Yellowing Base on a Mexican Fencepost Cactus (Pachycereus marginatus)

A 60-inch Mexican fencepost cactus shows yellowing from the base up 15 inches. The yellow tissue may be softer than the healthy green above. This pattern typically indicates rot originating in the roots and moving upward—caused by overwatering, trauma, or root mealybugs.

Unpot and inspect roots. If the base is soft or roots are black/stringy, cut above all rot with a sterilized knife to healthy tissue. Let the cutting callous, then root in barely moist sand. The remaining rooted base (if healthy) may produce new arms in a smaller pot.

Act quickly—rot spreads internally before it's visible. A photo would help confirm the diagnosis.