Brown leaves on Dieffenbachia can be caused by a variety of factors, including overwatering, underwatering, lighting, temperature, or pest infestation. Here's a detailed guide on why your Dieffenbachia has brown leaves and how to solve it:

1. Overwatering Overwatering can cause Dieffenbachia leaves to turn brown and wilt. Ensure that the soil is well-draining and water the plant only when the top inch of soil feels dry to the touch. Avoid letting the plant sit in standing water.

2. Underwatering Underwatering can also cause Dieffenbachia leaves to turn brown and dry out. Ensure that the soil is evenly moist but not waterlogged, and water the plant when the top inch of soil feels dry to the touch.

3. Lighting Dieffenbachia requires bright, indirect light to thrive. Insufficient light can cause the leaves to turn brown and fall off. Place the plant near a window with filtered light or use a grow light to provide the necessary light.

4. Temperature Dieffenbachia prefers warm temperatures between 65°F and 75°F. Exposing the plant to temperatures below 60°F can cause damage to the leaves and lead to browning.

5. Pest Infestation Pests such as spider mites or mealybugs can infest Dieffenbachia plants and cause the leaves to turn brown and dry out. Inspect the plant regularly and treat any pest infestations with insecticidal soap or neem oil.

6. Fertilizer Over-fertilizing can cause Dieffenbachia leaves to turn brown and curl. Use a balanced liquid fertilizer once a month during the growing season (spring and summer) and avoid fertilizing the plant during the winter months when it's dormant.

In summary, brown leaves on Dieffenbachia can be caused by overwatering, underwatering, lighting, temperature, pest infestation, or over-fertilizing. Ensure that the plant is receiving adequate water, light, and temperature, and watch for pest infestations or over-fertilization. By providing the appropriate care and addressing any issues promptly, you can help your Dieffenbachia recover and prevent further leaf browning.