improvement may take some time.
• Maintain consistent care, keeping in mind that Plumeria’s needs change with seasons. In winter, they go dormant and require less water.

If the problem persists or the plant’s condition worsens, consider consulting a local nursery or a plant specialist for more personalized advice. Sometimes, specific issues require a hands-on examination to diagnose accurately.