A browning stem on your rubber plant (Ficus elastica) can be a sign of distress. Common causes include overwatering, poor drainage, or fungal infections. Overwatering often leads to root rot, which can spread to the stem, turning it brown and soft. Ensure the soil is well-draining, and allow it to dry out between waterings.
Fungal infections, caused by excessive moisture or poor air circulation, can also damage the plant. Prune the affected areas and treat with a fungicide.

Environmental stress, like extreme temperature changes or low humidity, can weaken the stem. Place your plant in bright, indirect light and maintain consistent humidity.

For more insights on troubleshooting plant problems, check out this gardening guide. Taking timely action will help your rubber plant recover and thrive.